ATmega328/P
AVR® Microcontroller with picoPower® Technology
Introduction
The picoPower® ATmega328/P is a low-power CMOS 8-bit microcontroller based on the AVR® enhanced
RISC architecture. By executing powerful instructions in a single clock cycle, the ATmega328/P achieves
throughputs close to 1 MIPS per MHz. This empowers system designers to optimize the device for power
consumption versus processing speed.
Feature
High Performance, Low-Power AVR® 8-Bit Microcontroller Family
•Advanced RISC Architecture
– 131 Powerful instructions
– Most single clock cycle execution
– 32 x 8 General purpose working registers
– Fully static operation
– Up to 20 MIPS throughput at 20 MHz
– On-chip 2-cycle multiplier
• High Endurance Nonvolatile Memory Segments
– 32K Bytes of in-system self-programmable Flash program memory
– 1K Bytes EEPROM
– 2K Bytes internal SRAM
– Write/erase cycles: 10,000 Flash/100,000 EEPROM
– Data retention: 20 years at 85°C/100 years at 25°C(1)
– Optional boot code section with independent lock bits
• In-system programming by on-chip boot program
• True read-while-write operation
– Programming lock for software security
• QTouch Library Support
– Capacitive touch buttons, sliders and wheels
– QTouch and QMatrix acquisition
– Up to 64 sense channels
• Peripheral Features
– Two 8-bit Timer/counters with separate prescaler and Compare mode
– One 16-bit Timer/counter with separate prescaler, Compare mode, and Capture mode
– Real time counter with separate oscillator
– Six PWM channels
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 1
– 8-channel 10-bit ADC in TQFP and QFN/MLF package
• Temperature measurement
– 6-channel 10-bit ADC in PDIP package
• Temperature measurement
– Two master/slave SPI serial interface
– One programmable serial USART
– One byte-oriented 2-wire serial interface (Philips I2C compatible)
– Programmable watchdog timer with separate on-chip oscillator
– One on-chip analog comparator
– Interrupt and wake-up on pin change
• Special Microcontroller Features
– Power-on Reset and programmable Brown-out Detection
– Internal calibrated oscillator
– External and internal interrupt sources
– Six sleep modes: idle, ADC noise reduction, power-save, power-down, standby, and extended
standby
• I/O and Packages
– 23 Programmable I/O lines
– 28-pin PDIP, 32-lead TQFP, 28-pad QFN/MLF and 32-pad QFN/MLF
• Operating Voltage:
– 1.8 - 5.5V
• Temperature Range:
– -40°C to 105°C
• Speed Grade:
– ATmega328/P: 0 - 4 MHz @ 1.8 - 5.5V, 0 - 10 MHz @ 2.7 - 5.5V, 0 - 20 MHz @ 4.5 - 5.5V
• Power Consumption at 1 MHz, 1.8V, 25°C
– Active mode: 0.2 mA
– Power-Down mode: 0.1 μA
– Power-Save mode: 0.75 μA (Including 32 kHz RTC)
ATmega328/P
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 2
Table of Contents
Introduction......................................................................................................................1
Feature............................................................................................................................ 1
1. Description.................................................................................................................9
2. Configuration Summary...........................................................................................10
3. Ordering Information ...............................................................................................11
3.1. ATmega328 ............................................................................................................................... 11
3.2. ATmega328P .............................................................................................................................11
4. Block Diagram......................................................................................................... 13
5. Pin Configurations................................................................................................... 14
5.1. Pinout......................................................................................................................................... 14
5.2. Pin Descriptions......................................................................................................................... 17
6. I/O Multiplexing........................................................................................................19
7. Resources............................................................................................................... 21
8. Data Retention.........................................................................................................22
9. About Code Examples.............................................................................................23
10. Capacitive Touch Sensing....................................................................................... 24
10.1. QTouch Library........................................................................................................................... 24
11. AVR CPU Core........................................................................................................ 25
11.1. Overview.................................................................................................................................... 25
11.2. Arithmetic Logic Unit (ALU)........................................................................................................ 26
11.3. Status Register...........................................................................................................................26
11.4. General Purpose Register File................................................................................................... 29
11.5. Stack Pointer..............................................................................................................................30
11.6. Instruction Execution Timing...................................................................................................... 32
11.7. Reset and Interrupt Handling..................................................................................................... 33
12. AVR Memories.........................................................................................................36
12.1. Overview.................................................................................................................................... 36
12.2. In-System Reprogrammable Flash Program Memory................................................................36
12.3. SRAM Data Memory.................................................................................................................. 37
12.4. EEPROM Data Memory............................................................................................................. 38
12.5. I/O Memory.................................................................................................................................39
12.6. Register Description................................................................................................................... 40
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 3
13. System Clock and Clock Options............................................................................ 49
13.1. Clock Systems and Their Distribution........................................................................................ 49
13.2. Clock Sources............................................................................................................................ 50
13.3. Low-Power Crystal Oscillator..................................................................................................... 52
13.4. Full Swing Crystal Oscillator.......................................................................................................54
13.5. Low-Frequency Crystal Oscillator.............................................................................................. 55
13.6. Calibrated Internal RC Oscillator................................................................................................56
13.7. 128 kHz Internal Oscillator......................................................................................................... 57
13.8. External Clock............................................................................................................................ 58
13.9. Timer/Counter Oscillator.............................................................................................................59
13.10. Clock Output Buffer....................................................................................................................59
13.11. System Clock Prescaler............................................................................................................. 59
13.12. Register Description...................................................................................................................60
14. Power Management and Sleep Modes................................................................... 64
14.1. Overview.................................................................................................................................... 64
14.2. Sleep Modes.............................................................................................................................. 64
14.3. BOD Disable...............................................................................................................................65
14.4. Idle Mode....................................................................................................................................65
14.5. ADC Noise Reduction Mode...................................................................................................... 65
14.6. Power-Down Mode.....................................................................................................................66
14.7. Power-Save Mode......................................................................................................................66
14.8. Standby Mode............................................................................................................................ 67
14.9. Extended Standby Mode............................................................................................................67
14.10. Power Reduction Register......................................................................................................... 67
14.11. Minimizing Power Consumption................................................................................................. 67
14.12. Register Description...................................................................................................................69
15. System Control and Reset.......................................................................................74
15.1. Resetting the AVR...................................................................................................................... 74
15.2. Reset Sources............................................................................................................................74
15.3. Power-on Reset..........................................................................................................................75
15.4. External Reset............................................................................................................................76
15.5. Brown-out Detection...................................................................................................................76
15.6. Watchdog System Reset............................................................................................................77
15.7. Internal Voltage Reference.........................................................................................................77
15.8. Watchdog Timer......................................................................................................................... 78
15.9. Register Description................................................................................................................... 80
16. Interrupts................................................................................................................. 84
16.1. Interrupt Vectors in ATmega328/P............................................................................................. 84
16.2. Register Description................................................................................................................... 86
17. EXTINT - External Interrupts................................................................................... 89
17.1. Pin Change Interrupt Timing.......................................................................................................89
17.2. Register Description................................................................................................................... 90
18. I/O-Ports.................................................................................................................. 99
ATmega328/P
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 4
18.1. Overview.................................................................................................................................... 99
18.2. Ports as General Digital I/O......................................................................................................100
18.3. Alternate Port Functions...........................................................................................................103
18.4. Register Description................................................................................................................. 115
19. 8-bit Timer/Counter0 (TC0) with PWM.................................................................. 127
19.1. Features................................................................................................................................... 127
19.2. Overview.................................................................................................................................. 127
19.3. Timer/Counter Clock Sources.................................................................................................. 129
19.4. Counter Unit............................................................................................................................. 129
19.5. Output Compare Unit............................................................................................................... 130
19.6. Compare Match Output Unit.....................................................................................................132
19.7. Modes of Operation..................................................................................................................134
19.8. Timer/Counter Timing Diagrams.............................................................................................. 138
19.9. Register Description................................................................................................................. 140
20. 16-bit Timer/Counter1 (TC1) with PWM................................................................ 152
20.1. Overview.................................................................................................................................. 152
20.2. Features................................................................................................................................... 152
20.3. Block Diagram.......................................................................................................................... 152
20.4. Definitions.................................................................................................................................153
20.5. Registers.................................................................................................................................. 154
20.6. Accessing 16-bit Timer/Counter Registers...............................................................................154
20.7. Timer/Counter Clock Sources.................................................................................................. 157
20.8. Counter Unit............................................................................................................................. 157
20.9. Input Capture Unit.................................................................................................................... 158
20.10. Output Compare Units............................................................................................................. 160
20.11. Compare Match Output Unit.....................................................................................................162
20.12. Modes of Operation..................................................................................................................163
20.13. Timer/Counter 0, 1 Prescalers................................................................................................. 171
20.14. Timer/Counter Timing Diagrams.............................................................................................. 171
20.15. Register Description.................................................................................................................173
21. Timer/Counter 0, 1 Prescalers...............................................................................186
21.1. Internal Clock Source............................................................................................................... 186
21.2. Prescaler Reset........................................................................................................................186
21.3. External Clock Source..............................................................................................................186
21.4. Register Description................................................................................................................. 188
22. 8-bit Timer/Counter2 (TC2) with PWM and Asynchronous Operation...................190
22.1. Features................................................................................................................................... 190
22.2. Overview.................................................................................................................................. 190
22.3. Timer/Counter Clock Sources.................................................................................................. 192
22.4. Counter Unit............................................................................................................................. 192
22.5. Output Compare Unit............................................................................................................... 193
22.6. Compare Match Output Unit.....................................................................................................195
22.7. Modes of Operation..................................................................................................................196
22.8. Timer/Counter Timing Diagrams.............................................................................................. 200
ATmega328/P
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 5
22.9. Asynchronous Operation of Timer/Counter2............................................................................201
22.10. Timer/Counter Prescaler.......................................................................................................... 203
22.11. Register Description................................................................................................................. 203
23. Serial Peripheral Interface (SPI)............................................................................218
23.1. Features................................................................................................................................... 218
23.2. Overview.................................................................................................................................. 218
23.3. SS Pin Functionality................................................................................................................. 222
23.4. Data Modes..............................................................................................................................222
23.5. Register Description................................................................................................................. 223
24. Universal Synchronous Asynchronous Receiver Transceiver (USART)............... 228
24.1. Features................................................................................................................................... 228
24.2. Overview.................................................................................................................................. 228
24.3. Block Diagram.......................................................................................................................... 228
24.4. Clock Generation......................................................................................................................229
24.5. Frame Formats.........................................................................................................................232
24.6. USART Initialization................................................................................................................. 233
24.7. Data Transmission – The USART Transmitter......................................................................... 234
24.8. Data Reception – The USART Receiver.................................................................................. 236
24.9. Asynchronous Data Reception.................................................................................................240
24.10. Multi-Processor Communication Mode.................................................................................... 243
24.11. Examples of Baud Rate Setting............................................................................................... 243
24.12. Register Description.................................................................................................................246
25. USART in SPI (USARTSPI) Mode.........................................................................256
25.1. Features................................................................................................................................... 256
25.2. Overview.................................................................................................................................. 256
25.3. Clock Generation......................................................................................................................256
25.4. SPI Data Modes and Timing.....................................................................................................257
25.5. Frame Formats.........................................................................................................................257
25.6. Data Transfer............................................................................................................................259
25.7. AVR USART MSPIM vs. AVR SPI............................................................................................260
25.8. Register Description................................................................................................................. 261
26. Two-Wire Serial Interface (TWI)............................................................................ 262
26.1. Features................................................................................................................................... 262
26.2. Two-Wire Serial Interface Bus Definition..................................................................................262
26.3. Data Transfer and Frame Format.............................................................................................263
26.4. Multi-Master Bus Systems, Arbitration, and Synchronization...................................................266
26.5. Overview of the TWI Module.................................................................................................... 268
26.6. Using the TWI...........................................................................................................................270
26.7. Transmission Modes................................................................................................................ 273
26.8. Multi-Master Systems and Arbitration...................................................................................... 291
26.9. Register Description................................................................................................................. 292
27. Analog Comparator (AC).......................................................................................300
27.1. Overview.................................................................................................................................. 300
ATmega328/P
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 6
27.2. Analog Comparator Multiplexed Input...................................................................................... 300
27.3. Register Description................................................................................................................. 301
28. Analog-to-Digital Converter (ADC)........................................................................ 305
28.1. Features................................................................................................................................... 305
28.2. Overview.................................................................................................................................. 305
28.3. Starting a Conversion...............................................................................................................307
28.4. Prescaling and Conversion Timing...........................................................................................308
28.5. Changing Channel or Reference Selection.............................................................................. 310
28.6. ADC Noise Canceler................................................................................................................ 312
28.7. ADC Conversion Result........................................................................................................... 315
28.8. Temperature Measurement...................................................................................................... 316
28.9. Register Description................................................................................................................. 316
29. debugWIRE On-chip Debug System.....................................................................325
29.1. Features................................................................................................................................... 325
29.2. Overview.................................................................................................................................. 325
29.3. Physical Interface..................................................................................................................... 325
29.4. Software Breakpoints............................................................................................................... 326
29.5. Limitations of debugWIRE........................................................................................................326
29.6. Register Description................................................................................................................. 326
30. Boot Loader Support – Read-While-Write Self-programming (BTLDR)................ 328
30.1. Features................................................................................................................................... 328
30.2. Overview.................................................................................................................................. 328
30.3. Application and Boot Loader Flash Sections............................................................................328
30.4. Read-While-Write and No Read-While-Write Flash Sections...................................................329
30.5. Boot Loader Lock Bits.............................................................................................................. 331
30.6. Entering the Boot Loader Program...........................................................................................332
30.7. Addressing the Flash During Self-Programming...................................................................... 333
30.8. Self-Programming the Flash.....................................................................................................334
30.9. Register Description................................................................................................................. 342
31. Memory Programming (MEMPROG).....................................................................345
31.1. Program And Data Memory Lock Bits...................................................................................... 345
31.2. Fuse Bits.................................................................................................................................. 346
31.3. Signature Bytes........................................................................................................................348
31.4. Calibration Byte........................................................................................................................349
31.5. Serial Number.......................................................................................................................... 349
31.6. Page Size.................................................................................................................................349
31.7. Parallel Programming Parameters, Pin Mapping, and Commands..........................................349
31.8. Parallel Programming...............................................................................................................351
31.9. Serial Downloading.................................................................................................................. 359
32. Electrical Characteristics....................................................................................... 364
32.1. Absolute Maximum Ratings......................................................................................................364
32.2. Common DC Characteristics....................................................................................................364
32.3. Speed Grades.......................................................................................................................... 367
ATmega328/P
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 7
32.4. Clock Characteristics................................................................................................................368
32.5. System and Reset Characteristics........................................................................................... 369
32.6. SPI Timing Characteristics....................................................................................................... 370
32.7. Two-Wire Serial Interface Characteristics................................................................................ 371
32.8. ADC Characteristics................................................................................................................. 373
32.9. Parallel Programming Characteristics...................................................................................... 374
33. Typical Characteristics (TA = -40°C to 85°C).........................................................377
33.1. ATmega328 Typical Characteristics......................................................................................... 377
34. Typical Characteristics (TA = -40°C to 105°C).......................................................402
34.1. ATmega328P Typical Characteristics.......................................................................................402
35. Register Summary.................................................................................................427
35.1. Note..........................................................................................................................................429
36. Instruction Set Summary....................................................................................... 431
37. Packaging Information...........................................................................................436
37.1. 32-pin 32A................................................................................................................................436
37.2. 32-pin 32M1-A..........................................................................................................................437
37.3. 28-pin 28M1............................................................................................................................. 438
37.4. 28-pin 28P3..............................................................................................................................438
38. Errata.....................................................................................................................440
38.1. Errata ATmega328/P................................................................................................................440
39. Datasheet Revision History................................................................................... 441
39.1. Rev. A – 2/2018........................................................................................................................441
39.2. Pre Microchip Revisions...........................................................................................................441
The Microchip Web Site.............................................................................................. 442
Customer Change Notification Service........................................................................442
Customer Support....................................................................................................... 442
Microchip Devices Code Protection Feature............................................................... 442
Legal Notice.................................................................................................................443
Trademarks................................................................................................................. 443
Quality Management System Certified by DNV...........................................................444
Worldwide Sales and Service......................................................................................445
ATmega328/P
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 8
1. Description
The AVR® core combines a rich instruction set with 32 general purpose working registers. All the 32
registers are directly connected to the Arithmetic Logic Unit (ALU), allowing two independent registers to
be accessed in a single instruction executed in one clock cycle. The resulting architecture is more code
efficient while achieving throughputs up to ten times faster than conventional CISC microcontrollers.
The ATmega328/P provides the following features: 32Kbytes of in-system programmable Flash with read-
while-write capabilities, 1Kbytes EEPROM, 2Kbytes SRAM, 23 general purpose I/O lines, 32 general
purpose working registers, Real Time Counter (RTC), three flexible timer/counters with Compare modes
and PWM, 1 serial programmable USARTs , 1 byte-oriented 2-wire Serial Interface (I2C), a 6-channel 10-
bit ADC (8 channels in TQFP and QFN/MLF packages), a programmable watchdog timer with internal
oscillator, an SPI serial port, and six software selectable power saving modes. The Idle mode stops the
CPU while allowing the SRAM, timer/counters, SPI port, and interrupt system to continue functioning. The
Power-Down mode saves the register contents but freezes the oscillator, disabling all other chip functions
until the next interrupt or hardware Reset. In Power-Save mode, the asynchronous timer continues to run,
allowing the user to maintain a timer base while the rest of the device is sleeping. The ADC Noise
Reduction mode stops the CPU and all I/O modules except asynchronous timer and ADC to minimize
switching noise during ADC conversions. In Standby mode, the crystal/resonator oscillator is running
while the rest of the device is sleeping. This allows very fast start-up combined with low-power
consumption. In Extended Standby mode, both the main oscillator and the asynchronous timer continue
to run.
Microchip offers the QTouch® library for embedding capacitive touch buttons, sliders and wheels
functionality into AVR microcontrollers. The patented charge-transfer signal acquisition offers robust
sensing and includes fully debounced reporting of touch keys and includes Adjacent Key Suppression™
(AKS™) technology for unambiguous detection of key events. The easy-to-use QTouch Suite toolchain
allows you to explore, develop and debug your own touch applications.
The device is manufactured using Microchip’s high density nonvolatile memory technology. The on-chip
ISP Flash allows the program memory to be reprogrammed in-system through an SPI serial interface, by
a conventional nonvolatile memory programmer, or by an on-chip boot program running on the AVR core.
The boot program can use any interface to download the application program in the application Flash
memory. Software in the boot Flash section will continue to run while the application Flash section is
updated, providing true read-while-write operation. By combining an 8-bit RISC CPU with in-system self-
programmable Flash on a monolithic chip, the ATmega328/P is a powerful microcontroller that provides a
highly flexible and cost effective solution to many embedded control applications.
The ATmega328/P is supported with a full suite of program and system development tools including: C
compilers, macro assemblers, program debugger/simulators, in-circuit emulators, and evaluation kits.
ATmega328/P
Description
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 9
2. Configuration Summary
Features ATmega328/P
Pin Count 28/32
Flash (Bytes) 32K
SRAM (Bytes) 2K
EEPROM (Bytes) 1K
General Purpose I/O Lines 23
SPI 2
TWI (I2C) 1
USART 1
ADC 10-bit 15 kSPS
ADC Channels 8
8-bit Timer/Counters 2
16-bit Timer/Counters 1
ATmega328/P
Configuration Summary
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 10
3. Ordering Information
3.1 ATmega328
Speed [MHz](3) Power Supply [V] Ordering Code(2) Package(1) Operational Range
20 1.8 - 5.5 ATmega328-AU
ATmega328-AUR(5)
ATmega328-MMH(4)
ATmega328-MMHR(4)(5)
ATmega328-MU
ATmega328-MUR(5)
ATmega328-PU
32A
32A
28M1
28M1
32M1-A
32M1-A
28P3
Industrial
(-40°C to 85°C)
Note:
1. This device can also be supplied in wafer form. Please contact your local Microchip sales office for
detailed ordering information and minimum quantities.
2. Pb-free packaging, complies to the European Directive for Restriction of Hazardous Substances
(RoHS directive). Also Halide free and fully Green.
3. Please refer to Speed Grades for Speed vs. VCC
4. Tape & Reel.
5. NiPdAu Lead Finish.
Package Type
28M1 28-pad, 4 x 4 x 1.0 body, Lead Pitch 0.45mm Quad Flat No-Lead/Micro Lead Frame Package (QFN/
MLF)
28P3 28-lead, 0.300” Wide, Plastic Dual Inline Package (PDIP)
32M1-A 32-pad, 5 x 5 x 1.0 body, Lead Pitch 0.50mm Quad Flat No-Lead/Micro Lead Frame Package (QFN/
MLF)
32A 32-lead, Thin (1.0mm) Plastic Quad Flat Package (TQFP)
3.2 ATmega328P
Speed [MHz](3) Power Supply [V] Ordering Code(2) Package(1) Operational Range
20 1.8 - 5.5 ATmega328P-AU
ATmega328P-AUR(5)
ATmega328P-MMH(4)
ATmega328P-MMHR(4)(5)
ATmega328P-MU
32A
32A
28M1
28M1
32M1-A
Industrial
(-40°C to 85°C)
ATmega328/P
Ordering Information
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 11
Speed [MHz](3) Power Supply [V] Ordering Code(2) Package(1) Operational Range
ATmega328P-MUR(5)
ATmega328P-PU
32M1-A
28P3
ATmega328P-AN
ATmega328P-ANR(5)
ATmega328P-MN
ATmega328P-MNR(5)
ATmega328P-PN
32A
32A
32M1-A
32M1-A
28P3
Industrial
(-40°C to 105°C)
Note:
1. This device can also be supplied in wafer form. Please contact your local Microchip sales office for
detailed ordering information and minimum quantities.
2. Pb-free packaging, complies to the European Directive for Restriction of Hazardous Substances
(RoHS directive). Also Halide free and fully Green.
3. Please refer to Speed Grades for Speed vs. VCC
4. Tape & Reel.
5. NiPdAu Lead Finish.
Package Type
28M1 28-pad, 4 x 4 x 1.0 body, Lead Pitch 0.45mm Quad Flat No-Lead/Micro Lead Frame Package (QFN/
MLF)
28P3 28-lead, 0.300” Wide, Plastic Dual Inline Package (PDIP)
32M1-A 32-pad, 5 x 5 x 1.0 body, Lead Pitch 0.50mm Quad Flat No-Lead/Micro Lead Frame Package (QFN/
MLF)
32A 32-lead, Thin (1.0mm) Plastic Quad Flat Package (TQFP)
ATmega328/P
Ordering Information
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 12
4. Block Diagram
Figure 4-1. Block Diagram
CPU
USART
ADC
ADC[7:0]
AREF
RxD0
TxD0
XCK0
I/O
PORTS
D
A
T
A
B
U
S
GPIOR[2:0]
SRAM
OCD
EXTINT
FLASH
NVM
programming
debugWire
I
N
/
O
U
T
D
A
T
A
B
U
S
TC 0
(8-bit)
SPI
AC
AIN0
AIN1
ADCMUX
EEPROM
EEPROMIF
TC 1
(16-bit)
OC1A/B
T1
ICP1
TC 2
(8-bit async)
TWI SDA0
SCL0
Internal
Reference
Watchdog
Timer
Power
management
and clock
control
VCC
GND
Clock generation
8MHz
Calib RC
128kHz int
osc
32.768kHz
XOSC
External
clock
Power
Supervision
POR/BOD &
RESET
XTAL2 /
TOSC2
RESET
XTAL1 /
TOSC1
16MHz LP
XOSC
PCINT[23:0]
INT[1:0]
T0
OC0A
OC0B
MISO0
MOSI0
SCK0
SS0
OC2A
OC2B
PB[7:0]
PC[6:0]
PD[7:0]
ADC6,ADC7,PC[5:0]
AREF
PD[7:0], PC[6:0], PB[7:0]
PD3, PD2
PB1, PB2
PD5
PB0
PB3
PD3
PD4
PD6
PD5
PB4
PB3
PB5
PB2
PD6
PD7
ADC6, ADC7
PC[5:0]
PD0
PD1
PD4
PC4
PC5
ATmega328/P
Block Diagram
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 13
5. Pin Configurations
5.1 Pinout
Figure 5-1. 28-pin PDIP
Power
Ground
Programming/debug
Digital
Analog
Crystal/Osc
(PCINT14/RESET) PC6
(PCINT16/RXD) PD0
(PCINT17/TXD) PD1
(PCINT18/INT0) PD2
(PCINT19/OC2B/INT1) PD3
(PCINT20/XCK/T0) PD4
VCC
GND
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7
(PCINT21/OC0B/T1) PD5
(PCINT22/OC0A/AIN0) PD6
(PCINT23/AIN1) PD7
(PCINT0/CLKO/ICP1) PB0
PC5 (ADC5/SCL/PCINT13)
PC4 (ADC4/SDA/PCINT12)
PC3 (ADC3/PCINT11)
PC2 (ADC2/PCINT10)
PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
GND
AREF
AVCC
PB5 (SCK/PCINT5)
PB4 (MISO/PCINT4)
PB3 (MOSI/OC2A/PCINT3)
PB2 (SS/OC1B/PCINT2)
PB1 (OC1A/PCINT1)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
28
27
26
25
24
23
22
21
20
19
18
17
16
15
ATmega328/P
Pin Configurations
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 14
Figure 5-2. 28-pin MLF Top View
1
2
3
4
5
6
7
8
9
10
11
12
13
14
PD2 (INT0/PCINT18)
PD1 (TXD/PCINT17)
PD0 (RXD/PCINT16)
PC6 (RESET/PCINT14)
PC5 (ADC5/SCL/PCINT13)
PC4 (ADC4/SDA/PCINT12)
PC3 (ADC3/PCINT11)
PC2 (ADC2/PCINT10)
PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
GND
AREF
AVCC
PB5 (SCK/PCINT5)
(PCINT22/OC0A/AIN0) PD6
(PCINT23/AIN1) PD7
(PCINT0/CLKO/ICP1) PB0
(PCINT1/OC1A) PB1
(PCINT2/SS/OC1B) PB2
(PCINT3/OC2A/MOSI) PB3
(PCINT4/MISO) PB4
(PCINT19/OC2B/INT1) PD3
(PCINT20/XCK/T0) PD4
VCC
GND
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7
(PCINT21/OC0B/T1) PD5
Bottom pad should be
soldered to ground
Power
Ground
Programming/debug
Digital
Analog
Crystal/CLK
21
20
19
18
17
16
15
28
27
26
25
24
23
22
ATmega328/P
Pin Configurations
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 15
Figure 5-3. 32-pin TQFP Top View
1
2
3
4
32
31
30
29
28
27
26
5
6
7
8
24
23
22
21
20
19
18
17
25
9
10
11
12
13
14
15
16
Power
Ground
Programming/debug
Digital
Analog
Crystal/CLK
(PCINT19/OC2B/INT1) PD3
(PCINT20/XCK/T0) PD4
GND
VCC
GND
VCC
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7
PD2 (INT0/PCINT18)
PD1 (TXD/PCINT17)
PD0 (RXD/PCINT16)
PC6 (RESET/PCINT14)
PC5 (ADC5/SCL/PCINT13)
PC4 (ADC4/SDA/PCINT12)
PC3 (ADC3/PCINT11)
PC2 (ADC2/PCINT10)
PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
ADC7
GND
AREF
ADC6
AVCC
PB5 (SCK/PCINT5)
(PCINT21/OC0B/T1) PD5
(PCINT22/OC0A/AIN0) PD6
(PCINT23/AIN1) PD7
(PCINT0/CLKO/ICP1) PB0
(PCINT1/OC1A) PB1
(PCINT2/SS/OC1B) PB2
(PCINT3/OC2A/MOSI) PB3
(PCINT4/MISO) PB4
ATmega328/P
Pin Configurations
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 16
Figure 5-4. 32-pin MLF Top View
1
2
3
4
32
31
30
29
28
27
26
5
6
7
8
24
23
22
21
20
19
18
17
25
9
10
11
12
13
14
15
16
PD2 (INT0/PCINT18)
PD1 (TXD/PCINT17)
PD0 (RXD/PCINT16)
PC6 (RESET/PCINT14)
PC5 (ADC5/SCL/PCINT13)
PC4 (ADC4/SDA/PCINT12)
PC3 (ADC3/PCINT11)
PC2 (ADC2/PCINT10)
PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
ADC7
GND
AREF
ADC6
AVCC
PB5 (SCK/PCINT5)
(PCINT21/OC0B/T1) PD5
(PCINT22/OC0A/AIN0) PD6
(PCINT23/AIN1) PD7
(PCINT0/CLKO/ICP1) PB0
(PCINT1/OC1A) PB1
(PCINT2/SS/OC1B) PB2
(PCINT3/OC2A/MOSI) PB3
(PCINT4/MISO) PB4
(PCINT19/OC2B/INT1) PD3
(PCINT20/XCK/T0) PD4
GND
VCC
GND
VCC
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7
Bottom pad should be
soldered to ground
Power
Ground
Programming/debug
Digital
Analog
Crystal/CLK
5.2 Pin Descriptions
5.2.1 VCC
Digital supply voltage pin.
5.2.2 GND
Ground.
5.2.3 Port B (PB[7:0]) XTAL1/XTAL2/TOSC1/TOSC2
Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each pin). The Port B
output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs,
Port B pins that are externally pulled low will source current if the pull-up resistors are activated. The Port
B pins are tri-stated during a Reset condition even if the clock is not running.
ATmega328/P
Pin Configurations
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 17
Depending on the clock selection fuse settings, PB6 can be used as input to the inverting oscillator
amplifier and input to the internal clock operating circuit.
Depending on the clock selection fuse settings, PB7 can be used as output from the inverting oscillator
amplifier.
If the internal calibrated RC oscillator is used as chip clock source, PB[7:6] is used as TOSC[2:1] input for
the asynchronous timer/counter2 if the AS2 bit in ASSR is set.
5.2.4 Port C (PC[5:0])
Port C is a 7-bit bi-directional I/O port with internal pull-up resistors (selected for each pin). The PC[5:0]
output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs,
Port C pins that are externally pulled low will source current if the pull-up resistors are activated. The Port
C pins are tri-stated during a Reset condition even if the clock is not running.
5.2.5 PC6/RESET
If the RSTDISBL fuse is programmed, PC6 is used as an I/O pin. Note that the electrical characteristics of
PC6 differ from those of the other pins of Port C.
If the RSTDISBL fuse is unprogrammed, PC6 is used as a Reset input. A low level on this pin for longer
than the minimum pulse length will generate a Reset, even if the clock is not running. Shorter pulses are
not guaranteed to generate a Reset.
The various special features of Port C are elaborated in the Alternate Functions of Port C section.
5.2.6 Port D (PD[7:0])
Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each pin). The Port D
output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs,
Port D pins that are externally pulled low will source current if the pull-up resistors are activated. The Port
D pins are tri-stated during a Reset condition even if the clock is not running.
5.2.7 AVCC
AVCC is the supply voltage pin for the A/D Converter (ADC), PC[3:0], and PE[3:2]. It should be externally
connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through
a low-pass filter. Note that PC[6:4] use digital supply voltage, VCC.
5.2.8 AREF
AREF is the analog reference pin for the A/D Converter.
5.2.9 ADC[7:6]
In the TQFP and VFQFN package, ADC[7:6] serve as analog inputs to the A/D converter. These pins are
powered by the analog supply and serve as 10-bit ADC channels.
ATmega328/P
Pin Configurations
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 18
6. I/O Multiplexing
Each pin is by default controlled by the PORT as a general purpose I/O and alternatively it can be
assigned to one of the peripheral functions.
The following table describes the peripheral signals multiplexed to the PORT I/O pins.
Table 6-1. PORT Function Multiplexing
(32-pin
MLF/TQFP)
Pin#
(28-pin
MLF) Pin#
(28-pin
PIPD) Pin#
PAD EXTINT PCINT ADC/AC OSC T/C #0 T/C
#1
USART 0 I2C 0 SPI 0
1 1 5 PD3 INT1 PCINT19 OC2B
2 2 6 PD4 PCINT20 T0 XCK0
4 3 7 VCC
3 4 8 GND
6 - - VCC
5 - - GND
7 5 9 PB6 PCINT6 XTAL1/
TOSC1
8 6 10 PB7 PCINT7 XTAL2/
TOSC2
9 7 11 PD5 PCINT21 OC0B T1
10 8 12 PD6 PCINT22 AIN0 OC0A
11 9 13 PD7 PCINT23 AIN1
12 10 14 PB0 PCINT0 CLKO ICP1
13 11 15 PB1 PCINT1 OC1A
14 12 16 PB2 PCINT2 OC1B SS0
15 13 17 PB3 PCINT3 OC2A MOSI0
16 14 18 PB4 PCINT4 MISO0
17 15 19 PB5 PCINT5 SCK0
18 16 20 AVCC
19 - - ADC6 ADC6
20 17 21 AREF
21 18 22 GND
22 - - ADC7 ADC7
23 19 13 PC0 PCINT8 ADC0
24 20 24 PC1 PCINT9 ADC1
25 21 25 PC2 PCINT10 ADC2
26 22 26 PC3 PCINT11 ADC3
27 23 27 PC4 PCINT12 ADC4 SDA0
28 24 28 PC5 PCINT13 ADC5 SCL0
ATmega328/P
I/O Multiplexing
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 19
(32-pin
MLF/TQFP)
Pin#
(28-pin
MLF) Pin#
(28-pin
PIPD) Pin#
PAD EXTINT PCINT ADC/AC OSC T/C #0 T/C
#1
USART 0 I2C 0 SPI 0
29 25 1 PC6/RESET PCINT14
30 26 2 PD0 PCINT16 RXD0
31 27 3 PD1 PCINT17 TXD0
32 28 4 PD2 INT0 PCINT18
ATmega328/P
I/O Multiplexing
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 20
8. Data Retention
Reliability qualification results show that the projected data retention failure rate is much less than 1 PPM
over 20 years at 85°C.
ATmega328/P
Data Retention
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 22
9. About Code Examples
This documentation contains simple code examples that briefly show how to use various parts of the
device. These code examples assume that the part specific header file is included before compilation. Be
aware that not all C compiler vendors include bit definitions in the header files and interrupt handling in C
is compiler dependent. Confirm with the C compiler documentation for more details.
For I/O registers located in extended I/O map, IN, OUT, SBIS, SBIC, CBI, and SBI instructions must be
replaced with instructions that allow access to extended I/O. Typically LDS and STS combined with SBRS,
SBRC, SBR, and CBR.
ATmega328/P
About Code Examples
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 23
10. Capacitive Touch Sensing
10.1 QTouch Library
The QTouch® library provides a simple to use solution to realize touch sensitive interfaces on most AVR®
microcontrollers. The QTouch library includes support for the QTouch and QMatrix™ acquisition methods.
Touch sensing can be added to any application by linking the appropriate QTouch library for the AVR
microcontroller. This is done by using a simple set of APIs to define the touch channels and sensors, and
then calling the touch sensing API’s to retrieve the channel information and determine the touch sensor
states.
The QTouch library is FREE and downloadable from QTouch Library . For implementation details and
other information, refer to the QTouch Library User Guide, also available for download from the Microchip
website.
ATmega328/P
Capacitive Touch Sensing
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 24
11. AVR CPU Core
11.1 Overview
This section discusses the AVR core architecture in general. The main function of the CPU core is to
ensure correct program execution. The CPU must, therefore, be able to access memories, perform
calculations, control peripherals, and handle interrupts.
Figure 11-1. Block Diagram of the AVR Architecture
Register file
Flash program
memory
Program
counter
Instruction
register
Instruction
decode
Data memory
ALU
Status
register
R0R1
R2R3
R4R5
R6R7
R8R9
R10R11
R12R13
R14R15
R16R17
R18R19
R20R21
R22R23
R24R25
R26 (XL)R27 (XH)
R28 (YL)R29 (YH)
R30 (ZL)R31 (ZH)
Stack
pointer
In order to maximize performance and parallelism, the AVR uses a Harvard architecture – with separate
memories and buses for program and data. Instructions in the program memory are executed with a
single level pipelining. While one instruction is being executed, the next instruction is pre-fetched from the
program memory. This concept enables instructions to be executed in every clock cycle. The program
memory is In-System Reprogrammable Flash memory.
The fast-access register file contains 32 x 8-bit general purpose working registers with a single clock
cycle access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typical ALU
operation, two operands are output from the register file, the operation is executed, and the result is
stored back in the register file – in one clock cycle.
Six of the 32 registers can be used as three 16-bit indirect address register pointers for data space
addressing – enabling efficient address calculations. One of these address pointers can be used as an
ATmega328/P
AVR CPU Core
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 25
address pointer for lookup tables in Flash program memory. These added function registers are the 16-bit
X-, Y-, and Z-register, described later in this section.
The ALU supports arithmetic and logic operations between registers or between a constant and a
register. Single register operations can also be executed in the ALU. After an arithmetic operation, the
Status register is updated to reflect information about the result of the operation.
Program flow is provided by conditional and unconditional jump and call instructions, able to directly
address the whole address space. Most AVR instructions have a single 16-bit word format. Every
program memory address contains a 16- or 32-bit instruction.
Program Flash memory space is divided into two sections, the Boot Program section and the Application
Program section. Both sections have dedicated Lock bits for write and read/write protection. The SPM
instruction that writes into the Application Flash memory section must reside in the Boot Program section.
During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the Stack.
The Stack is effectively allocated in the general data SRAM, and consequently, the Stack size is only
limited by the total SRAM size and the usage of the SRAM. All user programs must initialize the Stack
Pointer (SP) in the Reset routine (before subroutines or interrupts are executed). The SP is read/write
accessible in the I/O space. The data SRAM can easily be accessed through the five different addressing
modes supported in the AVR architecture.
The memory spaces in the AVR architecture are all linear and regular memory maps.
A flexible interrupt module has its control registers in the I/O space with an additional global interrupt
enable bit in the Status register. All interrupts have a separate interrupt vector in the interrupt vector table.
The interrupts have priority in accordance with their interrupt vector position. The lower the interrupt
vector address, the higher the priority.
The I/O memory space contains 64 addresses for CPU peripheral functions as Control registers, SPI, and
other I/O functions. The I/O memory can be accessed directly, or as the data space locations following
those of the register file, 0x20 - 0x5F. In addition, this device has extended I/O space from 0x60 - 0xFF in
SRAM where only the ST/STS/STD and LD/LDS/LDD instructions can be used.
11.2 Arithmetic Logic Unit (ALU)
The high-performance AVR ALU operates in direct connection with all the 32 general purpose working
registers. Within a single clock cycle, arithmetic operations between general purpose registers or
between a register and an immediate are executed. The ALU operations are divided into three main
categories: arithmetic, logical, and bit-functions. Some implementations of the architecture provide a
powerful multiplier supporting both signed/unsigned multiplication and fractional format. See Instruction
Set Summary section for a detailed description.
Related Links
Instruction Set Summary
11.3 Status Register
The Status register contains information about the result of the most recently executed arithmetic
instruction. This information can be used for altering program flow in order to perform conditional
operations. The Status register is updated after all ALU operations, as specified in the instruction set
reference. This will in many cases remove the need for using the dedicated compare instructions,
resulting in faster and more compact code.
ATmega328/P
AVR CPU Core
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 26
The Status register is not automatically stored when entering an interrupt routine and restored when
returning from an interrupt. This must be handled by software.
ATmega328/P
AVR CPU Core
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 27
11.3.1 Status Register
Name: SREG
Offset: 0x5F
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x3F
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
I T H S V N Z C
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – I Global Interrupt Enable
The global interrupt enable bit must be set for the interrupts to be enabled. The individual interrupt enable
control is then performed in separate control registers. If the Global Interrupt Enable register is cleared,
none of the interrupts are enabled independent of the individual interrupt enable settings. The I-bit is
cleared by hardware after an interrupt has occurred, and is set by the RETI instruction to enable
subsequent interrupts. The I-bit can also be set and cleared by the application with the SEI and CLI
instructions, as described in the instruction set reference.
Bit 6 – T Copy Storage
The bit copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or destination for
the operated bit. A bit from a register in the register file can be copied into T by the BST instruction, and a
bit in T can be copied into a bit in a register in the register file by the BLD instruction.
Bit 5 – H Half Carry Flag
The half carry flag H indicates a half carry in some arithmetic operations. Half carry flag is useful in BCD
arithmetic. See the Instruction Set Description for detailed information.
Bit 4 – S Sign Flag, S = N ㊉ V
The S-bit is always an exclusive or between the negative flag N and the two’s complement overflow flag
V. See the Instruction Set Description for detailed information.
Bit 3 – V Two’s Complement Overflow Flag
The two’s complement overflow flag V supports two’s complement arithmetic. See the Instruction Set
Description for detailed information.
Bit 2 – N Negative Flag
The negative flag N indicates a negative result in an arithmetic or logic operation. See the Instruction Set
Description for detailed information.
Bit 1 – Z Zero Flag
The zero flag Z indicates a zero result in an arithmetic or logic operation. See the Instruction Set
Description for detailed information.
ATmega328/P
AVR CPU Core
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 28
Bit 0 – C Carry Flag
The carry flag C indicates a carry in an arithmetic or logic operation. See the Instruction Set Description
for detailed information.
11.4 General Purpose Register File
The register file is optimized for the AVR Enhanced RISC instruction set. In order to achieve the required
performance and flexibility, the following input/output schemes are supported by the register file:
• One 8-bit output operand and one 8-bit result input
• Two 8-bit output operands and one 8-bit result input
• Two 8-bit output operands and one 16-bit result input
• One 16-bit output operand and one 16-bit result input
Figure 11-2. AVR CPU General Purpose Working Registers
7
0
Addr.
R0
0x00
R1
0x01
R2
0x02
…
R13
0x0D
General
R14
0x0E
Purpose
R15
0x0F
Working
R16
0x10
Registers
R17
0x11
…
R26
0x1A
X-register Low Byte
R27
0x1B
X-register High Byte
R28
0x1C
Y-register Low Byte
R29
0x1D
Y-register High Byte
R30
0x1E
Z-register Low Byte
R31
0x1F
Z-register High Byte
Most of the instructions operating on the register file have direct access to all registers, and most of them
are single cycle instructions. As shown in the figure, each register is also assigned a data memory
address, mapping them directly into the first 32 locations of the user data space. Although not being
physically implemented as SRAM locations, this memory organization provides great flexibility in access
of the registers, as the X-, Y-, and Z-pointer registers can be set to index any register in the file.
11.4.1 The X-register, Y-register, and Z-register
The registers R26...R31 have some added functions to their general purpose usage. These registers are
16-bit address pointers for indirect addressing of the data space. The three indirect address registers X,
Y, and Z are defined as described in the figure.
ATmega328/P
AVR CPU Core
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 29
Figure 11-3. The X-, Y-, and Z-registers
15
XH
XL
0
X-register
7
0
7
0
R27
R26
15
YH
YL
0
Y-register
7
0
7
0
R29
R28
15
ZH
ZL
0
Z-register
7
0
7
0
R31
R30
In the different addressing modes, these address registers have functions as fixed displacement,
automatic increment, and automatic decrement (see the instruction set reference for details).
Related Links
Instruction Set Summary
11.5 Stack Pointer
The stack is mainly used for storing temporary data, local variables, and return addresses after interrupts
and subroutine calls. The stack is implemented as growing from higher to lower memory locations. The
Stack Pointer register always points to the top of the stack.
The stack pointer points to the data SRAM stack area where the subroutine and interrupt stacks are
located. A stack PUSH command will decrease the stack pointer. The stack in the data SRAM must be
defined by the program before any subroutine calls are executed or interrupts are enabled. Initial stack
pointer value equals the last address of the internal SRAM and the stack pointer must be set to point
above start of the SRAM. See the table for stack pointer details.
Table 11-1. Stack Pointer Instructions
Instruction Stack Pointer Description
PUSH Decremented by 1 Data is pushed onto the stack
CALL
ICALL
RCALL
Decremented by 2 Return address is pushed onto the stack with a subroutine call or
interrupt
POP Incremented by 1 Data is popped from the stack
RET
RETI
Incremented by 2 Return address is popped from the stack with return from subroutine or
return from interrupt
ATmega328/P
AVR CPU Core
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 30
The AVR stack pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually
used is implementation dependent. Note that the data space in some implementations of the AVR
architecture is so small that only SPL is needed. In this case, the SPH register will not be present.
ATmega328/P
AVR CPU Core
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 31
11.5.1 Stack Pointer Register Low and High byte
Name: SPL and SPH
Offset: 0x5D
Reset: 0x4FF
Property: When addressing I/O registers as data space the offset address is 0x3D
The SPL and SPH register pair represents the 16-bit value, SP. The low byte [7:0] (suffix L) is accessible
at the original offset. The high byte [15:8] (suffix H) can be accessed at offset + 0x01. For more details on
reading and writing 16-bit registers, refer to Accessing 16-bit Timer/Counter Registers.
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses.
Bit 15 14 13 12 11 10 9 8
SP11 SP10 SP9 SP8
Access R R R R RW RW RW RW
Reset 0 0 0 0 0 1 0 0
Bit 7 6 5 4 3 2 1 0
SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0
Access RW RW RW RW RW RW RW RW
Reset 1 1 1 1 1 1 1 1
Bits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 – SP Stack Pointer Register
SPL and SPH are combined into SP.
Related Links
Accessing 16-bit Timer/Counter Registers
11.6 Instruction Execution Timing
This section describes the general access timing concepts for instruction execution. The AVR CPU is
driven by the CPU clock clkCPU, directly generated from the selected clock source for the chip. No internal
clock division is used. The figure below shows the parallel instruction fetches and instruction executions
enabled by the Harvard architecture and the fast-access register file concept. This is the basic pipelining
concept to obtain up to 1 MIPS per MHz with the corresponding unique results for functions per cost,
functions per clocks, and functions per power unit.
ATmega328/P
AVR CPU Core
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 32
Figure 11-4. The Parallel Instruction Fetches and Instruction Executions
clk
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
T1 T2 T3 T4
CPU
The following figure shows the internal timing concept for the register file. In a single clock cycle, an ALU
operation using two register operands is executed and the result is stored back to the destination register.
Figure 11-5. Single Cycle ALU Operation
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
T1 T2 T3 T4
clkCPU
11.7 Reset and Interrupt Handling
The AVR provides several different interrupt sources. These interrupts and the separate Reset vector
each have a separate program vector in the program memory space. All interrupts are assigned
individual enable bits, which must be written logic one together with the global interrupt enable bit in the
Status register in order to enable the interrupt. Depending on the program counter value, interrupts may
be automatically disabled when Boot Lock bits BLB02 or BLB12 are programmed. This feature improves
software security.
The lowest addresses in the program memory space are by default defined as the Reset and interrupt
vectors. They have determined priority levels: The lower the address the higher is the priority level.
RESET has the highest priority, and next is INT0 – the External Interrupt Request 0. The interrupt vectors
can be moved to the start of the boot Flash section by setting the IVSEL bit in the MCU Control Register
(MCUCR). The Reset vector can be moved to the start of the boot Flash section by programming the
BOOTRST Fuse.
When an interrupt occurs, the global interrupt enable I-bit is cleared and all interrupts are disabled. The
user software can write logic one to the I-bit to enable nested interrupts. All enabled interrupts can then
interrupt the current interrupt routine. The I-bit is automatically set when a return from interrupt instruction
– RETI – is executed.
There are basically two types of interrupts:
The first type is triggered by an event that sets the interrupt flag. For these interrupts, the program
counter is vectored to the actual interrupt vector in order to execute the interrupt handling routine, and
ATmega328/P
AVR CPU Core
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 33
hardware clears the corresponding interrupt flag. Interrupt flags can be cleared by writing a logic one to
the flag bit position(s) to be cleared. If an interrupt condition occurs while the corresponding interrupt
enable bit is cleared, the interrupt flag will be set and remembered until the interrupt is enabled, or the
flag is cleared by software. Similarly, if one or more interrupt conditions occur while the global interrupt
enable bit is cleared, the corresponding interrupt flag(s) will be set and remembered until the global
interrupt enable bit is set and will then be executed by order of priority.
The second type of interrupts will trigger as long as the interrupt condition is present. These interrupts do
not necessarily have interrupt flags. If the interrupt condition disappears before the interrupt is enabled,
the interrupt will not be triggered. When the AVR exits from an interrupt, it will always return to the main
program and execute one more instruction before any pending interrupt is served.
The Status register is not automatically stored when entering an interrupt routine, nor restored when
returning from an interrupt routine. This must be handled by software.
When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled. No
interrupt will be executed after the CLI instruction, even if it occurs simultaneously with the CLI
instruction. The following example shows how this can be used to avoid interrupts during the timed
EEPROM write sequence.
Assembly Code Example(1)
in r16, SREG ; store SREG value
cli ; disable interrupts during timed sequence
sbi EECR, EEMPE ; start EEPROM write
sbi EECR, EEPE
out SREG, r16 ; restore SREG value (I-bit)
C Code Example(1)
char cSREG;
cSREG = SREG; /* store SREG value */
/* disable interrupts during timed sequence */
_CLI();
EECR |= (1<<EEMPE); /* start EEPROM write */
EECR |= (1<<EEPE);
SREG = cSREG; /* restore SREG value (I-bit) */
1. Refer to About Code Examples.
When using the SEI instruction to enable interrupts, the instruction following SEI will be executed before
any pending interrupts, as shown in this example.
Assembly Code Example(1)
sei ; set Global Interrupt Enable
sleep ; enter sleep, waiting for interrupt
; note: will enter sleep before any pending interrupt(s)
C Code Example(1)
__enable_interrupt(); /* set Global Interrupt Enable */
__sleep(); /* enter sleep, waiting for interrupt */
/* note: will enter sleep before any pending interrupt(s) */
1. Refer to About Code Examples.
Related Links
Memory Programming
Boot Loader Support – Read-While-Write Self-Programming
ATmega328/P
AVR CPU Core
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 34
11.7.1 Interrupt Response Time
The interrupt execution response for all the enabled AVR interrupts is four clock cycles minimum. After
four clock cycles, the program vector address for the actual interrupt handling routine is executed. During
this four clock cycle period, the program counter is pushed onto the stack. The vector is normally a jump
to the interrupt routine, and this jump takes three clock cycles. If an interrupt occurs during execution of a
multi-cycle instruction, this instruction is completed before the interrupt is served. If an interrupt occurs
when the microcontroller (MCU) is in Sleep mode, the interrupt execution response time is increased by
four clock cycles. This increase comes in addition to the start-up time from the selected Sleep mode. A
return from an interrupt handling routine takes four clock cycles. During these four clock cycles, the
program counter (two bytes) is popped back from the Stack, the Stack Pointer is incremented by two, and
the I-bit in SREG is set.
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AVR CPU Core
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12. AVR Memories
12.1 Overview
This section describes the different memory types in the device. The AVR architecture has two main
memory spaces, the Data Memory and the Program Memory space. In addition, the device features an
EEPROM Memory for data storage. All memory spaces are linear and regular.
12.2 In-System Reprogrammable Flash Program Memory
The ATmega328/P contains 32Kbytes on-chip in-system reprogrammable Flash memory for program
storage. Since all AVR instructions are 16 or 32 bits wide, the Flash is organized as 16 K x 16. For
software security, the Flash Program memory space is divided into two sections - Boot Loader Section
and Application Program Section in the device .
The ATmega328/P Program Counter (PC) is 14 bits wide, thus addressing the 16 K program memory
locations. The operation of the Boot Program section and associated Boot Lock bits for software
protection are described in detail in Boot Loader Support – Read-While-Write Self-Programming. Refer to
Memory Programming for the description of Flash data serial downloading using the SPI pins.
Constant tables can be allocated within the entire program memory address space, using the Load
Program Memory (LPM) instruction.
Timing diagrams for instruction fetch and execution are presented in Instruction Execution Timing.
Figure 12-1. Program Memory Map ATmega328/P
0x0000
0x3FFF
Program Memory
Application Flash Section
Boot Flash Section
Related Links
Boot Loader Support – Read-While-Write Self-programming (BTLDR)
Memory Programming (MEMPROG)
Instruction Execution Timing
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AVR Memories
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12.3 SRAM Data Memory
The following figure shows how the device SRAM memory is organized.
The device is a complex microcontroller with more peripheral units than can be supported within the 64
locations reserved in the Opcode for the IN and OUT instructions. For the extended I/O space from 0x60 -
0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
The lower 2303 data memory locations address both the register file, the I/O memory, extended I/O
memory, and the internal data SRAM. The first 32 locations address the register file, the next 64 location
the standard I/O memory, then 160 locations of extended I/O memory, and the next 2 K locations address
the internal data SRAM.
The five different addressing modes for the data memory cover:
• Direct
– The direct addressing reaches the entire data space.
• Indirect with Displacement
– The indirect with displacement mode reaches 63 address locations from the base address
given by the Y- or Z-register.
• Indirect
– In the register file, registers R26 to R31 feature the indirect addressing pointer registers.
• Indirect with Pre-decrement
– The address registers X, Y, and Z are decremented.
• Indirect with Post-increment
– The address registers X, Y, and Z are incremented.
The 32 general purpose working registers, 64 I/O registers, 160 extended I/O registers, and the 2K bytes
of internal data SRAM in the device are all accessible through all these addressing modes.
Figure 12-2. Data Memory Map with 2048 Byte Internal Data SRAM
Load/Store
IN/OUT
0x0000 – 0x001F
0x0100
0x08FF
160 Ext I/O registers
64 I/O registers
32 registers
Internal SRAM
(2048x8)
0x0020 – 0x005F
0x0060 – 0x00FF
0x0000 – 0x001F
12.3.1 Data Memory Access Times
The internal data SRAM access is performed in two clkCPU cycles as described in the following Figure.
ATmega328/P
AVR Memories
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 37
Figure 12-3. On-chip Data SRAM Access Cycles
clk
WR
RD
Data
Data
Address Address valid
T1 T2 T3
Compute Address
Read Write
CPU
Memory Access Instruction Next Instruction
12.4 EEPROM Data Memory
The ATmega328/P contains 1KB of data EEPROM memory. It is organized as a separate data space, in
which single bytes can be read and written. The access between the EEPROM and the CPU is described
in the following, specifying the EEPROM Address registers, the EEPROM Data register, and the
EEPROM Control register.
See the related links for a detailed description on EEPROM Programming in SPI or Parallel Programming
mode.
Related Links
Memory Programming (MEMPROG)
12.4.1 EEPROM Read/Write Access
The EEPROM access registers are accessible in the I/O space.
The write access time for the EEPROM is given in Table 12-2. A self-timing function, however, lets the
user software detect when the next byte can be written. If the user code contains instructions that write
the EEPROM, some precautions must be taken. In heavily filtered power supplies, VCC is likely to rise or
fall slowly on power-up/down. This causes the device for some period of time to run at a voltage lower
than specified as a minimum for the clock frequency used. Refer to Preventing EEPROM Corruption for
details on how to avoid problems in these situations.
In order to prevent unintentional EEPROM writes, a specific write procedure must be followed. Refer to
the description of the EEPROM Control register for details on this.
When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is
executed. When the EEPROM is written, the CPU is halted for two clock cycles before the next instruction
is executed.
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12.4.2 Preventing EEPROM Corruption
During periods of low VCC, the EEPROM data can be corrupted because the supply voltage is too low for
the CPU and the EEPROM to operate properly. These issues are the same as for board level systems
using EEPROM, and the same design solutions should be applied.
An EEPROM data corruption can be caused by two situations when the voltage is too low. First, a regular
write sequence to the EEPROM requires a minimum voltage to operate correctly. Secondly, the CPU itself
can execute instructions incorrectly, if the supply voltage is too low.
EEPROM data corruption can easily be avoided by following this design recommendation:
Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done
by enabling the internal Brown-out Detector (BOD). If the detection level of the internal BOD does not
match the needed detection level, an external low VCC Reset protection circuit can be used. If a Reset
occurs while a write operation is in progress, the write operation will be completed provided that the
power supply voltage is sufficient.
12.5 I/O Memory
The I/O space definition of the device is shown in the Register Summary.
All device I/Os and peripherals are placed in the I/O space. All I/O locations may be accessed by the
LD/LDS/LDD and ST/STS/STD instructions, transferring data between the 32 general purpose working
registers and the I/O space. I/O registers within the address range 0x00-0x1F are directly bit-accessible
using the SBI and CBI instructions. In these registers, the value of single bits can be checked by using
the SBIS and SBIC instructions.
When using the I/O specific commands IN and OUT, the I/O addresses 0x00-0x3F must be used. When
addressing I/O registers as data space using LD and ST instructions, 0x20 must be added to these
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 locations reserved in Opcode for the IN and OUT instructions. For the extended I/O space
from 0x60..0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O
memory addresses should never be written.
Some of the status flags are cleared by writing a '1' to them; this is described in the flag descriptions.
Note that, unlike most other AVRs, the CBI and SBI instructions will only operate on the specified bit, and
can, therefore, be used on registers containing such status flags. The CBI and SBI instructions work with
registers 0x00-0x1F only.
The I/O and peripherals control registers are explained in later sections.
Related Links
Memory Programming (MEMPROG)
Register Summary
Instruction Set Summary
12.5.1 General Purpose I/O Registers
The device contains three general purpose I/O registers; General purpose I/O register 0/1/2 (GPIOR
0/1/2). These registers can be used for storing any information, and they are particularly useful for storing
global variables and status flags. General purpose I/O registers within the address range 0x00 - 0x1F are
directly bit-accessible using the SBI, CBI, SBIS, and SBIC instructions.
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12.6 Register Description
12.6.1 Accessing 16-Bit Registers
The AVR data bus is 8-bits wide, so accessing 16-bit registers requires atomic operations. These
registers must be byte-accessed using two read or write operations. 16-bit registers are connected to the
8-bit bus and a temporary register using a 16-bit bus.
For a write operation, the high byte of the 16-bit register must be written before the low byte. The high
byte is then written into the temporary register. When the low byte of the 16-bit register is written, the
temporary register is copied into the high byte of the 16-bit register in the same clock cycle.
For a read operation, the low byte of the 16-bit register must be read before the high byte. When the low
byte register is read by the CPU, the high byte of the 16-bit register is copied into the temporary register
in the same clock cycle as the low byte is read. When the high byte is read, it is then read from the
temporary register.
This ensures that the low and high bytes of 16-bit registers are always accessed simultaneously when
reading or writing the register.
Interrupts can corrupt the timed sequence if an interrupt is triggered and accesses the same 16-bit
register during an atomic 16-bit read/write operation. To prevent this, interrupts can be disabled when
writing or reading 16-bit registers.
The temporary registers can be read and written directly from user software.
Note: For more information, refer to Accessing 16-bit Timer/Counter registers.
Related Links
Accessing 16-bit Timer/Counter Registers
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AVR Memories
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12.6.2 EEPROM Address Register Low and High Byte
Name: EEARL and EEARH
Offset: 0x41 [ID-000004d0]
Reset: 0xXX
Property: When addressing as I/O Register: address offset is 0x21
The EEARL and EEARH register pair represents the 16-bit value, EEAR. The low byte [7:0] (suffix L) is
accessible at the original offset. The high byte [15:8] (suffix H) can be accessed at offset + 0x01. For
more details on reading and writing 16-bit registers, refer to accessing 16-bit registers in the section
above.
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 15 14 13 12 11 10 9 8
EEAR[9:8]
Access R/W R/W
Reset x x
Bit 7 6 5 4 3 2 1 0
EEAR[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x x
Bits 9:0 – EEAR[9:0] EEPROM Address
The EEPROM Address Registers, EEARH and EEARL, specify the EEPROM address in the 1KB
EEPROM space. The EEPROM data bytes are addressed linearly between 0 and 255/511/511. The initial
value of EEAR is undefined. A proper value must be written before the EEPROM may be accessed.
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AVR Memories
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12.6.3 EEPROM Data Register
Name: EEDR
Offset: 0x40 [ID-000004d0]
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x20
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
EEDR[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – EEDR[7:0] EEPROM Data
For the EEPROM write operation, the EEDR register contains the data to be written to the EEPROM in
the address given by the EEAR register. For the EEPROM read operation, the EEDR contains the data
read out from the EEPROM at the address given by EEAR.
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AVR Memories
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12.6.4 EEPROM Control Register
Name: EECR
Offset: 0x3F [ID-000004d0]
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x1F
Bit 7 6 5 4 3 2 1 0
EEPM[1:0] EERIE EEMPE EEPE EERE
Access R/W R/W R/W R/W R/W R/W
Reset x x 0 0 x 0
Bits 5:4 – EEPM[1:0] EEPROM Programming Mode Bits
The EEPROM Programming mode bit setting defines which programming action will be triggered when
writing EEPE. It is possible to program data in one atomic operation (erase the old value and program the
new value) or to split the erase and write operations into two different operations. The programming times
for the different modes are shown in the table below. While EEPE is set, any write to EEPMn will be
ignored. During reset, the EEPMn bits will be reset to 0b00 unless the EEPROM is busy programming.
Table 12-1. EEPROM Mode Bits
EEPM[1:0] Typ. Programming Time Operation
00 3.4ms Erase and Write in one operation (Atomic Operation)
01 1.8ms Erase Only
10 1.8ms Write Only
11 - Reserved for future use
Bit 3 – EERIE EEPROM Ready Interrupt Enable
Writing EERIE to '1' enables the EEPROM ready interrupt if the I bit in SREG is set. Writing EERIE to
zero disables the interrupt. The EEPROM ready interrupt generates a constant interrupt when EEPE is
cleared. The interrupt will not be generated during EEPROM write or SPM.
Bit 2 – EEMPE EEPROM Master Write Enable
The EEMPE bit determines whether writing EEPE to '1' causes the EEPROM to be written.
When EEMPE is '1', setting EEPE within four clock cycles will write data to the EEPROM at the selected
address.
If EEMPE is zero, setting EEPE will have no effect. When EEMPE has been written to '1' by software,
hardware clears the bit to zero after four clock cycles. See the description of the EEPE bit for an
EEPROM write procedure.
Bit 1 – EEPE EEPROM Write Enable
The EEPROM write enable signal EEPE is the write strobe to the EEPROM. When address and data are
correctly set up, the EEPE bit must be written to '1' to write the value into the EEPROM. The EEMPE bit
must be written to '1' before EEPE is written to '1', otherwise, no EEPROM write takes place. The
following procedure should be followed when writing the EEPROM (the order of steps 3 and 4 is not
essential):
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AVR Memories
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 43
1. Wait until EEPE becomes zero.
2. Wait until SPMEN in SPMCSR becomes zero.
3. Write new EEPROM address to EEAR (optional).
4. Write new EEPROM data to EEDR (optional).
5. Write a '1' to the EEMPE bit while writing a zero to EEPE in EECR.
6. Within four clock cycles after setting EEMPE, write a '1' to EEPE.
The EEPROM cannot be programmed during a CPU write to the Flash memory. The software must check
that the Flash programming is completed before initiating a new EEPROM write. Step 2 is only relevant if
the software contains a Boot Loader allowing the CPU to program the Flash. If the Flash is never being
updated by the CPU, step 2 can be omitted.
CAUTION
An interrupt between step 5 and step 6 will make the write cycle fail, since the EEPROM Master
Write Enable will time-out. If an interrupt routine accessing the EEPROM is interrupting another
EEPROM access, the EEAR or EEDR register will be modified, causing the interrupted
EEPROM access to fail. It is recommended to have the global interrupt flag cleared during all
the steps to avoid these problems.
When the write access time has elapsed, the EEPE bit is cleared by hardware. The user
software can poll this bit and wait for a zero before writing the next byte. When EEPE has been
set, the CPU is halted for two cycles before the next instruction is executed.
Bit 0 – EERE EEPROM Read Enable
The EEPROM read enable signal EERE is the read strobe to the EEPROM. When the correct address is
set up in the EEAR register, the EERE bit must be written to a '1' to trigger the EEPROM read. The
EEPROM read access takes one instruction, and the requested data is available immediately. When the
EEPROM is read, the CPU is halted for four cycles before the next instruction is executed.
The user should poll the EEPE bit before starting the read operation. If a write operation is in progress, it
is neither possible to read the EEPROM, nor to change the EEAR register.
The calibrated oscillator is used to time the EEPROM accesses. See the following table for typical
programming times for EEPROM access from the CPU.
Table 12-2. EEPROM Programming Time
Symbol Number of Calibrated RC Oscillator Cycles Typ. Programming Time
EEPROM write (from CPU) 26,368 3.3ms
The following code examples show one assembly and one C function for writing to the EEPROM. The
examples assume that interrupts are controlled (e.g. by disabling interrupts globally) so that no interrupts
will occur during execution of these functions. The examples also assume that no Flash Boot Loader is
present in the software. If such code is present, the EEPROM write function must also wait for any
ongoing SPM command to finish.
Assembly Code Example(1)
EEPROM_write:
; Wait for completion of previous write
sbic EECR,EEPE
rjmp EEPROM_write
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AVR Memories
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 44
; Set up address (r18:r17) in address register
out EEARH, r18
out EEARL, r17
; Write data (r16) to Data Register
out EEDR,r16
; Write logical one to EEMPE
sbi EECR,EEMPE
; Start eeprom write by setting EEPE
sbi EECR,EEPE
ret
C Code Example(1)
void EEPROM_write(unsigned int uiAddress, unsigned char ucData)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set up address and Data Registers */
EEAR = uiAddress;
EEDR = ucData;
/* Write logical one to EEMPE */
EECR |= (1<<EEMPE);
/* Start eeprom write by setting EEPE */
EECR |= (1<<EEPE);
}
Note: (1) Refer to About Code Examples
The following code examples show assembly and C functions for reading the EEPROM. The examples
assume that interrupts are controlled so that no interrupts will occur during execution of these functions.
Assembly Code Example(1)
EEPROM_read:
; Wait for completion of previous write
sbic EECR,EEPE
rjmp EEPROM_read
; Set up address (r18:r17) in address register
out EEARH, r18
out EEARL, r17
; Start eeprom read by writing EERE
sbi EECR,EERE
; Read data from Data Register
in r16,EEDR
ret
C Code Example(1)
unsigned char EEPROM_read(unsigned int uiAddress)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set up address register */
EEAR = uiAddress;
/* Start eeprom read by writing EERE */
EECR |= (1<<EERE);
/* Return data from Data Register */
return EEDR;
}
1. Refer to About Code Examples.
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AVR Memories
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 45
12.6.5 GPIOR2 – General Purpose I/O Register 2
Name: GPIOR2
Offset: 0x4B [ID-000004d0]
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x2B
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
GPIOR2[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – GPIOR2[7:0] General Purpose I/O
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12.6.6 GPIOR1 – General Purpose I/O Register 1
Name: GPIOR1
Offset: 0x4A [ID-000004d0]
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x2A
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
GPIOR1[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – GPIOR1[7:0] General Purpose I/O
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12.6.7 GPIOR0 – General Purpose I/O Register 0
Name: GPIOR0
Offset: 0x3E [ID-000004d0]
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x1E
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
GPIOR0[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – GPIOR0[7:0] General Purpose I/O
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13. System Clock and Clock Options
13.1 Clock Systems and Their Distribution
The following figure illustrates the principal clock systems in the device and their distribution. All the
clocks need not be active at a given time. In order to reduce power consumption, the clocks to modules
not being used can be halted by using different sleep modes. The clock systems are described in the
following sections.
The system clock frequency refers to the frequency generated from the system clock prescaler. All clock
outputs from the AVR clock control unit runs in the same frequency.
Figure 13-1. Clock Distribution
Watchdog
Oscillator
ATmega328/P
System Clock and Clock Options
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 49
13.1.1 CPU Clock – clkCPU
The CPU clock is routed to parts of the system concerned with operation of the AVR core. Examples of
such modules are the general purpose register file, the Status register, and the data memory holding the
stack pointer. Halting the CPU clock inhibits the core from performing general operations and
calculations.
13.1.2 I/O Clock – clkI/O
The I/O clock is used by the majority of the I/O modules, like timer/counters, SPI, and USART. The I/O
clock is also used by the External Interrupt module, but the start condition detection in the USI module is
carried out asynchronously when clkI/O is halted, TWI address recognition in all Sleep modes.
Note: If a level triggered interrupt is used for wake-up from power-down, the required level must be held
long enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears
before the end of the start-up time, the MCU will still wake up, but no interrupt will be generated. The
start-up time is defined by the SUT and CKSEL fuses.
13.1.3 Flash Clock – clkFLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously
with the CPU clock.
13.1.4 Asynchronous Timer Clock – clkASY
The asynchronous timer clock allows asynchronous timer/counters to be clocked directly from an external
clock or an external 32 kHz clock crystal. The dedicated clock domain allows using this timer/counter as a
real-time counter even when the device is in Sleep mode.
13.1.5 ADC Clock – clkADC
The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks in order
to reduce noise generated by digital circuitry. This gives more accurate ADC conversion results.
13.2 Clock Sources
The device has the following clock source options, selectable by Flash fuse bits as shown below. The
clock from the selected source is input to the AVR clock generator and routed to the appropriate modules.
Table 13-1. Device Clocking Options Select
Device Clocking Option CKSEL[3:0]
Low-Power Crystal Oscillator 1111 - 1000
Full Swing Crystal Oscillator 0111 - 0110
Low Frequency Crystal Oscillator 0101 - 0100
Internal 128 kHz RC Oscillator 0011
Calibrated Internal RC Oscillator 0010
External Clock 0000
Reserved 0001
Note: For all fuses, '1' means unprogrammed while '0' means programmed.
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© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 50
13.2.1 Default Clock Source
The device is shipped with internal RC oscillator at 8.0 MHz and with the fuse CKDIV8 programmed,
resulting in 1.0 MHz system clock. The start-up time is set to maximum, and the time-out period is
enabled: CKSEL=0010, SUT=10, CKDIV8=0. This default setting ensures that all users can make their
desired clock source setting using any available programming interface.
13.2.2 Clock Start-Up Sequence
Any clock source needs a sufficient VCC to start oscillating and a minimum number of oscillating cycles
before it can be considered stable.
To ensure sufficient VCC, the device issues an internal Reset with a time-out delay (tTOUT) after the device
Reset is released by all other Reset sources. See the Related Links for a description of the start
conditions for the internal Reset. The delay (tTOUT) is timed from the Watchdog oscillator and the number
of cycles in the delay is set by the SUTx and CKSELx fuse bits. The selectable delays are shown in the
table below. The frequency of the Watchdog oscillator is voltage dependent.
Table 13-2. Number of Watchdog Oscillator Cycles
Typ. Time-out (VCC = 5.0V) Typ. Time-out (VCC = 3.0V)
0 ms 0 ms
4 ms 4.3 ms
65 ms 69 ms
Main purpose of the delay is to keep the device in Reset until it is supplied with minimum VCC. The delay
will not monitor the actual voltage, so it is required to select a delay longer than the VCC rise time. If this is
not possible, an internal or external Brown-out Detection (BOD) circuit should be used. A BOD circuit will
ensure sufficient VCC before it releases the reset, and the time out delay can be disabled. Disabling the
time-out delay without utilizing a BOD circuit is not recommended.
The oscillator is required to oscillate for a minimum number of cycles before the clock is considered
stable. An internal ripple counter monitors the oscillator output clock, and keeps the internal Reset active
for a given number of clock cycles. The Reset is then released and the device will start to execute. The
recommended oscillator start-up time is dependent on the clock type, and varies from six cycles for an
externally applied clock to 32K cycles for a low frequency crystal.
The start-up sequence for the clock includes both the time-out delay and the start-up time when the
device starts up from Reset. When starting up from Power-save or Power-down mode, VCC is assumed to
be at a sufficient level and only the start-up time is included.
13.2.3 Clock Source Connections
Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier that can be configured
for use as an on-chip oscillator, as shown in the figure below. Either a quartz crystal or a ceramic
resonator may be used.
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the capacitors
depends on the crystal or resonator in use, the amount of stray capacitance, and the electromagnetic
noise of the environment. Some initial guidelines for choosing capacitors for use with crystals are given in
the next table. For ceramic resonators, the capacitor values given by the manufacturer should be used.
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Figure 13-2. Crystal Oscillator Connections
XTAL2
XTAL1
GND
C2
C1
Note: XTALn share the same pins as TOSCn
Related Links
Low-Power Crystal Oscillator
Full Swing Crystal Oscillator
Low-Frequency Crystal Oscillator
13.3 Low-Power Crystal Oscillator
This crystal oscillator is a low-power oscillator, with reduced voltage swing on the XTAL2 output. It gives
the lowest power consumption, but is not capable of driving other clock inputs, and may be more
susceptible to noise in noisy environments.
The crystal should be connected as described in Clock Source Connections. When selecting crystals,
load capacitance must be taken into consideration. The capacitance (Ce+Ci) needed at each TOSC pin
can be calculated by using:
+= 2
where:
•Ce - is optional external capacitors. (= C1, C2 as shown in the schematics.)
•Ci - is the pin capacitance in the following table.
•CL - is the load capacitance specified by the crystal vendor.
•CS - is the total stray capacitance for one XTAL pin.
32kHz Osc. Type Internal Pad Capacitance
(XTAL1)
Internal Pad Capacitance
(XTAL2)
Ci of system oscillator (XTAL pins) 18 pF 8 pF
The low-power oscillator can operate in three different modes, each optimized for a specific frequency
range. The operating mode is selected by the fuses CKSEL[3:1], as shown in the following table:
Table 13-3. Low-Power Crystal Oscillator Operating Modes(1)
Frequency Range
[MHz]
CKSEL[3:1](2) Absolute limits for total capacitance of C1 and C2 [pF](4)
0.4 - 0.9 100(3) –
0.9 - 3.0 101 12 - 22
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Frequency Range
[MHz]
CKSEL[3:1](2) Absolute limits for total capacitance of C1 and C2 [pF](4)
3.0 - 8.0 110 12 - 22
8.0 - 16.0 111 12 - 22
Note:
1. This is the recommended CKSEL settings for the difference frequency ranges.
2. This option should not be used with crystals, only with ceramic resonators.
3. If the crystal frequency exceeds the specification of the device (depends on VCC), the CKDIV8 fuse
can be programmed in order to divide the internal frequency by 8. It must be ensured that the
resulting divided clock meets the frequency specification of the device.
4. When selecting the external capacitor value, the stray capacitance from the PCB and device should
be deducted.
The CKSEL0 Fuse together with the SUT[1:0] fuses select the start-up times, as shown in the following
table:
Table 13-4. Start-up Times for the Low-Power Crystal Oscillator Clock Selection
Oscillator Source / Power
Conditions
Start-up Time
from Power-down
and Power-save
Additional Delay from
Reset
(VCC = 5.0V)
CKSEL0 SUT[1:0]
Ceramic resonator, fast rising
power
258 CK 14 CK + 4.1 ms(1) 0 00
Ceramic resonator, slowly
rising power
258 CK 14 CK + 65 ms(1) 0 01
Ceramic resonator, BOD
enabled
1K CK 14 CK(2) 0 10
Ceramic resonator, fast rising
power
1K CK 14 CK + 4.1 ms(2) 0 11
Ceramic resonator, slowly
rising power
1K CK 14 CK + 65 ms(2) 1 00
Crystal Oscillator, BOD
enabled
16K CK 14 CK 1 01
Crystal Oscillator, fast rising
power
16K CK 14 CK + 4.1 ms 1 10
Crystal Oscillator, slowly rising
power
16K CK 14 CK + 65 ms 1 11
Note:
1. These options should only be used when not operating close to the maximum frequency of the
device, and only if frequency stability at start-up is not important for the application. These options
are not suitable for crystals.
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2. These options are intended for use with ceramic resonators and will ensure frequency stability at
start-up. They can also be used with crystals when not operating close to the maximum frequency
of the device, and if frequency stability at start-up is not important for the application.
Related Links
Clock Source Connections
Full Swing Crystal Oscillator
13.4 Full Swing Crystal Oscillator
This crystal oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 output. This is useful for
driving other clock inputs and in noisy environments. The current consumption is higher than for the low-
power crystal oscillator. Note that the full swing crystal oscillator will only operate for VCC=2.7-5.5V.
Some initial guidelines for choosing capacitors for use with crystals are given in Table 13-6. The crystal
should be connected as described in Clock Source Connections”.
The Operating mode is selected based on the fuses CKSEL[3:1] as shown in the table:
Table 13-5. Full Swing Crystal Oscillator Operating Modes
Frequency Range(1) [MHz] CKSEL[3:1] Absolute limits for Capacitors C1 and C2 [pF]
0.4 - 20 011 12 - 22
Note:
1. If the crystal frequency exceeds the specification of the device (depends on VCC), the CKDIV8 fuse
can be programmed in order to divide the internal frequency by 8. It must be ensured that the
resulting divided clock meets the frequency specification of the device.
For the crystal oscillator connections refer to Low Power Crystal Oscillator in the previous section.
Table 13-6. Start-Up Times for the Full Swing Crystal Oscillator Clock Selection
Oscillator Source / Power
Conditions
Start-Up Time from
Power-down and
Power-save
Additional Delay from Reset
(VCC = 5.0V)
CKSEL0 SUT[1:0]
Ceramic resonator, fast rising
power
258 CK 14 CK + 4.1 ms(1) 0 00
Ceramic resonator, slowly rising
power
258 CK 14 CK + 65 ms(1) 0 01
Ceramic resonator, BOD enabled 1K CK 14 CK(2) 0 10
Ceramic resonator, fast rising
power
1K CK 14 CK + 4.1 ms(2) 0 11
Ceramic resonator, slowly rising
power
1K CK 14 CK + 65 ms(2) 1 00
Crystal Oscillator, BOD enabled 16K CK 14 CK 1 01
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Oscillator Source / Power
Conditions
Start-Up Time from
Power-down and
Power-save
Additional Delay from Reset
(VCC = 5.0V)
CKSEL0 SUT[1:0]
Crystal oscillator, fast rising power 16K CK 14 CK + 4.1 ms 1 10
Crystal oscillator, slowly rising
power
16K CK 14 CK + 65 ms 1 11
Note:
1. These options should only be used when not operating close to the maximum frequency of the
device, and only if frequency stability at start-up is not important for the application. These options
are not suitable for crystals.
2. These options are intended for use with ceramic resonators and will ensure frequency stability at
start-up. They can be used with crystals when not operating close to the maximum frequency of the
device, and if frequency stability at start-up is not important for the application.
Related Links
Clock Source Connections
Low-Power Crystal Oscillator
13.5 Low-Frequency Crystal Oscillator
The low-frequency crystal oscillator is optimized for use with a 32.768 kHz watch crystal. When selecting
crystals, load capacitance and crystal’s Equivalent Series Resistance (ESR) must be taken into
consideration. Both values are specified by the crystal vendor. The oscillator is optimized for very low
power consumption, and thus when selecting crystals, consider the maximum ESR recommendations:
Table 13-7. Maximum ESR Recommendation for 32.768kHz Crystal
Crystal CL [pF] Max. ESR [kΩ](1)
6.5 75
9.0 65
12.5 30
Note:
1. Maximum ESR is typical value based on characterization.
The low-frequency crystal oscillator provides an internal load capacitance at each TOSC pin:
Table 13-8. Capacitance for Low-Frequency Oscillator
32kHz Osc. Type Cap. (XTAL1/TOSC1) Cap. (XTAL2/TOSC2)
Ci of system oscillator (XTAL pins) 18 pF 8 pF
Ci of timer oscillator (TOSC pins) 18 pF 8 pF
The capacitance (Ce+Ci) needed at each TOSC pin can be calculated by using:
= 2C
where:
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• Ce - is optional external capacitors as described in Figure 13-2.
• Ci - is the pin capacitance in the above table.
• CL - is the load capacitance for a 32.768 kHz crystal specified by the crystal vendor.
•CS - is the total stray capacitance for one TOSC pin.
Crystals specifying a load capacitance (CL) higher than 6pF require external capacitors applied as
described in Low-Power Crystal Oscillator.
The low-frequency crystal oscillator must be selected by setting the CKSEL fuses to 0110 or 0111.
Table 13-9. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
CKSEL[3:0] Start-up Time from Power-down and Power-save Recommended Usage
0100(1) 1K CK
0101 32K CK Stable frequency at start-up
Note:
1. This option should only be used if frequency stability at start-up is not important for the application.
Start-up times are determined by the SUT Fuses as shown in the following table.
Table 13-10. Start-up Times for the Low-Frequency Crystal Oscillator Clock Selection
SUT[1:0] Additional Delay from Reset (VCC = 5.0V) Power Conditions
00 14 CK BOD enabled
01 14 CK + 4.1 ms Fast rising power
10 14 CK + 65 ms Slowly rising power
11 Reserved
Related Links
Clock Source Connections
Timer/Counter Oscillator
13.6 Calibrated Internal RC Oscillator
By default, the internal RC oscillator provides an 8.0 MHz clock. Though voltage and temperature
dependent, this clock can be very accurately calibrated by the user. The device is shipped with the
CKDIV8 fuse unprogrammed.
This clock may be selected as the system clock by programming the CKSEL fuses as shown in the
following table. During Reset, hardware loads the pre-programmed calibration value into the OSCCAL
register and thereby automatically calibrates the RC oscillator.
By changing the OSCCAL register from SW, it is possible to get a higher calibration accuracy than by
using the factory calibration.
When this oscillator is used as the chip clock, the Watchdog oscillator will still be used for the watchdog
timer and for the Reset time out.
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Table 13-11. Internal Calibrated RC Oscillator Operating Modes
Frequency Range(1) [MHz] CKSEL[3:0]
7.3 - 8.1 0010(2)
Note:
1. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8 fuse can
be programmed in order to divide the internal frequency by 8.
2. The device is shipped with this option selected.
When this oscillator is selected, start-up times are determined by the SUT fuses:
Table 13-12. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection - SUT
Power Conditions Start-up Time from Power-down
and Power-save
Additional Delay from Reset (VCC = 5.0V) SUT[1:0]
BOD enabled 6 CK 14 CK 00
Fast rising power 6 CK 14 CK + 4 ms 01
Slow rising power 6 CK 14 CK + 65 ms 10(1)
Reserved 11
Note:
1. The device is shipped with this option selected.
Related Links
Clock Characteristics
System Clock Prescaler
Calibration Byte
OSCCAL
OSCCAL
13.7 128 kHz Internal Oscillator
The 128 kHz internal oscillator is a low-power oscillator providing a clock of 128 kHz. This clock may be
selected as the system clock by programming the CKSEL fuses to '0011' as shown in the following table.
Warning: Using the 128 kHz internal oscillator as the system oscillator and Watchdog timer
simultaneously is not recommended as this defeats one of the purposes of the Watchdog timer.
Table 13-13. 128kHz Internal Oscillator Operating Modes
Nominal Frequency(1) CKSEL[3:0]
128 kHz 0011
Note:
1. The 128 kHz oscillator is a very low-power clock source and is not designed for high accuracy.
When this clock source is selected, start-up times are determined by the SUT fuses:
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Table 13-14. Start-Up Times for the 128kHz Internal Oscillator
Power Conditions Start-Up Time from Power-Down and Power-
Save
Additional Delay from Reset SUT[1:0]
BOD enabled 6 CK 14 CK 00
Fast rising power 6 CK 14 CK + 4 ms 01
Slowly rising power 6 CK 14 CK + 65 ms 10
Reserved 11
13.8 External Clock
To drive the device from an external clock source, EXTCLK should be driven as shown in the figure
below. To run the device on an external clock, the CKSEL fuses must be programmed to '0000':
Table 13-15. External Clock Frequency
Frequency(1) CKSEL[3:0]
0 - 20 MHz 0000
Note:
1. If the crystal frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse
can be programmed in order to divide the internal frequency by 8. It must be ensured that the
resulting divided clock meets the frequency specification of the device.
Figure 13-3. External Clock Drive Configuration
EXTERNAL
CLOCK
SIGNAL
EXTCLK
GND
When this clock source is selected, start-up times are determined by the SUT fuses:
Table 13-16. Start-Up Times for the External Clock Selection - SUT
Power Conditions Start-Up Time from Power-Down
and Power-Save
Additional Delay from Reset (VCC = 5.0V) SUT[1:0]
BOD enabled 6 CK 14 CK 00
Fast rising power 6 CK 14 CK + 4 ms 01
Slowly rising power 6 CK 14 CK + 65 ms 10
Reserved 11
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When applying an external clock, it is required to avoid sudden changes in the applied clock frequency to
ensure stable operation of the MCU. A variation in frequency of more than 2% from one clock cycle to the
next can lead to unpredictable behavior. If changes of more than 2% is required, ensure that the MCU is
kept in Reset during the changes.
The system clock prescaler can be used to implement run-time changes of the internal clock frequency
while still ensuring stable operation.
Related Links
System Clock Prescaler
13.9 Timer/Counter Oscillator
The device uses the same crystal oscillator for low-frequency oscillator and Timer/Counter oscillator. See
Low Frequency Crystal Oscillator for details on the oscillator and crystal requirements.
On this device, the Timer/Counter Oscillator Pins (TOSC1 and TOSC2) are shared with XTAL1 and
XTAL2. When using the Timer/Counter oscillator, the system clock needs to be four times the oscillator
frequency. Due to this and the pin sharing, the Timer/Counter oscillator can only be used when the
calibrated internal RC oscillator is selected as system clock source.
Applying an external clock source to TOSC1 can be done if the Enable External Clock Input bit in the
Asynchronous Status Register (ASSR.EXCLK) is written to '1'. See the description of the Asynchronous
Operation of Timer/Counter2 for further description on selecting external clock as input instead of a
32.768 kHz watch crystal.
Related Links
Low-Frequency Crystal Oscillator
OCR2B
ASSR
13.10 Clock Output Buffer
The device can output the system clock on the CLKO pin. To enable the output, the CKOUT fuse has to
be programmed. This mode is suitable when the chip clock is used to drive other circuits on the system.
The clock also will be output during Reset, and the normal operation of I/O pin will be overridden when
the fuse is programmed. Any clock source, including the internal RC oscillator, can be selected when the
clock is output on CLKO. If the system clock prescaler is used, it is the divided system clock that is
output.
13.11 System Clock Prescaler
The device has a system clock prescaler and the system clock can be divided by configuring the Clock
Prescale Register (CLKPR). This feature can be used to decrease the system clock frequency and the
power consumption when the requirement for processing power is low. This can be used with all clock
source options, and it will affect the clock frequency of the CPU and all synchronous peripherals. clkI/O,
clkADC, clkCPU, and clkFLASH are divided by a factor as shown in the CLKPR description.
When switching between prescaler settings, the system clock prescaler ensures that no glitches occur in
the clock system. It also ensures that no intermediate frequency is higher than neither the clock
frequency corresponding to the previous setting nor the clock frequency corresponding to the new setting.
The ripple counter that implements the prescaler runs at the frequency of the undivided clock, which may
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be faster than the CPU's clock frequency. Hence, it is not possible to determine the state of the prescaler
- even if it were readable, the exact time it takes to switch from one clock division to the other cannot be
exactly predicted. From the time the Clock Prescaler Selection bits (CLKPS[3:0]) values are written, it
takes between T1 + T2 and T1 + 2 * T2 before the new clock frequency is active. In this interval, two
active clock edges are produced. Here, T1 is the previous clock period, and T2 is the period
corresponding to the new prescaler setting.
To avoid unintentional changes of clock frequency, a special write procedure must be followed to change
the CLKPS bits:
1. Write the Clock Prescaler Change Enable (CLKPCE) bit to '1' and all other bits in CLKPR to zero:
CLKPR=0x80.
2. Within four cycles, write the desired value to CLKPS[3:0] while writing a zero to CLKPCE:
CLKPR=0x0N.
Interrupts must be disabled when changing prescaler setting to make sure the write procedure is not
interrupted.
Related Links
Calibrated Internal RC Oscillator
External Clock
CLKPR
13.12 Register Description
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13.12.1 Oscillator Calibration Register
Name: OSCCAL
Offset: 0x66
Reset: Device Specific Calibration Value
Property: -
Bit 7 6 5 4 3 2 1 0
CAL[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x x
Bits 7:0 – CAL[7:0] Oscillator Calibration Value
The oscillator calibration register is used to trim the calibrated internal RC oscillator to remove process
variations away from the oscillator frequency. A preprogrammed calibration value is automatically written
to this register during chip reset, giving the factory calibrated frequency as specified in the Clock
Characteristics section of chapter Electrical Characteristics.The application software can write this
register to change the oscillator frequency. The oscillator can be calibrated to frequencies as specified in
the Clock Characteristics section of chapter Electrical Characteristics. Calibration outside that range is not
recommended.
Note that this oscillator is used to time EEPROM and Flash write accesses, and these write times will be
affected accordingly. If the EEPROM or Flash are written, do not calibrate to more than 8.8 MHz.
Otherwise, the EEPROM or Flash write may fail.
The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the lowest
frequency range, setting this bit to 1 gives the highest frequency range. The two frequency ranges are
overlapping, in other words, a setting of OSCCAL=0x7F gives a higher frequency than OSCCAL=0x80.
The CAL[6:0] bits are used to tune the frequency within the selected range. A setting of 0x00 gives the
lowest frequency in that range and a setting of 0x7F gives the highest frequency in the range.
Related Links
Calibrated Internal RC Oscillator
Clock Characteristics
Calibrated Internal RC Oscillator Accuracy
External Clock Drive Waveforms
External Clock Drive
Calibrated Internal RC Oscillator
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13.12.2 Clock Prescaler Register
Name: CLKPR
Offset: 0x61
Reset: Refer to the bit description
Property: -
Bit 7 6 5 4 3 2 1 0
CLKPCE CLKPS[3:0]
Access R/W R/W R/W R/W R/W
Reset 0 x x x x
Bit 7 – CLKPCE Clock Prescaler Change Enable
The CLKPCE bit must be written to logic one to enable change of the CLKPS bits. The CLKPCE bit is
only updated when the other bits in CLKPR are simultaneously written to zero. CLKPCE is cleared by
hardware four cycles after it is written or when CLKPS bits are written. Rewriting the CLKPCE bit within
this time-out period does neither extend the time-out period nor clear the CLKPCE bit.
Bits 3:0 – CLKPS[3:0] Clock Prescaler Select
These bits define the division factor between the selected clock source and the internal system clock.
These bits can be written run-time to vary the clock frequency to suit the application requirements. As the
divider divides the master clock input to the MCU, the speed of all synchronous peripherals is reduced
when a division factor is used. The division factors are given in the table below.
The CKDIV8 Fuse determines the initial value of the CLKPS bits. If CKDIV8 is unprogrammed, the
CLKPS bits will be reset to “0000”. If CKDIV8 is programmed, CLKPS bits are reset to “0011”, giving a
division factor of 8 at start-up. This feature should be used if the selected clock source has a higher
frequency than the maximum frequency of the device at the present operating conditions. Note that any
value can be written to the CLKPS bits regardless of the CKDIV8 Fuse setting. The Application software
must ensure that a sufficient division factor is chosen if the selected clock source has a higher frequency
than the maximum frequency of the device at the present operating conditions. The device is shipped
with the CKDIV8 Fuse programmed.
Table 13-17. Clock Prescaler Select
CLKPS[3:0] Clock Division Factor
0000 1
0001 2
0010 4
0011 8
0100 16
0101 32
0110 64
0111 128
1000 256
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14. Power Management and Sleep Modes
14.1 Overview
Sleep modes enable the application to shut down unused modules in the MCU, thereby saving power.
The device provides various sleep modes allowing the user to tailor the power consumption to the
application requirements.
When enabled, the Brown-out Detector (BOD) actively monitors the power supply voltage during the
sleep periods. To further save power, it is possible to disable the BOD in some sleep modes. See also
BOD Disable.
Note: BOD disable is only available for ATmega328P.
14.2 Sleep Modes
The following table shows the different sleep modes, BOD disable ability, and their wake-up sources.
Table 14-1. Active Clock Domains and Wake-Up Sources in the Different Sleep Modes
Sleep Mode Active Clock Domains Oscillators Wake-Up Sources
Software
BOD Disable
clkCPU
clkFLASH
clkIO
clkADC
clkASY
Main Clock
Source Enabled
Timer Oscillator
Enabled
INT and PCINT
TWI Address
Match
Timer2
SPM/EEPROM
Ready
ADC
WDT
Other I/O
Idle Yes Yes Yes Yes Yes(2) Yes Yes Yes Yes Yes Yes Yes
ADC Noise
Reduction
Yes Yes Yes Yes(2) Yes(3) Yes Yes(2) Yes Yes Yes
Power-Down Yes(3) Yes Yes Yes
Power-Save Yes Yes(2) Yes(3) Yes Yes Yes Yes
Standby(1) Yes Yes(3) Yes Yes Yes
Extended Standby Yes(2) Yes Yes(2) Yes(3) Yes Yes Yes Yes
Note:
1. Only recommended with external crystal or resonator selected as the clock source.
2. If Timer/Counter2 is running in Asynchronous mode.
3. For INT1 and INT0, only level interrupt.
To enter any of the six sleep modes, the sleep enable bit in the Sleep Mode Control Register (SMCR.SE)
must be written to '1' and a SLEEP instruction must be executed. Sleep Mode Select bits
(SMCR.SM[2:0]) select which sleep mode (Idle, ADC Noise Reduction, Power-Down, Power-Save,
Standby, or Extended Standby) will be activated by the SLEEP instruction.
Note: The block diagram in the section System Clock and Clock Options provides an overview over the
different clock systems in the device and their distribution. This figure is helpful in selecting an appropriate
Sleep mode.
If an enabled interrupt occurs while the MCU is in a Sleep mode, the MCU wakes up. The MCU is then
halted for four cycles in addition to the start-up time, executes the interrupt routine, and resumes
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execution from the instruction following SLEEP. The contents of the register file and SRAM are unaltered
when the device wakes up from sleep. If a reset occurs during Sleep mode, the MCU wakes up and
executes from the Reset vector.
Related Links
System Clock and Clock Options
14.3 BOD Disable
When the Brown-out Detector (BOD) is enabled by BODLEVEL fuses, the BOD is actively monitoring the
power supply voltage during a sleep period. To save power, it is possible to disable the BOD by use of
software for some of the sleep modes. The sleep mode power consumption will then be at the same level
as when BOD is globally disabled by fuses. If BOD is disabled in software, the BOD function is turned off
immediately after entering the sleep mode. Upon wake-up from sleep, BOD is automatically enabled
again. This ensures safe operation in case the VCC level has dropped during the sleep period.
When the BOD has been disabled, the wake-up time from sleep mode will be approximately 60 μs to
ensure that the BOD is working correctly before the MCU continues executing code.
BOD disable is controlled by the BOD Sleep bit in the MCU Control Register (MCUCR.BODS). Writing
this bit to '1' turns off the BOD in relevant sleep modes, while a zero in this bit keeps BOD active. The
default setting, BODS=0, keeps BOD active.
Note: Writing to the BODS bit is controlled by a timed sequence and an enable bit.
Note: BOD disable is only available for ATmega328P.
Related Links
MCUCR
14.4 Idle Mode
When the SM[2:0] bits are written to '000', the SLEEP instruction makes the MCU enter Idle mode,
stopping the CPU but allowing the SPI, USART, analog comparator, two-wire serial interface, timer/
counters, watchdog, and the interrupt system to continue operating. This Sleep mode basically halts
clkCPU and clkFLASH, while allowing the other clocks to run.
The Idle mode enables the MCU to wake-up from external triggered interrupts as well as internal ones
like the timer overflow and USART transmit complete interrupts. If wake-up from the analog comparator
interrupt is not required, the analog comparator can be powered-down by setting the ACD bit in the
Analog Comparator Control and Status Register – ACSR. This will reduce power consumption in Idle
mode.
14.5 ADC Noise Reduction Mode
When the SM[2:0] bits are written to '001', the SLEEP instruction makes the MCU enter ADC Noise
Reduction mode, stopping the CPU but allowing the ADC, the external interrupts, the two-wire serial
interface address watch, Timer/Counter(1), and the Watchdog to continue operating (if enabled). This
sleep mode basically halts clkI/O, clkCPU, and clkFLASH, while allowing the other clocks to run.
This improves the noise environment for the ADC, enabling higher resolution measurements. If the ADC
is enabled, a conversion starts automatically when this mode is entered. Apart from the ADC conversion
complete interrupt, only these events can wake-up the MCU from ADC Noise Reduction mode:
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• External Reset
• Watchdog System Reset
• Watchdog Interrupt
• Brown-out Reset
• Two-wire Serial Interface Address Match
• Timer/Counter Interrupt
• SPM/EEPROM Ready Interrupt
• External Level Interrupt on INT
• Pin Change Interrupt
Note: 1. Timer/Counter will only keep running in Asynchronous mode.
Related Links
8-bit Timer/Counter2 with PWM and Asynchronous Operation
14.6 Power-Down Mode
When the SM[2:0] bits are written to '010', the SLEEP instruction makes the MCU enter the Power-Down
mode. In this mode, the external oscillator is stopped, while the external interrupts, the two-wire serial
interface address watch, and the Watchdog continue operating (if enabled).
Only one of these events can wake up the MCU:
• External Reset
• Watchdog System Reset
• Watchdog Interrupt
• Brown-out Reset
• Two-wire Serial Interface Address Match
• External level Interrupt on INT
• Pin Change Interrupt
This sleep mode basically halts all generated clocks, allowing operation of asynchronous modules only.
Note: If a level triggered interrupt is used for wake-up from power-down, the required level must be held
long enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears
before the end of the start-up time, the MCU will still wake up, but no interrupt will be generated. The
start-up time is defined by the SUT and CKSEL Fuses.
When waking up from the Power-Down mode, there is a delay from the wake-up condition occurs until
the wake-up becomes effective. This allows the clock to restart and become stable after having been
stopped. The wake-up period is defined by the same CKSEL fuses that define the Reset time-out period.
Related Links
System Clock and Clock Options
14.7 Power-Save Mode
When the SM[2:0] bits are written to 011, the SLEEP instruction makes the MCU enter Power-Save
mode. This mode is identical to power-down, except:
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If timer/counter2 is enabled, it will keep running during sleep. The device can wake-up from either timer
overflow or output compare event from timer/counter2 if the corresponding timer/counter2 interrupt
enable bits are set in TIMSK2, and the global interrupt enable bit in SREG is set.
If timer/counter2 is not running, the Power-Down mode is recommended instead of the Power-Save
mode.
The timer/counter2 can be clocked both synchronously and asynchronously in Power-Save mode. If
timer/counter2 is not using the asynchronous clock, the timer/counter oscillator is stopped during sleep. If
timer/counter2 is not using the synchronous clock, the clock source is stopped during sleep. Even if the
synchronous clock is running in power-save, this clock is only available for timer/counter2.
14.8 Standby Mode
When the SM[2:0] bits are written to '110' and an external crystal/resonator clock option is selected, the
SLEEP instruction makes the MCU enter Standby mode. This mode is identical to the Power-Down mode
with the exception that the oscillator is kept running. From Standby mode, the device wakes up in six
clock cycles.
14.9 Extended Standby Mode
When the SM[2:0] bits are written to '111' and an external crystal/resonator clock option is selected, the
SLEEP instruction makes the MCU enter Extended Standby mode. This mode is identical to Power-Save
mode with the exception that the oscillator is kept running. From Extended Standby mode, the device
wakes up in six clock cycles.
14.10 Power Reduction Register
The Power Reduction Register (PRR) provides a method to stop the clock to individual peripherals to
reduce power consumption. The current state of the peripheral is frozen and the I/O registers cannot be
read or written. Resources used by the peripheral when stopping the clock will remain occupied, hence
the peripheral should in most cases be disabled before stopping the clock. Waking up a module, which is
done by clearing the corresponding bit in the PRR, puts the module in the same state as before
shutdown.
Module shutdown can be used in Idle mode and Active mode to significantly reduce the overall power
consumption. In all other sleep modes, the clock is already stopped.
14.11 Minimizing Power Consumption
There are several possibilities to consider when trying to minimize the power consumption in an AVR
controlled system. In general, sleep modes should be used as much as possible, and the sleep mode
should be selected so that as few as possible of the device’s functions are operating. All functions not
needed should be disabled. In particular, the following modules may need special consideration when
trying to achieve the lowest possible power consumption.
14.11.1 Analog-to-Digital Converter
If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should be disabled
before entering any sleep mode. When the ADC is turned off and on again, the next conversion will be an
extended conversion.
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Related Links
Analog-to-Digital Converter
14.11.2 Analog Comparator
When entering Idle mode, the analog comparator should be disabled if not used. When entering ADC
Noise Reduction mode, the analog comparator should be disabled. In other sleep modes, the analog
comparator is automatically disabled. However, if the analog comparator is set up to use the internal
voltage reference as input, the analog comparator should be disabled in all sleep modes. Otherwise, the
internal voltage reference will be enabled, independent of the sleep mode.
Related Links
Analog Comparator
14.11.3 Brown-Out Detector
If the Brown-Out Detector (BOD) is not needed by the application, this module should be turned off. If the
BOD is enabled by the BODLEVEL fuses, it will be enabled in all sleep modes, and hence, always
consume power. In the deeper sleep modes, this will contribute significantly to the total current
consumption.
Related Links
System Control and Reset
14.11.4 Internal Voltage Reference
The internal voltage reference will be enabled when needed by the Brown-out Detection, the analog
comparator or the Analog-to-Digital Converter (ADC). If these modules are disabled as described in the
sections above, the internal voltage reference will be disabled and it will not be consuming power. When
turned on again, the user must allow the reference to start-up before the output is used. If the reference is
kept on in Sleep mode, the output can be used immediately.
Related Links
System Control and Reset
14.11.5 Watchdog Timer
If the watchdog timer is not needed in the application, the module should be turned off. If the watchdog
timer is enabled, it will be enabled in all sleep modes and hence always consume power. In the deeper
sleep modes, this will contribute significantly to the total current consumption.
Related Links
System Control and Reset
14.11.6 Port Pins
When entering a sleep mode, all port pins should be configured to use minimum power. The most
important is then to ensure that no pins drive resistive loads. In sleep modes where both the I/O clock
(clkI/O) and the ADC clock (clkADC) are stopped, the input buffers of the device will be disabled. This
ensures that no power is consumed by the input logic when not needed. In some cases, the input logic is
needed for detecting wake-up conditions, and it will then be enabled. Refer to the section Digital Input
Enable and Sleep Modes for details on which pins are enabled. If the input buffer is enabled and the input
signal is left floating or have an analog signal level close to VCC/2, the input buffer will use excessive
power.
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For analog input pins, the digital input buffer should be disabled at all times. An analog signal level close
to VCC/2 on an input pin can cause significant current even in active mode. Digital input buffers can be
disabled by writing to the Digital Input Disable Registers (DIDR0 for ADC, DIDR1 for AC).
Related Links
Digital Input Enable and Sleep Modes
14.11.7 On-chip Debug System
If the on-chip debug system is enabled by the fuse and the chip enters Sleep mode, the main clock
source is enabled and hence always consumes power. In the deeper Sleep modes, this will contribute
significantly to the total current consumption.
14.12 Register Description
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14.12.1 Sleep Mode Control Register
Name: SMCR
Offset: 0x53
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x33
The Sleep Mode Control register contains control bits for power management.
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
SM[2:0] SE
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bits 3:1 – SM[2:0] Sleep Mode Select
The SM[2:0] bits select between the five available sleep modes.
Table 14-2. Sleep Mode Select
SM[2:0] Sleep Mode
000 Idle
001 ADC Noise Reduction
010 Power-down
011 Power-save
100 Reserved
101 Reserved
110 Standby(1)
111 Extended Standby(1)
Note:
1. Standby mode is only recommended for use with external crystals or resonators.
Bit 0 – SE Sleep Enable
The SE bit must be written to logic one to make the MCU enter the sleep mode when the SLEEP
instruction is executed. To avoid the MCU entering the sleep mode unless it is the programmer’s purpose,
it is recommended to write the Sleep Enable (SE) bit to one just before the execution of the SLEEP
instruction and to clear it immediately after waking up.
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14.12.2 MCU Control Register
Name: MCUCR
Offset: 0x55
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x35
The MCU Control register controls the placement of the interrupt vector table in order to move interrupts
between application and boot space.
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
BODS BODSE PUD IVSEL IVCE
Access R/W R/W R/W R/W R/W
Reset 0 0 0 0 0
Bit 6 – BODS BOD Sleep
The BODS bit must be written to '1' in order to turn off BOD during sleep. Writing to the BODS bit is
controlled by a timed sequence and the enable bit BODSE. To disable BOD in relevant sleep modes, both
BODS and BODSE must first be written to '1'. Then, BODS must be written to '1' and BODSE must be
written to zero within four clock cycles.
The BODS bit is active three clock cycles after it is set. A sleep instruction must be executed while BODS
is active in order to turn off the BOD for the actual sleep mode. The BODS bit is automatically cleared
after three clock cycles.
Note: BOD disable is only available for ATmega328P.
Bit 5 – BODSE BOD Sleep Enable
BODSE enables setting of BODS control bit, as explained in BODS bit description. BOD disable is
controlled by a timed sequence.
Note: BOD disable is only available for ATmega328P.
Bit 4 – PUD Pull-up Disable
When this bit is written to one, the pull ups in the I/O ports are disabled even if the DDxn and PORTxn
registers are configured to enable the pull ups ({DDxn, PORTxn} = 0b01).
Bit 1 – IVSEL Interrupt Vector Select
When the IVSEL bit is cleared (zero), the interrupt vectors are placed at the start of the Flash memory.
When this bit is set (one), the interrupt vectors are moved to the beginning of the boot loader section of
the Flash. The actual address of the start of the boot Flash section is determined by the BOOTSZ fuses.
To avoid unintentional changes of interrupt vector tables, a special write procedure must be followed to
change the IVSEL bit:
1. Write the Interrupt Vector Change Enable (IVCE) bit to one.
2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.
Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled in the
same cycle as IVCE is written, and interrupts remain disabled until after the instruction following the write
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to IVSEL. If IVSEL is not written, interrupts remain disabled for four cycles. The I-bit in the Status register
is unaffected by the automatic disabling.
Note: If interrupt vectors are placed in the boot loader section and Boot Lock bit BLB02 is programmed,
interrupts are disabled while executing from the application section. If interrupt vectors are placed in the
application section and Boot Lock bit BLB12 is programmed, interrupts are disabled while executing from
the boot loader section.
Bit 0 – IVCE Interrupt Vector Change Enable
The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by hardware
four cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable interrupts, as
explained in the IVSEL description above. See the code example below.
Assembly Code Example
Move_interrupts:
; Get MCUCR
in r16, MCUCR
mov r17, r16
; Enable change of Interrupt Vectors
ori r16, (1<<IVCE)
out MCUCR, r16
; Move interrupts to Boot Flash section
ori r17, (1<<IVSEL)
out MCUCR, r17
ret
C Code Example
void Move_interrupts(void)
{
uchar temp;
/* GET MCUCR*/
temp = MCUCR;
/* Enable change of Interrupt Vectors */
MCUCR = temp|(1<<IVCE);
/* Move interrupts to Boot Flash section */
MCUCR = temp|(1<<IVSEL);
}
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14.12.3 Power Reduction Register
Name: PRR
Offset: 0x64
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
PRTWI0 PRTIM2 PRTIM0 PRTIM1 PRSPI0 PRUSART0 PRADC
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bit 7 – PRTWI0 Power Reduction TWI0
Writing a logic one to this bit shuts down the TWI 0 by stopping the clock to the module. When waking up
the TWI again, the TWI should be reinitialized to ensure proper operation.
Bit 6 – PRTIM2 Power Reduction Timer/Counter2
Writing a logic one to this bit shuts down the Timer/Counter2 module in synchronous mode (AS2 is 0).
When the Timer/Counter2 is enabled, the operation will continue like before the shutdown.
Bit 5 – PRTIM0 Power Reduction Timer/Counter0
Writing a logic one to this bit shuts down the Timer/Counter0 module. When the Timer/Counter0 is
enabled, the operation will continue like before the shutdown.
Bit 3 – PRTIM1 Power Reduction Timer/Counter1
Writing a logic one to this bit shuts down the Timer/Counter1 module. When the Timer/Counter1 is
enabled, the operation will continue like before the shutdown.
Bit 2 – PRSPI0 Power Reduction Serial Peripheral Interface 0
If using debugWIRE on-chip debug system, this bit should not be written to one. Writing a logic one to this
bit shuts down the Serial Peripheral Interface (SPI) by stopping the clock to the module. When waking up
the SPI again, the SPI should be reinitialized to ensure proper operation.
Bit 1 – PRUSART0 Power Reduction USART0
Writing a logic one to this bit shuts down the USART by stopping the clock to the module. When waking
up the USART again, the USART should be reinitialized to ensure proper operation.
Bit 0 – PRADC Power Reduction ADC
Writing a logic one to this bit shuts down the ADC. The ADC must be disabled before shut down. The
analog comparator cannot use the ADC input MUX when the ADC is shut down.
Related Links
Supply Current of IO Modules
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Power Management and Sleep Modes
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15. System Control and Reset
15.1 Resetting the AVR
During Reset, all I/O registers are set to their initial values, and the program starts execution from the
Reset vector. The instruction placed at the Reset vector must be an Absolute Jump instruction (JMP) to
the reset handling routine for . If the program never enables an interrupt source, the interrupt vectors are
not used, and regular program code can be placed at these locations. This is also the case if the Reset
vector is in the application section while the interrupt vectors are in the boot section or vice versa. The
circuit diagram in the next section shows the reset logic.
The I/O ports of the AVR are immediately reset to their initial state when a Reset source goes active. This
does not require any clock source to be running.
After all Reset sources have gone inactive, a delay counter is invoked, stretching the internal Reset. This
allows the power to reach a stable level before the normal operation starts. The time-out period of the
delay counter is defined by the user through the SUT and CKSEL fuses. The different selections for the
delay period are presented in the System Clock and Clock Options chapter.
Related Links
System Clock and Clock Options
15.2 Reset Sources
The device has the following sources of Reset:
• Power-on Reset. The MCU is Reset when the supply voltage is less than the Power-on Reset
threshold (VPOT).
• External Reset. The MCU is Reset when a low level is present on the RESET pin for longer than
the minimum pulse length.
• Watchdog System Reset. The MCU is Reset when the Watchdog Timer period expires and the
Watchdog System Reset mode is enabled.
• Brown-out Reset. The MCU is Reset when the supply voltage VCC is less than the Brown-out Reset
threshold (VBOT) and the Brown-out Detector is enabled.
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© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 74
Figure 15-1. Reset Logic
MCU Status
Register (MCUSR)
Brown-out
Reset Circuit
BODLEVEL [2..0]
Delay Counters
CKSEL[3:0]
CK
TIMEOUT
WDRF
BORF
EXTRF
PORF
DATA BUS
Clock
Generator
SPIKE
FILTER
Pull-up Resistor
Watchdog
Oscillator
SUT[1:0]
Power-on Reset
Circuit
RSTDISBL
15.3 Power-on Reset
A Power-on Reset (POR) pulse is generated by an on-chip detection circuit. The POR is activated
whenever VCC is below the detection level. The POR circuit can be used to trigger the start-up Reset, as
well as to detect a failure in supply voltage.
A POR circuit ensures that the device is reset from power-on. Reaching the POR threshold voltage
invokes the delay counter, which determines how long the device is kept in Reset after VCC rise. The
Reset signal is activated again, without any delay, when VCC decreases below the detection level.
Figure 15-2. MCU Start-up, RESET Tied to VCC
V
RESET
TIME-OUT
INTERNAL
RESET
tTOUT
VPOT
VRST
CC
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System Control and Reset
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 75
Figure 15-3. MCU Start-up, RESET Extended Externally
RESET
TIME-OUT
INTERNAL
RESET
tTOUT
VPOT
VRST
VCC
15.4 External Reset
An external Reset is generated by a low level on the RESET pin. Reset pulses longer than the minimum
pulse width will generate a Reset, even if the clock is not running. Shorter pulses are not guaranteed to
generate a Reset. When the applied signal reaches the Reset Threshold Voltage (VRST) on its positive
edge, the delay counter starts the MCU after the Time-out period (tTOUT ) has expired. The external Reset
can be disabled by the RSTDISBL fuse.
Figure 15-4. External Reset During Operation
CC
15.5 Brown-out Detection
The device has an on-chip Brown-out Detection (BOD) circuit for monitoring the VCC level during
operation by comparing it to a fixed trigger level. The trigger level for the BOD can be selected by the
BODLEVEL Fuses. The trigger level has a hysteresis to ensure spike-free BOD. The hysteresis on the
detection level should be interpreted as VBOT+ = VBOT + VHYST/2 and VBOT- = VBOT - VHYST/2. When the
BOD is enabled, and VCC decreases to a value below the trigger level (VBOT- in the following figure), the
Brown-out Reset is immediately activated. When VCC increases above the trigger level (VBOT+ in the
following figure), the delay counter starts the MCU after the Time-out period tTOUT has expired.
The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for longer than
tBOD.
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System Control and Reset
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 76
Figure 15-5. Brown-out Reset During Operation
VCC
RESET
TIME-OUT
INTERNALRESET
VBOT-
VBOT+
tTOUT
15.6 Watchdog System Reset
When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On the falling
edge of this pulse, the delay timer starts counting the Time-out period tTOUT.
Figure 15-6. Watchdog System Reset During Operation
CK
CC
15.7 Internal Voltage Reference
The device features an internal bandgap reference. This reference is used for Brown-out Detection, and it
can be used as an input to the analog comparator or the ADC.
15.7.1 Voltage Reference Enable Signals and Start-up Time
The voltage reference has a start-up time that may influence the way it should be used. To save power,
the reference is not always turned ON. The reference is ON during the following situations:
1. When the BOD is enabled (by programming the BODLEVEL [2:0] Fuses).
2. When the bandgap reference is connected to the Analog Comparator (by setting the ACBG bit in
ACSR (ACSR.ACBG)).
3. When the ADC is enabled.
Thus, when the BOD is not enabled, after setting ACSR.ACBG or enabling the ADC, the user must
always allow the reference to start-up before the output from the analog comparator or ADC is used. To
reduce power consumption in the Power-Down mode, the user can avoid the three conditions above to
ensure that the reference is turned OFF before entering Power-Down mode.
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15.8 Watchdog Timer
If the watchdog timer is not needed in the application, the module should be turned OFF. If the watchdog
timer is enabled, it will be enabled in all sleep modes and hence always consume power. In the deeper
sleep modes, this will contribute significantly to the total current consumption.
Refer to Watchdog System Reset for details on how to configure the watchdog timer.
15.8.1 Features
• Clocked from Separate On-chip Oscillator
• Three Operating modes:
– Interrupt
– System Reset
– Interrupt and System Reset
• Selectable Time-out Period from 16 ms to 8s
• Possible Hardware Fuse Watchdog Always ON (WDTON) for Fail-safe mode
15.8.2 Overview
The device has an Enhanced Watchdog Timer (WDT). The WDT is a timer counting cycles of a separate
on-chip 128 kHz oscillator. The WDT gives an interrupt or a system reset when the counter reaches a
given time-out value. In normal operation mode, it is required that the system uses the Watchdog Timer
Reset (WDR) instruction to restart the counter before the time-out value is reached. If the system doesn't
restart the counter, an interrupt or system reset will be issued.
Figure 15-7. Watchdog Timer
OSC/2K
OSC/4K
OSC/8K
OSC/16K
OSC/32K
OSC/64K
OSC/128K
OSC/256K
OSC/512K
OSC/1024K
WDP[3:0]
WATCHDOG
RESET
WDE
WDIF
WDIE
MCU RESET
INTERRUPT
128 kHz
OSCILLATOR
In Interrupt mode, the WDT gives an interrupt when the timer expires. This interrupt can be used to wake
the device from Sleep modes, and as a general system timer. One example is to limit the maximum time
allowed for certain operations, giving an interrupt when the operation has run longer than expected. In
System Reset mode, the WDT gives a reset when the timer expires. This is typically used to prevent
system hang-up in case of runaway code. The third mode, Interrupt and System Reset mode, combines
the other two modes by first giving an interrupt and then switch to System Reset mode. This mode will for
instance allow a safe shutdown by saving critical parameters before a system Reset.
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The Watchdog always on (WDTON) fuse, if programmed, will force the Watchdog Timer to System Reset
mode. With the fuse programmed the System Reset mode bit (WDE) and Interrupt mode bit (WDIE) are
locked to 1 and 0 respectively. To further ensure program security, alterations to the Watchdog set-up
must follow timed sequences. The sequence for clearing WDE and changing time out configuration is as
follows:
1. In the same operation, write a logic one to the Watchdog change enable bit (WDCE) and Watchdog
System Reset Enable (WDE) in Watchdog Timer Control Register (WDTCSR.WDCE and
WDTCSR.WDE). A logic one must be written to WDTCSR.WDE regardless of the previous value of
the WDTCSR.WDE.
2. Within the next four clock cycles, write the WDTCSR.WDE and Watchdog prescaler bits group
(WDTCSR.WDP) as desired, but with the WDTCSR.WDCE cleared. This must be done in one
operation.
The following examples show a function for turning off the Watchdog Timer. The
examples assume that interrupts are controlled (e.g. by disabling interrupts globally) so
that no interrupts will occur during the execution of these functions.
Assembly Code Example
WDT_off:
; Turn off global interrupt
cli
; Reset Watchdog Timer
wdr
; Clear WDRF in MCUSR
in r16, MCUSR
andi r16, (0xff & (0<<WDRF))
out MCUSR, r16
; Write '1' to WDCE and WDE
; Keep old prescaler setting to prevent unintentional time-out
lds r16, WDTCSR
ori r16, (1<<WDCE) | (1<<WDE)
sts WDTCSR, r16
; Turn off WDT
ldi r16, (0<<WDE)
sts WDTCSR, r16
; Turn on global interrupt
sei
ret
C Code Example
void WDT_off(void)
{
__disable_interrupt();
__watchdog_reset();
/* Clear WDRF in MCUSR */
MCUSR &= ~(1<<WDRF);
/* Write logical one to WDCE and WDE */
/* Keep old prescaler setting to prevent unintentional time-out */
WDTCSR |= (1<<WDCE) | (1<<WDE);
/* Turn off WDT */
WDTCSR = 0x00;
__enable_interrupt();
}
Note: If the Watchdog is accidentally enabled, for example by a runaway pointer or
brown-out condition, the device will be reset and the Watchdog Timer will stay enabled. If
the code is not set up to handle the Watchdog, this might lead to an eternal loop of time-
out resets. To avoid this situation, the application software should always clear the
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Watchdog System Reset Flag (WDRF) and the WDE control bit in the initialization
routine, even if the Watchdog is not in use.
The following code examples shows how to change the time-out value of the Watchdog
Timer.
Assembly Code Example
WDT_Prescaler_Change:
; Turn off global interrupt
cli
; Reset Watchdog Timer
wdr
; Start timed sequence
lds r16, WDTCSR
ori r16, (1<<WDCE) | (1<<WDE)
sts WDTCSR, r16
; -- Got four cycles to set the new values from here -
; Set new prescaler(time-out) value = 64K cycles (~0.5 s)
ldi r16, (1<<WDE) | (1<<WDP2) | (1<<WDP0)
sts WDTCSR, r16
; -- Finished setting new values, used 2 cycles -
; Turn on global interrupt
sei
ret
C Code Example
void WDT_Prescaler_Change(void)
{
__disable_interrupt();
__watchdog_reset();
/* Start timed sequence */
WDTCSR |= (1<<WDCE) | (1<<WDE);
/* Set new prescaler(time-out) value = 64K cycles (~0.5 s) */
WDTCSR = (1<<WDE) | (1<<WDP2) | (1<<WDP0);
__enable_interrupt();
}
Note: The Watchdog Timer should be reset before any change of the WDTCSR.WDP
bits, since a change in the WDTCSR.WDP bits can result in a time out when switching to
a shorter time-out period.
15.9 Register Description
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© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 80
15.9.1 MCU Status Register
Name: MCUSR
Offset: 0x54 [ID-000004d0]
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x34
To make use of the Reset flags to identify a reset condition, the user should read and then Reset the
MCUSR as early as possible in the program. If the register is cleared before another reset occurs, the
source of the reset can be found by examining the Reset Flags.
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
WDRF BORF EXTRF PORF
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 3 – WDRF Watchdog System Reset Flag
This bit is set if a Watchdog system Reset occurs. The bit is reset by a Power-on Reset, or by writing a '0'
to it.
Bit 2 – BORF Brown-out Reset Flag
This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a '0' to it.
Bit 1 – EXTRF External Reset Flag
This bit is set if an external Reset occurs. The bit is reset by a Power-on Reset, or by writing a '0' to it.
Bit 0 – PORF Power-on Reset Flag
This bit is set if a Power-on Reset occurs. The bit is reset only by writing a '0' to it.
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15.9.2 WDTCSR – Watchdog Timer Control Register
Name: WDTCSR
Offset: 0x60 [ID-000004d0]
Reset: 0x00
Bit 7 6 5 4 3 2 1 0
WDIF WDIE WDP[3] WDCE WDE WDP[2:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – WDIF Watchdog Interrupt Flag
This bit is set when a time out occurs in the Watchdog Timer and the Watchdog Timer is configured for
interrupt. WDIF is cleared by hardware when executing the corresponding interrupt handling vector.
Alternatively, WDIF is cleared by writing a '1' to it. When the I-bit in SREG and WDIE are set, the
Watchdog Timeout Interrupt is executed.
Bit 6 – WDIE Watchdog Interrupt Enable
When this bit is written to '1' and the I-bit in the Status register is set, the Watchdog Interrupt is enabled. If
WDE is cleared in combination with this setting, the Watchdog Timer is in Interrupt mode, and the
corresponding interrupt is executed if timeout in the Watchdog Timer occurs. If WDE is set, the Watchdog
Timer is in Interrupt and System Reset mode. The first timeout in the Watchdog Timer will set WDIF.
Executing the corresponding interrupt vector will clear WDIE and WDIF automatically by hardware (the
Watchdog goes to System Reset mode).
This is useful for keeping the Watchdog Timer security while using the interrupt. To stay in Interrupt and
System Reset mode, WDIE must be set after each interrupt. This should not be done within the interrupt
service routine itself, as this might compromise the safety function of the Watchdog System Reset mode.
If the interrupt is not executed before the next timeout, a System Reset will be applied.
Table 15-1. Watchdog Timer Configuration
WDTON(1) WDE WDIE Mode Action on Time-out
1 0 0 Stopped None
1 0 1 Interrupt mode Interrupt
1 1 0 System Reset mode Reset
1 1 1 Interrupt and System Reset mode Interrupt, then go to System Reset mode
0 x x System Reset mode Reset
Note: 1. WDTON Fuse set to '0' means programmed and '1' means unprogrammed.
Bit 5 – WDP[3] Watchdog Timer Prescaler 3
Bit 4 – WDCE Watchdog Change Enable
This bit is used in timed sequences for changing WDE and prescaler bits. To clear the WDE bit, and/or
change the prescaler bits, WDCE must be set. Once written to '1', hardware will clear WDCE after four
clock cycles. Refer to Overview in section Watchdog Timer for information on how to use WDCE.
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Bit 3 – WDE Watchdog System Reset Enable
WDE is overridden by WDRF in MCUSR. This means that WDE is always set when WDRF is set. To
clear WDE, WDRF must be cleared first. This feature ensures multiple resets during conditions causing
failure, and a safe start-up after the failure.
Bits 2:0 – WDP[2:0] Watchdog Timer Prescaler 2, 1, and 0
The WDP[3:0] bits determine the Watchdog Timer prescaling when the Watchdog Timer is running. The
different prescaling values and their corresponding time out periods are shown in the following table.
Table 15-2. Watchdog Timer Prescale Select
WDP[3] WDP[2] WDP[1] WDP[0] Number of WDT Oscillator (Cycles) Oscillator
0 0 0 0 2K (2048) 16 ms
0 0 0 1 4K (4096) 32 ms
0 0 1 0 8K (8192) 64 ms
0 0 1 1 16K (16384) 0.125s
0 1 0 0 32K (32768) 0.25s
0 1 0 1 64K (65536) 0.5s
0 1 1 0 128K (131072) 1.0s
0 1 1 1 256K (262144) 2.0s
1 0 0 0 512K (524288) 4.0s
1 0 0 1 1024K (1048576) 8.0s
1 0 1 0 Reserved
1 0 1 1
1100
1101
1110
1111
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16. Interrupts
This section describes the specifics of the interrupt handling of the device. For a general explanation of
the AVR interrupt handling, refer to the description of Reset and Interrupt Handling.
• Each interrupt vector occupies two instruction words .
• Reset vector is affected by the BOOTRST fuse, and the interrupt vector start address is affected by
the IVSEL bit in MCUCR
16.1 Interrupt Vectors in ATmega328/P
Table 16-1. Reset and Interrupt Vectors in ATmega328/P
Vector No Program Address(2) Source Interrupts definition
1 0x0000(1) RESET External Pin, Power-on Reset, Brown-out Reset and Watchdog System
Reset
2 0x0002 INT0 External Interrupt Request 0
3 0x0004 INT1 External Interrupt Request 1
4 0x0006 PCINT0 Pin Change Interrupt Request 0
5 0x0008 PCINT1 Pin Change Interrupt Request 1
6 0x000A PCINT2 Pin Change Interrupt Request 2
7 0x000C WDT Watchdog Time-out Interrupt
8 0x000E TIMER2_COMPA Timer/Counter2 Compare Match A
9 0x0010 TIMER2_COMPB Timer/Coutner2 Compare Match B
10 0x0012 TIMER2_OVF Timer/Counter2 Overflow
11 0x0014 TIMER1_CAPT Timer/Counter1 Capture Event
12 0x0016 TIMER1_COMPA Timer/Counter1 Compare Match A
13 0x0018 TIMER1_COMPB Timer/Coutner1 Compare Match B
14 0x001A TIMER1_OVF Timer/Counter1 Overflow
15 0x001C TIMER0_COMPA Timer/Counter0 Compare Match A
16 0x001E TIMER0_COMPB Timer/Coutner0 Compare Match B
17 0x0020 TIMER0_OVF Timer/Counter0 Overflow
18 0x0022 SPI STC SPI Serial Transfer Complete
19 0x0024 USART_RX USART Rx Complete
20 0x0026 USART_UDRE USART Data Register Empty
21 0x0028 USART_TX USART Tx Complete
22 0x002A ADC ADC Conversion Complete
23 0x002C EE READY EEPROM Ready
24 0x002E ANALOG COMP Analog Comparator
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Vector No Program Address(2) Source Interrupts definition
25 0x0030 TWI 2-wire Serial Interface (I2C)
26 0x0032 SPM READY Store Program Memory Ready
Note:
1. When the BOOTRST fuse is programmed, the device will jump to the boot loader address at Reset,
see “Boot Loader Support – Read-While-Write Self- Programming”
2. When the IVSEL bit in MCUCR is set, Interrupt Vectors will be moved to the start of the boot Flash
section. The address of each Interrupt Vector will then be the address in this table added to the
start address of the boot Flash section.
The table below shows reset and Interrupt Vectors placement for the various combinations of BOOTRST
and IVSEL settings. If the program never enables an interrupt source, the Interrupt Vectors are not used,
and regular program code can be placed at these locations. This is the case if the Reset vector is in the
application section while the interrupt vectors are in the boot section or vice versa.
Table 16-2. Reset and Interrupt Vectors Placement
BOOTRST(1) IVSEL Reset Address Interrupt Vectors Start Address
1 0 0x000 0x002
1 1 0x000 Boot Reset Address + 0x0002
0 0 Boot Reset Address 0x002
0 1 Boot Reset Address Boot Reset Address + 0x0002
Note: 1. For the BOOTRST fuse “1” means unprogrammed while “0” means programmed.
The most typical and general program setup for the Reset and Interrupt Vector addresses is:
Address Labels Code Comments
0x0000 jmp RESET ; Reset
0x0002 jmp INT0 ; IRQ0
0x0004 jmp INT1 ; IRQ1
0x0006 jmp PCINT0 ; PCINT0
0x0008 jmp PCINT1 ; PCINT1
0x000A jmp PCINT2 ; PCINT2
0x000C jmp WDT ; Watchdog Timeout
0x000E jmp TIM2_COMPA ; Timer2 CompareA
0x0010 jmp TIM2_COMPB ; Timer2 CompareB
0x0012 jmp TIM2_OVF ; Timer2 Overflow
0x0014 jmp TIM1_CAPT ; Timer1 Capture
0x0016 jmp TIM1_COMPA ; Timer1 CompareA
0x0018 jmp TIM1_COMPB ; Timer1 CompareB
0x001A jmp TIM1_OVF ; Timer1 Overflow
0x001C jmp TIM0_COMPA ; Timer0 CompareA
0x001E jmp TIM0_COMPB ; Timer0 CompareB
0x0020 jmp TIM0_OVF ; Timer0 Overflow
0x0022 jmp SPI_STC ; SPI Transfer Complete
0x0024 jmp USART_RXC ; USART RX Complete
0x0026 jmp USART_UDRE ; USART UDR Empty
0x0028 jmp USART_TXC ; USART TX Complete
0x002A jmp ADC ; ADC Conversion Complete
0x002C jmp EE_RDY ; EEPROM Ready
0x002E jmp ANA_COMP ; Analog Comparator
0x0030 jmp TWI ; 2-wire Serial
0x0032 jmp SPM_RDY ; SPM Ready
;
0x0034 RESET: ldi r16,high(RAMEND) ; Main program start
0x0035 out SPH,r16 ; Set Stack Pointer to top of RAM
0x0036 ldi r16,low(RAMEND)
0x0037 out SPL,r16
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0x0038 sei ; Enable interrupts
0x0039 <instr> xxx
... ... ... ...
When the BOOTRST fuse is unprogrammed, the Boot section size set to 2K bytes and the
MCUCR.IVSEL is set before any interrupts are enabled, the most typical and general program setup for
the Reset and Interrupt Vector addresses is:
Address Labels Code Comments
0x0000 RESET: ldi r16,high(RAMEND) ; Main program start
0x0001 out SPH,r16 ; Set Stack Pointer to top of RAM
0x0002 ldi r16,low(RAMEND)
0x0003 out SPL,r16
0x0004 sei ; Enable interrupts
0x0005 <instr> xxx
;
.org 0x3C02
0x3C02 jmp EXT_INT0 ; IRQ0 Handler
0x3C04 jmp EXT_INT1 ; IRQ1 Handler
... ... ... ;
0x3C32 jmp SPM_RDY ; SPM Ready Handler
When the BOOTRST fuse is programmed and the boot section size set to 2K bytes, the most typical and
general program setup for the Reset and Interrupt Vector addresses is:
Address Labels Code Comments
.org 0x0002
0x0002 jmp EXT_INT0 ; IRQ0 Handler
0x0004 jmp EXT_INT1 ; IRQ1 Handler
... ... ... ;
0x0032 jmp SPM_RDY ; SPM Ready Handler
;
.org 0x3C00
0x3C00 RESET: ldi r16,high(RAMEND) ; Main program start
0x3C01 out SPH,r16 ; Set Stack Pointer to top of RAM
0x3C02 ldi r16,low(RAMEND)
0x3C03 out SPL,r16
0x3C04 sei ; Enable interrupts
0x3C05 <instr> xxx
When the BOOTRST fuse is programmed, the boot section size set to 2K bytes and the MCUCR.IVSEL
is set before any interrupts are enabled, the most typical and general program setup for the Reset and
Interrupt Vector addresses is:
Address Labels Code Comments
;
.org 0x3C00
0x3C00 jmp RESET ; Reset handler
0x3C02 jmp EXT_INT0 ; IRQ0 Handler
0x3C04 jmp EXT_INT1 ; IRQ1 Handler
... ... ... ;
0x3C32 jmp SPM_RDY ; SPM Ready Handler
;
0x3C34 RESET: ldi r16,high(RAMEND) ; Main program start
0x3C35 out SPH,r16 ; Set Stack Pointer to top of RAM
0x3C36 ldi r16,low(RAMEND)
0x3C37 out SPL,r16
0x3C38 sei ; Enable interrupts
0x3C39 <instr> xxx
16.2 Register Description
16.2.1 Moving Interrupts Between Application and Boot Space
The MCU Control register controls the placement of the interrupt vector table.
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16.2.2 MCU Control Register
Name: MCUCR
Offset: 0x55
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x35
The MCU Control register controls the placement of the interrupt vector table in order to move interrupts
between application and boot space.
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
BODS BODSE PUD IVSEL IVCE
Access R/W R/W R/W R/W R/W
Reset 0 0 0 0 0
Bit 6 – BODS BOD Sleep
The BODS bit must be written to '1' in order to turn off BOD during sleep. Writing to the BODS bit is
controlled by a timed sequence and the enable bit BODSE. To disable BOD in relevant sleep modes, both
BODS and BODSE must first be written to '1'. Then, BODS must be written to '1' and BODSE must be
written to zero within four clock cycles.
The BODS bit is active three clock cycles after it is set. A sleep instruction must be executed while BODS
is active in order to turn off the BOD for the actual sleep mode. The BODS bit is automatically cleared
after three clock cycles.
Note: BOD disable is only available for ATmega328P.
Bit 5 – BODSE BOD Sleep Enable
BODSE enables setting of BODS control bit, as explained in BODS bit description. BOD disable is
controlled by a timed sequence.
Note: BOD disable is only available for ATmega328P.
Bit 4 – PUD Pull-up Disable
When this bit is written to one, the pull ups in the I/O ports are disabled even if the DDxn and PORTxn
registers are configured to enable the pull ups ({DDxn, PORTxn} = 0b01).
Bit 1 – IVSEL Interrupt Vector Select
When the IVSEL bit is cleared (zero), the interrupt vectors are placed at the start of the Flash memory.
When this bit is set (one), the interrupt vectors are moved to the beginning of the boot loader section of
the Flash. The actual address of the start of the boot Flash section is determined by the BOOTSZ fuses.
To avoid unintentional changes of interrupt vector tables, a special write procedure must be followed to
change the IVSEL bit:
1. Write the Interrupt Vector Change Enable (IVCE) bit to one.
2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.
Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled in the
same cycle as IVCE is written, and interrupts remain disabled until after the instruction following the write
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to IVSEL. If IVSEL is not written, interrupts remain disabled for four cycles. The I-bit in the Status register
is unaffected by the automatic disabling.
Note: If interrupt vectors are placed in the boot loader section and Boot Lock bit BLB02 is programmed,
interrupts are disabled while executing from the application section. If interrupt vectors are placed in the
application section and Boot Lock bit BLB12 is programmed, interrupts are disabled while executing from
the boot loader section.
Bit 0 – IVCE Interrupt Vector Change Enable
The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by hardware
four cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable interrupts, as
explained in the IVSEL description above. See the code example below.
Assembly Code Example
Move_interrupts:
; Get MCUCR
in r16, MCUCR
mov r17, r16
; Enable change of Interrupt Vectors
ori r16, (1<<IVCE)
out MCUCR, r16
; Move interrupts to Boot Flash section
ori r17, (1<<IVSEL)
out MCUCR, r17
ret
C Code Example
void Move_interrupts(void)
{
uchar temp;
/* GET MCUCR*/
temp = MCUCR;
/* Enable change of Interrupt Vectors */
MCUCR = temp|(1<<IVCE);
/* Move interrupts to Boot Flash section */
MCUCR = temp|(1<<IVSEL);
}
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17. EXTINT - External Interrupts
The external interrupts are triggered by the INT pins or any of the PCINT pins. Observe that, if enabled,
the interrupts will trigger even if the INT or PCINT pins are configured as outputs. This feature provides a
way of generating a software interrupt.
The Pin Change Interrupt Request 2 (PCI2) will trigger if any enabled PCINT[23:16] pin toggles. The Pin
Change Interrupt Request 1 (PCI1) will trigger if any enabled PCINT[14:8] pin toggles. The Pin Change
Interrupt Request 0 (PCI0) will trigger if any enabled PCINT[7:0] pin toggles. The PCMSK2, PCMSK1 and
PCMSK0 registers control which pins contribute to the pin change interrupts. Pin change interrupts on
PCINT are detected asynchronously. This implies that these interrupts can be used for waking the part
from sleep modes other than Idle mode.
The external interrupts can be triggered by a falling or rising edge or a low level. This is set up as
indicated in the specification for the External Interrupt Control Register A (EICRA). When the external
interrupts are enabled and are configured as level-triggered, the interrupts will trigger as long as the pin is
held low. Note that recognition of falling or rising edge interrupts on INT requires the presence of an I/O
clock. Low level interrupt on INT is detected asynchronously. This implies that this interrupt can be used
for waking the part from sleep modes other than Idle mode. The I/O clock is halted in all sleep modes
except Idle mode.
Note: If a level triggered interrupt is used for wake-up from power-down, the required level must be held
long enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears
before the end of the start-up time, the MCU will still wake up, but no interrupt will be generated. The
start-up time is defined by the SUT and CKSEL Fuses.
Related Links
System Control and Reset
Clock Systems and Their Distribution
System Clock and Clock Options
17.1 Pin Change Interrupt Timing
An example of timing of a pin change interrupt is shown in the following figure.
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Figure 17-1. Timing of Pin Change Interrupts
LE
D Q D Q
clk
PCINT[i]
pin
PCINT[i] bit
(of PCMSKn)
D Q D Q D Q
clk
pcint_sync pcint_setflag
0
7
PCIFn
(interrupt
flag)
PCINT[i] pin
pin_lat
pin_sync
clk
pcint_in[i]
pcint_syn
pcint_setflag
PCIFn
pin_lat pin_sync pcint_in[i]
Related Links
System Control and Reset
Clock Systems and Their Distribution
System Clock and Clock Options
17.2 Register Description
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© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 90
17.2.1 External Interrupt Control Register A
Name: EICRA
Offset: 0x69
Reset: 0x00
Property: -
The External Interrupt Control Register A contains control bits for interrupt sense control.
Bit 7 6 5 4 3 2 1 0
ISC1[1:0] ISC0[1:0]
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bits 3:2 – ISC1[1:0] Interrupt Sense Control 1
The external interrupt 1 is activated by the external pin INT1 if the SREG I-flag and the corresponding
interrupt mask are set. The level and edges on the external INT1 pin that activates the interrupt are
defined in the table below. The value on the INT1 pin is sampled before detecting edges. If edge or toggle
interrupt is selected, pulses that last longer than one clock period will generate an interrupt. Shorter
pulses are not recommended to generate an interrupt. If the low-level interrupt is selected, the low level
must be held until the completion of the currently executing instruction to generate an interrupt.
Value Description
00 The low level of INT1 generates an interrupt request.
01 Any logical change on INT1 generates an interrupt request.
10 The falling edge of INT1 generates an interrupt request.
11 The rising edge of INT1 generates an interrupt request.
Bits 1:0 – ISC0[1:0] Interrupt Sense Control 0
The external interrupt 0 is activated by the external pin INT0 if the SREG I-flag and the corresponding
interrupt mask are set. The level and edges on the external INT0 pin that activates the interrupt are
defined in table below. The value on the INT0 pin is sampled before detecting edges. If edge or toggle
interrupt is selected, pulses that last longer than one clock period will generate an interrupt. Shorter
pulses are not recommended to generate an interrupt. If the low-level interrupt is selected, the low level
must be held until the completion of the currently executing instruction to generate an interrupt.
Value Description
00 The low level of INT0 generates an interrupt request.
01 Any logical change on INT0 generates an interrupt request.
10 The falling edge of INT0 generates an interrupt request.
11 The rising edge of INT0 generates an interrupt request.
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17.2.2 External Interrupt Mask Register
Name: EIMSK
Offset: 0x3D
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x1D
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
INT1 INT0
Access R/W R/W
Reset 0 0
Bit 1 – INT1 External Interrupt Request 1 Enable
When the INT1 bit is set and the I-bit in the Status Register (SREG) is set, the external pin interrupt is
enabled. The Interrupt Sense Control1 bits 1/0 (ISC11 and ISC10) in the External Interrupt Control
Register A (EICRA) define whether the external interrupt is activated on rising and/or falling edge of the
INT1 pin or level sensed. Activity on the pin will cause an interrupt request even if INT1 is configured as
an output. The corresponding interrupt of external interrupt request 1 is executed from the INT1 interrupt
vector.
Bit 0 – INT0 External Interrupt Request 0 Enable
When the INT0 bit is set and the I-bit in the Status Register (SREG) is set, the external pin interrupt is
enabled. The Interrupt Sense Control0 bits 1/0 (ISC01 and ISC00) in the External Interrupt Control
Register A (EICRA) define whether the external interrupt is activated on rising and/or falling edge of the
INT0 pin or level sensed. Activity on the pin will cause an interrupt request even if INT0 is configured as
an output. The corresponding interrupt of external interrupt request 0 is executed from the INT0 interrupt
vector.
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17.2.3 External Interrupt Flag Register
Name: EIFR
Offset: 0x3C
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x1C
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
INTF1 INTF0
Access R/W R/W
Reset 0 0
Bit 1 – INTF1 External Interrupt Flag 1
When an edge or logic change on the INT1 pin triggers an interrupt request, INTF1 will be set. If the I-bit
in SREG and the INT1 bit in EIMSK are set, the MCU will jump to the corresponding interrupt vector. The
flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing '1' to
it. This flag is always cleared when INT1 is configured as a level interrupt.
Bit 0 – INTF0 External Interrupt Flag 0
When an edge or logic change on the INT0 pin triggers an interrupt request, INTF0 will be set. If the I-bit
in SREG and the INT0 bit in EIMSK are set, the MCU will jump to the corresponding interrupt vector. The
flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing '1' to
it. This flag is always cleared when INT0 is configured as a level interrupt.
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17.2.4 Pin Change Interrupt Control Register
Name: PCICR
Offset: 0x68
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
PCIE2 PCIE1 PCIE0
Access R/W R/W R/W
Reset 0 0 0
Bit 2 – PCIE2 Pin Change Interrupt Enable 2
When the PCIE2 bit is set and the I-bit in the Status Register (SREG) is set, pin change interrupt 2 is
enabled. Any change on any enabled PCINT[23:16] pin will cause an interrupt. The corresponding
interrupt of pin change interrupt request is executed from the PCI2 Interrupt Vector. PCINT[23:16] pins
are enabled individually by the PCMSK2 register.
Bit 1 – PCIE1 Pin Change Interrupt Enable 1
When the PCIE1 bit is set and the I-bit in the Status Register (SREG) is set, pin change interrupt 1 is
enabled. Any change on any enabled PCINT[14:8] pin will cause an interrupt. The corresponding interrupt
of pin change interrupt request is executed from the PCI1 Interrupt Vector. PCINT[14:8] pins are enabled
individually by the PCMSK1 register.
Bit 0 – PCIE0 Pin Change Interrupt Enable 0
When the PCIE0 bit is set and the I-bit in the Status Register (SREG) is set, pin change interrupt 0 is
enabled. Any change on any enabled PCINT[7:0] pin will cause an interrupt. The corresponding interrupt
of pin change interrupt request is executed from the PCI0 Interrupt Vector. PCINT[7:0] pins are enabled
individually by the PCMSK0 register.
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17.2.5 Pin Change Interrupt Flag Register
Name: PCIFR
Offset: 0x3B
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x1B
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
PCIF2 PCIF1 PCIF0
Access R/W R/W R/W
Reset 0 0 0
Bit 2 – PCIF2 Pin Change Interrupt Flag 2
When a logic change on any PCINT[23:16] pin triggers an interrupt request, PCIF2 will be set. If the I-bit
in SREG and the PCIE2 bit in PCICR are set, the MCU will jump to the corresponding interrupt vector.
The flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing
'1' to it.
Bit 1 – PCIF1 Pin Change Interrupt Flag 1
When a logic change on any PCINT[14:8] pin triggers an interrupt request, PCIF1 will be set. If the I-bit in
SREG and the PCIE1 bit in PCICR are set, the MCU will jump to the corresponding interrupt vector. The
flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing '1' to
it.
Bit 0 – PCIF0 Pin Change Interrupt Flag 0
When a logic change on any PCINT[7:0] pin triggers an interrupt request, PCIF0 will be set. If the I-bit in
SREG and the PCIE0 bit in PCICR are set, the MCU will jump to the corresponding interrupt vector. The
flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing '1' to
it.
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17.2.6 Pin Change Mask Register 2
Name: PCMSK2
Offset: 0x6D
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
PCINT[23:16]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – PCINT[23:16] Pin Change Enable Mask
Each PCINT[23:16]-bit selects whether pin change interrupt is enabled on the corresponding I/O pin. If
PCINT[23:16] is set and the PCIE2 bit in PCICR is set, pin change interrupt is enabled on the
corresponding I/O pin. If PCINT[23:16] is cleared, pin change interrupt on the corresponding I/O pin is
disabled.
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17.2.7 Pin Change Mask Register 1
Name: PCMSK1
Offset: 0x6C
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
PCINT[14:8]
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bits 6:0 – PCINT[14:8] Pin Change Enable Mask
Each PCINT[15:8]-bit selects whether pin change interrupt is enabled on the corresponding I/O pin. If
PCINT[15:8] is set and the PCIE1 bit in PCICR is set, pin change interrupt is enabled on the
corresponding I/O pin. If PCINT[15:8] is cleared, pin change interrupt on the corresponding I/O pin is
disabled.
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17.2.8 Pin Change Mask Register 0
Name: PCMSK0
Offset: 0x6B
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
PCINT[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – PCINT[7:0] Pin Change Enable Mask
Each PCINT[7:0] bit selects whether pin change interrupt is enabled on the corresponding I/O pin. If
PCINT[7:0] is set and the PCIE0 bit in PCICR is set, pin change interrupt is enabled on the corresponding
I/O pin. If PCINT[7:0] is cleared, pin change interrupt on the corresponding I/O pin is disabled.
ATmega328/P
EXTINT - External Interrupts
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 98
18. I/O-Ports
18.1 Overview
All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports. This
means that the direction of one port pin can be changed without unintentionally changing the direction of
any other pin with the SBI and CBI instructions. The same applies when changing drive value (if
configured as an output) or enabling/disabling of pull-up resistors (if configured as an input). Each output
buffer has symmetrical drive characteristics with both high sink and source capability. The pin driver is
strong enough to drive LED displays directly. All port pins have individually selectable pull-up resistors
with a supply voltage invariant resistance. All I/O pins have protection diodes to both VCC and ground as
indicated in the following figure.
Figure 18-1. I/O Pin Equivalent Schematic
Cpin
Logic
Rpu
See Figure
"General Digital I/O" for
Details
Pxn
All registers and bit references in this section are written in general form. A lower case “x” represents the
numbering letter for the port, and a lower case “n” represents the bit number. However, when using the
register or bit defines in a program, the precise form must be used. For example, PORTB3 for bit number
3 in Port B, here documented generally as PORTxn.
I/O memory address locations are allocated for each port, one each for the Data Register (Portx), Data
Direction Register (DDRx), and the Port Input Pins (PINx). The port input pins I/O location is read-only,
while the data register and the data direction register are read/write. However, writing '1' to a bit in the
PINx register will result in a toggle in the corresponding bit in the data register. In addition, the Pull-up
Disable (PUD) bit in MCUCR disables the pull-up function for all pins in all ports when set.
Using the I/O port as general digital I/O is described in next section. Most port pins are multiplexed with
alternate functions for the peripheral features on the device. How each alternate function interferes with
the port pin is described in Alternate Port Functions section in this chapter. Refer to the individual module
sections for a full description of the alternate functions.
Enabling the alternate function of some of the port pins does not affect the use of the other pins in the
port as general digital I/O.
ATmega328/P
I/O-Ports
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 99
18.2 Ports as General Digital I/O
The ports are bi-directional I/O ports with optional internal pull-ups. The following figure shows the
functional description of one I/O-port pin, here generically called Pxn.
Figure 18-2. General Digital I/O(1)
clk
RPx
RRx
RDx
WDx
PUD
SYNCHRONIZER
WDx: WRITE DDRx
WRx: WRITE PORTx
RRx: READ PORTx REGISTER
RPx: READ PORTx PIN
PUD: PULLUP DISABLE
clkI/O: I/O CLOCK
RDx: READ DDRx
D
L
Q
Q
RESET
RESET
Q
QD
Q
Q D
CLR
PORTxn
Q
Q D
CLR
DDxn
PINxn
DATA BUS
SLEEP
SLEEP: SLEEP CONTROL
Pxn
I/O
WPx
0
1
WRx
WPx: WRITE PINx REGISTER
Note: 1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O,
SLEEP, and PUD are common to all ports.
18.2.1 Configuring the Pin
Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in the register
description, the DDxn bits are accessed at the DDRx I/O address, the PORTxn bits at the PORTx I/O
address, and the PINxn bits at the PINx I/O address.
The DDxn bit in the DDRx register selects the direction of this pin. If DDxn is written to '1', Pxn is
configured as an output pin. If DDxn is written to '0', Pxn is configured as an input pin.
If PORTxn is written to '1' when the pin is configured as an input pin, the pull-up resistor is activated. To
switch the pull-up resistor off, PORTxn has to be written to '0' or the pin has to be configured as an output
pin. The port pins are tri-stated when the reset condition becomes active, even if no clocks are running.
If PORTxn is written to '1' when the pin is configured as an output pin, the port pin is driven high. If
PORTxn is written logic zero when the pin is configured as an output pin, the port pin is driven low.
ATmega328/P
I/O-Ports
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 100
18.2.2 Toggling the Pin
Writing a '1' to PINxn toggles the value of PORTxn, independent on the value of DDRxn. The SBI
instruction can be used to toggle one single bit in a port.
18.2.3 Switching Between Input and Output
When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn} = 0b11),
an intermediate state with either pull-up enabled {DDxn, PORTxn} = 0b01) or output low ({DDxn,
PORTxn} = 0b10) must occur. Normally, the pull-up enabled state is fully acceptable, as a high-
impedance environment will not notice the difference between a strong high driver and a pull-up. If this is
not the case, the PUD bit in the MCUCR register can be set to disable all pull-ups in all ports.
Switching between input with pull-up and output low generates the same problem. The user must use
either the tri-state ({DDxn, PORTxn} = 0b00) or the output high state ({DDxn, PORTxn} = 0b11) as an
intermediate step.
The following table summarizes the control signals for the pin value.
Table 18-1. Port Pin Configurations
DDxn PORTxn PUD
(in MCUCR)
I/O Pull-up Comment
0 0 X Input No Tri-state (Hi-Z)
0 1 0 Input Yes Pxn will source current if ext. pulled low
0 1 1 Input No Tri-state (Hi-Z)
1 0 X Output No Output Low (Sink)
1 1 X Output No Output High (Source)
18.2.4 Reading the Pin Value
Independent of the setting of Data Direction bit DDxn, the port pin can be read through the PINxn register
bit. As shown in Ports as General Digital I/O, the PINxn register bit and the preceding latch constitute a
synchronizer. This is needed to avoid metastability if the physical pin changes value near the edge of the
internal clock, but it also introduces a delay. The following figure shows a timing diagram of the
synchronization when reading an externally applied pin value. The maximum and minimum propagation
delays are denoted tpd,max and tpd,min respectively.
Figure 18-3. Synchronization when Reading an Externally Applied Pin value
XXX in r17, PINx
0x00 0xFF
INSTRUCTIONS
SYNC LATCH
PINxn
r17
XXX
SYSTEM CLK
tpd, max
tpd, min
ATmega328/P
I/O-Ports
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 101
Consider the clock period starting shortly after the first falling edge of the system clock. The latch is
closed when the clock is low and goes transparent when the clock is high, as indicated by the shaded
region of the “SYNC LATCH” signal. The signal value is latched when the system clock goes low. It is
clocked into the PINxn register at the succeeding positive clock edge. As indicated by the two arrows
tpd,max and tpd,min, a single signal transition on the pin will be delayed between ½ and 1½ system clock
period depending upon the time of assertion.
When reading back a software-assigned pin value, a nop instruction must be inserted as indicated in the
following figure. The out instruction sets the “SYNC LATCH” signal at the positive edge of the clock. In
this case, the delay tpd through the synchronizer is one system clock period.
Figure 18-4. Synchronization when Reading a Software Assigned Pin Value
out PORTx, r16 nop in r17, PINx
0xFF
0x00 0xFF
SYSTEM CLK
r16
INSTRUCTIONS
SYNC LATCH
PINxn
r17
tpd
The following code example shows how to set port B pins 0 and 1 high, 2 and 3 low, and define the port
pins from 4 to 7 as input with pull-ups assigned to port pins 6 and 7. The resulting pin values are read
back again, but as previously discussed, a nop instruction is included to be able to read back the value
recently assigned to some of the pins.
Assembly Code Example(1)
...
; Define pull-ups and set outputs high
; Define directions for port pins
ldi r16,(1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0)
ldi r17,(1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0)
out PORTB,r16
out DDRB,r17
; Insert nop for synchronization
nop
; Read port pins
in r16,PINB
...
Note: 1. For the assembly program, two temporary registers are used to minimize the
time from pull-ups are set on pins 0, 1, 6, and 7, until the direction bits are correctly set,
defining bit 2 and 3 as low and redefining bits 0 and 1 as strong high drivers.
C Code Example
unsigned char i;
...
/* Define pull-ups and set outputs high */
/* Define directions for port pins */
PORTB = (1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0);
DDRB = (1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0);
ATmega328/P
I/O-Ports
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 102
/* Insert nop for synchronization*/
__no_operation();
/* Read port pins */
i = PINB;
...
18.2.5 Digital Input Enable and Sleep Modes
As shown in the figure of General Digital I/O, the digital input signal can be clamped to ground at the input
of the Schmitt Trigger. The signal denoted SLEEP in the figure, is set by the MCU sleep controller in
Power-Down mode and Standby mode to avoid high power consumption if some input signals are left
floating, or have an analog signal level close to VCC/2.
SLEEP is overridden for port pins enabled as external interrupt pins. If the external interrupt request is not
enabled, SLEEP is active for these pins. SLEEP is also overridden by various other alternate functions as
described in Alternate Port Functions section in this chapter.
If a logic high level is present on an asynchronous external interrupt pin configured as “Interrupt on Rising
Edge, Falling Edge, or Any Logic Change on Pin” while the external interrupt is not enabled, the
corresponding external interrupt flag will be set when resuming from the above mentioned Sleep mode,
as the clamping in these sleep mode produces the requested logic change.
18.2.6 Unconnected Pins
If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though
most of the digital inputs are disabled in the deep sleep modes as described above, floating inputs should
be avoided to reduce current consumption in all other modes where the digital inputs are enabled (Reset,
Active mode and Idle mode).
The simplest method to ensure a defined level of an unused pin is to enable the internal pull-up. In this
case, the pull-up will be disabled during reset. If low power consumption during reset is important, it is
recommended to use an external pull-up or pull-down. Connecting unused pins directly to VCC or GND is
not recommended, since this may cause excessive currents if the pin is accidentally configured as an
output.
18.3 Alternate Port Functions
Most port pins have alternate functions in addition to being general digital I/Os. The following figure
shows how the port pin control signals from the simplified Figure 18-2 can be overridden by alternate
functions. The overriding signals may not be present in all port pins, but the figure serves as a generic
description applicable to all port pins in the AVR microcontroller family.
ATmega328/P
I/O-Ports
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 103
Figure 18-5. Alternate Port Functions(1)
clk
RPx
RRx
WRx
RDx
WDx
PUD
SYNCHRONIZER
WDx: WRITE DDRx
WRx: WRITE PORTx
RRx: READ PORTx REGISTER
RPx: READ PORTx PIN
PUD: PULLUP DISABLE
clkI/O: I/O CLOCK
RDx: READ DDRx
D
L
Q
Q
SET
CLR
0
1
0
1
0
1
DIxn
AIOxn
DIEOExn
PVOVxn
PVOExn
DDOVxn
DDOExn
PUOExn
PUOVxn
PUOExn: Pxn PULL-UP OVERRIDE ENABLE
PUOVxn: Pxn PULL-UP OVERRIDE VALUE
DDOExn: Pxn DATA DIRECTION OVERRIDE ENABLE
DDOVxn: Pxn DATA DIRECTION OVERRIDE VALUE
PVOExn: Pxn PORT VALUE OVERRIDE ENABLE
PVOVxn: Pxn PORT VALUE OVERRIDE VALUE
DIxn: DIGITAL INPUT PIN n ON PORTx
AIOxn: ANALOG INPUT/OUTPUT PIN n ON PORTx
RESET
RESET
Q
Q D
CLR
Q
Q D
CLR
Q
QD
CLR
PINxn
PORTxn
DDxn
DATA BUS
0
1
DIEOVxn
SLEEP
DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE
DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE
SLEEP: SLEEP CONTROL
Pxn
I/O
0
1
WPx: WRITE PINx
WPx
Note: 1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O,
SLEEP, and PUD are common to all ports. All other signals are unique for each pin.
The following table summarizes the function of the overriding signals. The pin and port indexes from the
previous figure are not shown in the succeeding tables. The overriding signals are generated internally in
the modules having the alternate function.
ATmega328/P
I/O-Ports
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 104
Table 18-2. Generic Description of Overriding Signals for Alternate Functions
Signal
Name
Full Name Description
PUOE Pull-up Override
Enable
If this signal is set, the pull-up enable is controlled by the PUOV signal. If this
signal is cleared, the pull-up is enabled when {DDxn, PORTxn, PUD} = 0b010.
PUOV Pull-up Override
Value
If PUOE is set, the pull-up is enabled/disabled when PUOV is set/cleared,
regardless of the setting of the DDxn, PORTxn, and PUD Register bits.
DDOE Data Direction
Override Enable
If this signal is set, the Output Driver Enable is controlled by the DDOV signal. If
this signal is cleared, the Output driver is enabled by the DDxn Register bit.
DDOV Data Direction
Override Value
If DDOE is set, the Output Driver is enabled/disabled when DDOV is set/cleared,
regardless of the setting of the DDxn Register bit.
PVOE Port Value
Override Enable
If this signal is set and the Output Driver is enabled, the port value is controlled
by the PVOV signal. If PVOE is cleared, and the Output Driver is enabled, the
port Value is controlled by the PORTxn Register bit.
PVOV Port Value
Override Value
If PVOE is set, the port value is set to PVOV, regardless of the setting of the
PORTxn Register bit.
DIEOE Digital Input
Enable Override
Enable
If this bit is set, the Digital Input Enable is controlled by the DIEOV signal. If this
signal is cleared, the Digital Input Enable is determined by MCU state (Normal
mode, sleep mode).
DIEOV Digital Input
Enable Override
Value
If DIEOE is set, the Digital Input is enabled/disabled when DIEOV is set/cleared,
regardless of the MCU state (Normal mode, sleep mode).
DI Digital Input This is the Digital Input to alternate functions. In the figure, the signal is
connected to the output of the Schmitt Trigger but before the synchronizer.
Unless the Digital Input is used as a clock source, the module with the alternate
function will use its own synchronizer.
AIO Analog Input/
Output
This is the Analog Input/output to/from alternate functions. The signal is
connected directly to the pad and can be used bi-directionally.
The following subsections shortly describe the alternate functions for each port and relate the overriding
signals to the alternate function. Refer to the alternate function description for further details.
18.3.1 Alternate Functions of Port B
The Port B pins with alternate functions are shown in the table below:
Table 18-3. Port B Pins Alternate Functions
Port Pin Alternate Functions
PB7 XTAL2 (Chip Clock Oscillator pin 2)
TOSC2 (Timer Oscillator pin 2)
ATmega328/P
I/O-Ports
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Port Pin Alternate Functions
PCINT7 (Pin Change Interrupt 7)
PB6 XTAL1 (Chip Clock Oscillator pin 1 or External clock input)
TOSC1 (Timer Oscillator pin 1)
PCINT6 (Pin Change Interrupt 6)
PB5 SCK (SPI Bus Master clock Input)
PCINT5 (Pin Change Interrupt 5)
PB4 MISO (SPI Bus Master Input/Slave Output)
PCINT4 (Pin Change Interrupt 4)
PB3 MOSI (SPI Bus Master Output/Slave Input)
OC2A (Timer/Counter2 Output Compare Match A Output)
PCINT3 (Pin Change Interrupt 3)
PB2 SS (SPI Bus Master Slave select)
OC1B (Timer/Counter1 Output Compare Match B Output)
PCINT2 (Pin Change Interrupt 2)
PB1 OC1A (Timer/Counter1 Output Compare Match A Output)
PCINT1 (Pin Change Interrupt 1)
PB0 ICP1 (Timer/Counter1 Input Capture Input)
CLKO (Divided System Clock Output)
PCINT0 (Pin Change Interrupt 0)
The alternate pin configuration is as follows:
• XTAL2/TOSC2/PCINT7 – Port B, Bit 7
– XTAL2: Chip clock oscillator pin 2. Used as clock pin for crystal oscillator or low-frequency
crystal oscillator. When used as a clock pin, the pin can not be used as an I/O pin.
ATmega328/P
I/O-Ports
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 106
– TOSC2: Timer Oscillator pin 2. Used only if internal calibrated RC oscillator is selected as
chip clock source, and the asynchronous timer is enabled by the correct setting in ASSR.
When the AS2 bit in ASSR is set (one) and the EXCLK bit is cleared (zero) to enable
asynchronous clocking of Timer/Counter2 using the crystal oscillator, pin PB7 is disconnected
from the port, and becomes the inverting output of the oscillator amplifier. In this mode, a
crystal oscillator is connected to this pin, and the pin cannot be used as an I/O pin.
– PCINT7: Pin Change Interrupt source 7. The PB7 pin can serve as an external interrupt
source.
If PB7 is used as a clock pin, DDB7, PORTB7 and PINB7 will all read 0.
• XTAL1/TOSC1/PCINT6 – Port B, Bit 6
– XTAL1: Chip clock oscillator pin 1. Used for all chip clock sources except internal calibrated
RC oscillator. When used as a clock pin, the pin can not be used as an I/O pin.
– TOSC1: Timer Oscillator pin 1. Used only if internal calibrated RC oscillator is selected as
chip clock source, and the asynchronous timer is enabled by the correct setting in ASSR.
When the AS2 bit in ASSR is set (one) to enable asynchronous clocking of Timer/Counter2,
pin PB6 is disconnected from the port, and becomes the input of the inverting oscillator
amplifier. In this mode, a crystal oscillator is connected to this pin, and the pin can not be
used as an I/O pin.
– PCINT6: Pin Change Interrupt source 6. The PB6 pin can serve as an external interrupt
source.
If PB6 is used as a clock pin, DDB6, PORTB6 and PINB6 will all read 0.
• SCK/PCINT5 – Port B, Bit 5
– SCK: Master clock output, slave clock input pin for SPI channel. When the SPI is enabled as
a slave, this pin is configured as an input regardless of the setting of DDB5. When the SPI is
enabled as a master, the data direction of this pin is controlled by DDB5. When the pin is
forced by the SPI to be an input, the pull-up can still be controlled by the PORTB5 bit.
– PCINT5: Pin Change Interrupt source 5. The PB5 pin can serve as an external interrupt
source.
• MISO/PCINT4 – Port B, Bit 4
– MISO: Master data input, slave data output pin for SPI channel. When the SPI is enabled as a
master, this pin is configured as an input regardless of the setting of DDB4. When the SPI is
enabled as a slave, the data direction of this pin is controlled by DDB4. When the pin is forced
by the SPI to be an input, the pull-up can still be controlled by the PORTB4 bit.
– PCINT4: Pin Change Interrupt source 4. The PB4 pin can serve as an external interrupt
source.
• MOSI/OC2A/PCINT3 – Port B, Bit 3
– MOSI: SPI Master data output, slave data input for SPI channel. When the SPI is enabled as
a slave, this pin is configured as an input regardless of the setting of DDB3. When the SPI is
enabled as a master, the data direction of this pin is controlled by DDB3. When the pin is
forced by the SPI to be an input, the pull-up can still be controlled by the PORTB3 bit.
– OC2A: Output Compare Match output. The PB3 pin can serve as an external output for the
Timer/Counter2 Compare Match A. The PB3 pin has to be configured as an output (DDB3 set
'1') to serve this function. The OC2A pin is also the output pin for the PWM mode timer
function.
ATmega328/P
I/O-Ports
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 107
– PCINT3: Pin Change Interrupt source 3. The PB3 pin can serve as an external interrupt
source.
• SS/OC1B/PCINT2 – Port B, Bit 2
– SS: Slave Select input. When the SPI is enabled as a slave, this pin is configured as an input
regardless of the setting of DDB2. As slave, the SPI is activated when this pin is driven low.
When the SPI is enabled as a master, the data direction of this pin is controlled by DDB2.
When the pin is forced by the SPI to be an input, the pull-up can still be controlled by the
PORTB2 bit.
– OC1B: Output Compare Match output. The PB2 pin can serve as an external output for the
Timer/Counter1 Compare Match B. The PB2 pin has to be configured as an output (DDB2 set
(one)) to serve this function. The OC1B pin is also the output pin for the PWM mode timer
function.
– PCINT2: Pin Change Interrupt source 2. The PB2 pin can serve as an external interrupt
source.
• OC1A/PCINT1 – Port B, Bit 1
– OC1A: Output Compare Match output. The PB1 pin can serve as an external output for the
Timer/Counter1 Compare Match A. The PB1 pin has to be configured as an output (DDB1 set
(one)) to serve this function. The OC1A pin is also the output pin for the PWM mode timer
function.
– PCINT1: Pin Change Interrupt source 1. The PB1 pin can serve as an external interrupt
source.
• ICP1/CLKO/PCINT0 – Port B, Bit 0
– ICP1: Input Capture Pin. The PB0 pin can act as an Input Capture Pin for Timer/Counter1.
– CLKO: Divided System Clock. The divided system clock can be output on the PB0 pin. The
divided system clock will be output if the CKOUT Fuse is programmed, regardless of the
PORTB0 and DDB0 settings. It will also be output during reset.
– PCINT0: Pin Change Interrupt source 0. The PB0 pin can serve as an external interrupt
source.
The tables below relate the alternate functions of Port B to the overriding signals shown in Figure 18-5.
SPI MSTR INPUT and SPI SLAVE OUTPUT constitute the MISO signal, while MOSI is divided into SPI
MSTR OUTPUT and SPI SLAVE INPUT.
Table 18-4. Overriding Signals for Alternate Functions in PB7...PB4
Signal
Name
PB7/XTAL2/TOSC2/PCINT7(1) PB6/XTAL1/TOSC1/
PCINT6(1)
PB5/SCK/PCINT5 PB4/MISO/PCINT4
PUOE INTRC • EXTCK+ AS2 INTRC + AS2 SPE • MSTR SPE • MSTR
PUOV 0 0 PORTB5 • PUD PORTB4 • PUD
DDOE INTRC • EXTCK+ AS2 INTRC + AS2 SPE • MSTR SPE • MSTR
DDOV 0 0 0 0
PVOE 0 0 SPE • MSTR SPE • MSTR
PVOV 0 0 SCK OUTPUT SPI SLAVE
OUTPUT
ATmega328/P
I/O-Ports
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 108
Signal
Name
PB7/XTAL2/TOSC2/PCINT7(1) PB6/XTAL1/TOSC1/
PCINT6(1)
PB5/SCK/PCINT5 PB4/MISO/PCINT4
DIEOE INTRC • EXTCK + AS2 + PCINT7
• PCIE0
INTRC + AS2 + PCINT6 •
PCIE0
PCINT5 • PCIE0 PCINT4 • PCIE0
DIEOV (INTRC + EXTCK) • AS2 INTRC • AS2 1 1
DI PCINT7 INPUT PCINT6 INPUT PCINT5 INPUT
SCK INPUT
PCINT4 INPUT
SPI MSTR INPUT
AIO Oscillator Output Oscillator/Clock Input – –
Notes: 1. INTRC means that one of the internal RC oscillators are selected (by the CKSEL fuses),
EXTCK means that external clock is selected (by the CKSEL fuses).
Table 18-5. Overriding Signals for Alternate Functions in PB3...PB0
Signal
Name
PB3/MOSI/TXD1/OC2A/PCINT3 PB2/SS/OC1B/PCINT2 PB1/OC1A/PCINT1 PB0/ICP1/CLKO/
PCINT0
PUOE SPE • MSTR + TXEN1 SPE • MSTR 0 0
PUOV PORTB3 • PUD PORTB2 • PUD 0 0
DDOE SPE • MSTR + TXEN1 SPE • MSTR 0 0
DDOV 0 0 0 0
PVOE SPE • MSTR + OC2A ENABLE OC1B ENABLE OC1A ENABLE 0
PVOV SPI MSTR OUTPUT + OC2A +
TXD1
OC1B OC1A 0
DIEOE PCINT3 • PCIE0 PCINT2 • PCIE0 PCINT1 • PCIE0 PCINT0 • PCIE0
DIEOV 1 1 1 1
DI PCINT3 INPUT
SPI SLAVE INPUT
PCINT2 INPUT
SPI SS
PCINT1 INPUT PCINT0 INPUT
ICP1 INPUT
AIO – – – –
18.3.2 Alternate Functions of Port C
The Port C pins with alternate functions are shown in the table below:
ATmega328/P
I/O-Ports
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 109
Table 18-6. Port C Pins Alternate Functions
Port Pin Alternate Function
PC6 RESET (Reset pin)
PCINT14 (Pin Change Interrupt 14)
PC5 ADC5 (ADC Input Channel 5)
SCL (2-wire Serial Bus Clock Line)
PCINT13 (Pin Change Interrupt 13)
PC4 ADC4 (ADC Input Channel 4)
SDA (2-wire Serial Bus Data Input/Output Line)
PCINT12 (Pin Change Interrupt 12)
PC3 ADC3 (ADC Input Channel 3)
PCINT11 (Pin Change Interrupt 11)
PC2 ADC2 (ADC Input Channel 2)
PCINT10 (Pin Change Interrupt 10)
PC1 ADC1 (ADC Input Channel 1)
PCINT9 (Pin Change Interrupt 9)
PC0 ADC0 (ADC Input Channel 0)
PCINT8 (Pin Change Interrupt 8)
The alternate pin configuration is as follows:
• RESET/PCINT14 – Port C, Bit 6
– RESET: Reset pin. When the RSTDISBL Fuse is programmed, this pin functions as a normal
I/O pin, and the part will have to rely on Power-on Reset and Brown-out Reset as its reset
sources. When the RSTDISBL Fuse is unprogrammed, the reset circuitry is connected to the
pin, and the pin can not be used as an I/O pin.
– PCINT14: Pin Change Interrupt source 14. The PC6 pin can serve as an external interrupt
source.
If PC6 is used as a reset pin, DDC6, PORTC6 and PINC6 will all read 0.
• SCL/ADC5/PCINT13 – Port C, Bit 5
– SCL: 2-wire Serial Interface Clock. When the TWEN bit in TWCR is set (one) to enable the 2-
wire Serial Interface, pin PC5 is disconnected from the port and becomes the Serial Clock I/O
pin for the 2-wire Serial Interface. In this mode, there is a spike filter on the pin to suppress
spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver
with slew-rate limitation.
– PCINT13: Pin Change Interrupt source 13. The PC5 pin can serve as an external interrupt
source.
– PC5 can also be used as ADC input Channel 5. The ADC input channel 5 uses digital power.
• SDA/ADC4/PCINT12 – Port C, Bit 4
– SDA: 2-wire Serial Interface Data. When the TWEN bit in TWCR is set (one) to enable the 2-
wire Serial Interface, pin PC4 is disconnected from the port and becomes the Serial Data I/O
pin for the 2-wire Serial Interface. In this mode, there is a spike filter on the pin to suppress
spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver
with slew-rate limitation.
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– PCINT12: Pin Change Interrupt source 12. The PC4 pin can serve as an external interrupt
source.
– PC4 can also be used as ADC input Channel 4. The ADC input channel 4 uses digital power.
• ADC3/PCINT11 – Port C, Bit 3
– PC3 can also be used as ADC input Channel 3. The ADC input channel 3 uses analog power.
– PCINT11: Pin Change Interrupt source 11. The PC3 pin can serve as an external interrupt
source.
• ADC2/PCINT10 – Port C, Bit 2
– PC2 can also be used as ADC input Channel 2. The ADC input channel 2 uses analog power.
– PCINT10: Pin Change Interrupt source 10. The PC2 pin can serve as an external interrupt
source.
• ADC1/PCINT9 – Port C, Bit 1
– PC1 can also be used as ADC input Channel 1. The ADC input channel 1 uses analog power.
– PCINT9: Pin Change Interrupt source 9. The PC1 pin can serve as an external interrupt
source.
• ADC0//CINT8 – Port C, Bit 0
– PC0 can also be used as ADC input Channel 0. The ADC input channel 0 uses analog power.
– PCINT8: Pin Change Interrupt source 8. The PC0 pin can serve as an external interrupt
source.
The tables below relate the alternate functions of Port C to the overriding signals shown in Figure 18-5.
Table 18-7. Overriding Signals for Alternate Functions in PC6...PC4(1)
Signal
Name
PC6/RESET/PCINT14 PC5/SCL/ADC5/PCINT13 PC4/SDA/ADC4/PCINT12
PUOE RSTDISBL TWEN TWEN
PUOV 1 PORTC5 • PUD PORTC4 • PUD
DDOE RSTDISBL TWEN TWEN
DDOV 0 SCL_OUT SDA_OUT
PVOE 0 TWEN TWEN
PVOV 0 0 0
DIEOE RSTDISBL + PCINT14 •
PCIE1
PCINT13 • PCIE1 + ADC5D PCINT12 • PCIE1 + ADC4D
DIEOV RSTDISBL PCINT13 • PCIE1 PCINT12 • PCIE1
DI PCINT14 INPUT PCINT13 INPUT PCINT12 INPUT
AIO RESET INPUT ADC5 INPUT / SCL INPUT ADC4 INPUT / SDA INPUT
Note: 1. When enabled, the 2-wire Serial Interface enables slew-rate controls on the output pins PC4
and PC5. This is not shown in the figure. In addition, spike filters are connected between the AIO outputs
shown in the port figure and the digital logic of the TWI module.
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Table 18-8. Overriding Signals for Alternate Functions in PC3...PC0
Signal
Name
PC3/ADC3/
PCINT11
PC2/ADC2/
PCINT10
PC1/ADC1/
PCINT9
PC0/ADC0/
PCINT8
PUOE 0 0 0 0
PUOV 0 0 0 0
DDOE 0 0 0 0
DDOV 0 0 0 0
PVOE 0 0 0 0
PVOV 0 0 0 0
DIEOE PCINT11 • PCIE1 +
ADC3D
PCINT10 • PCIE1 +
ADC2D
PCINT9 • PCIE1 +
ADC1D
PCINT8 • PCIE1 +
ADC0D
DIEOV PCINT11 • PCIE1 PCINT10 • PCIE1 PCINT9 • PCIE1 PCINT8 • PCIE1
DI PCINT11 INPUT PCINT10 INPUT PCINT9 INPUT PCINT8 INPUT
AIO ADC3 INPUT ADC2 INPUT ADC1 INPUT ADC0 INPUT
18.3.3 Alternate Functions of Port D
The Port D pins with alternate functions are shown in the table below:
Table 18-9. Port D Pins Alternate Functions
Port Pin Alternate Function
PD7 AIN1 (Analog Comparator Negative Input)
PCINT23 (Pin Change Interrupt 23)
PD6 AIN0 (Analog Comparator Positive Input)
OC0A (Timer/Counter0 Output Compare Match A Output)
PCINT22 (Pin Change Interrupt 22)
PD5 T1 (Timer/Counter 1 External Counter Input)
OC0B (Timer/Counter0 Output Compare Match B Output)
PCINT21 (Pin Change Interrupt 21)
PD4 XCK (USART External Clock Input/Output)
T0 (Timer/Counter 0 External Counter Input)
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Port Pin Alternate Function
PCINT20 (Pin Change Interrupt 20)
PD3 INT1 (External Interrupt 1 Input)
OC2B (Timer/Counter2 Output Compare Match B Output)
PCINT19 (Pin Change Interrupt 19)
PD2 INT0 (External Interrupt 0 Input)
PCINT18 (Pin Change Interrupt 18)
PD1 TXD (USART Output Pin)
PCINT17 (Pin Change Interrupt 17)
PD0 RXD (USART Input Pin)
PCINT16 (Pin Change Interrupt 16)
The alternate pin configuration is as follows:
• AIN1/OC2B/PCINT23 – Port D, Bit 7
– AIN1: Analog Comparator1 Negative Input. Configure the port pin as input with the internal
pull-up switched off to avoid the digital port function from interfering with the function of the
Analog Comparator.
– PCINT23: Pin Change Interrupt source 23. The PD7 pin can serve as an external interrupt
source.
• AIN0/OC0A/PCINT22 – Port D, Bit 6
– AIN0: Analog Comparator0 Positive Input. Configure the port pin as input with the internal
pull-up switched off to avoid the digital port function from interfering with the function of the
Analog Comparator.
– OC0A: Output Compare Match output. The PD6 pin can serve as an external output for the
Timer/Counter0 Compare Match A. The PD6 pin has to be configured as an output (DDD6 set
(one)) to serve this function. The OC0A pin is also the output pin for the PWM mode timer
function.
– PCINT22: Pin Change Interrupt source 22. The PD6 pin can serve as an external interrupt
source.
• T1/OC0B/PCINT21 – Port D, Bit 5
– T1: Timer/Counter1 counter source.
– OC0B: Output Compare Match output. The PD5 pin can serve as an external output for the
Timer/Counter0 Compare Match B. The PD5 pin has to be configured as an output (DDD5 set
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(one)) to serve this function. The OC0B pin is also the output pin for the PWM mode timer
function.
– PCINT21: Pin Change Interrupt source 21. The PD5 pin can serve as an external interrupt
source.
• XCK/T0/PCINT20 – Port D, Bit 4
– XCK: USART external clock.
– T0: Timer/Counter0 counter source.
– PCINT20: Pin Change Interrupt source 20. The PD4 pin can serve as an external interrupt
source.
• INT1/OC2B/PCINT19 – Port D, Bit 3
– INT1: External Interrupt source 1. The PD3 pin can serve as an external interrupt source.
– OC2B: Output Compare Match output: The PD3 pin can serve as an external output for the
Timer/Counter2 Compare Match B. The PD3 pin has to be configured as an output (DDD3 set
(one)) to serve this function. The OC2B pin is also the output pin for the PWM mode timer
function.
– PCINT19: Pin Change Interrupt source 19. The PD3 pin can serve as an external interrupt
source.
• INT0/PCINT18 – Port D, Bit 2
– INT0: External Interrupt source 0. The PD2 pin can serve as an external interrupt source.
– PCINT18: Pin Change Interrupt source 18. The PD2 pin can serve as an external interrupt
source.
• TXD/PCINT17 – Port D, Bit 1
– TXD: Transmit Data (Data output pin for the USART). When the USART Transmitter is
enabled, this pin is configured as an output regardless of the value of DDD1.
– PCINT17: Pin Change Interrupt source 17. The PD1 pin can serve as an external interrupt
source.
• RXD/PCINT16 – Port D, Bit 0
– RXD: Receive Data (Data input pin for the USART). When the USART Receiver is enabled
this pin is configured as an input regardless of the value of DDD0. When the USART forces
this pin to be an input, the pull-up can still be controlled by the PORTD0 bit.
– PCINT16: Pin Change Interrupt source 16. The PD0 pin can serve as an external interrupt
source.
The tables below relate the alternate functions of Port D to the overriding signals shown in Figure 18-5.
Table 18-10. Overriding Signals for Alternate Functions PD7...PD4
Signal
Name
PD7/AIN1
/PCINT23
PD6/AIN0/
OC0A/PCINT22
PD5/T1/OC0B/
PCINT21
PD4/XCK/
T0/PCINT20
PUOE 0 0 0 0
PUO 0 0 0 0
DDOE 0 0 0 0
DDOV 0 0 0 0
PVOE 0 OC0A ENABLE OC0B ENABLE UMSEL
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Signal
Name
PD7/AIN1
/PCINT23
PD6/AIN0/
OC0A/PCINT22
PD5/T1/OC0B/
PCINT21
PD4/XCK/
T0/PCINT20
PVOV 0 OC0A OC0B XCK OUTPUT
DIEOE PCINT23 • PCIE2 PCINT22 • PCIE2 PCINT21 • PCIE2 PCINT20 • PCIE2
DIEOV 1 1 1 1
DI PCINT23 INPUT PCINT22 INPUT PCINT21 INPUT
/ T1 INPUT
PCINT20 INPUT
/ XCK INPUT
/ T0 INPUT
AIO AIN1 INPUT AIN0 INPUT – –
Table 18-11. Overriding Signals for Alternate Functions in PD3...PD0
Signal
Name
PD3/OC2B/INT1/
PCINT19
PD2/INT0/
PCINT18
PD1/TXD/
PCINT17
PD0/RXD/
PCINT16
PUOE 0 0 TXEN RXEN
PUO 0 0 0 PORTD0 • PUD
DDOE 0 0 TXEN RXEN
DDOV 0 0 1 0
PVOE OC2B ENABLE 0 TXEN 0
PVOV OC2B 0 TXD 0
DIEOE INT1 ENABLE + PCINT19 •
PCIE2
INT0 ENABLE + PCINT18 •
PCIE1
PCINT17 • PCIE2 PCINT16 • PCIE2
DIEOV 1 1 1 1
DI PCINT19 INPUT
/ INT1 INPUT
PCINT18 INPUT
/ INT0 INPUT
PCINT17 INPUT PCINT16 INPUT
/ RXD
AIO – – – –
18.4 Register Description
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18.4.1 MCU Control Register
Name: MCUCR
Offset: 0x55
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x35
The MCU Control register controls the placement of the interrupt vector table in order to move interrupts
between application and boot space.
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
BODS BODSE PUD IVSEL IVCE
Access R/W R/W R/W R/W R/W
Reset 0 0 0 0 0
Bit 6 – BODS BOD Sleep
The BODS bit must be written to '1' in order to turn off BOD during sleep. Writing to the BODS bit is
controlled by a timed sequence and the enable bit BODSE. To disable BOD in relevant sleep modes, both
BODS and BODSE must first be written to '1'. Then, BODS must be written to '1' and BODSE must be
written to zero within four clock cycles.
The BODS bit is active three clock cycles after it is set. A sleep instruction must be executed while BODS
is active in order to turn off the BOD for the actual sleep mode. The BODS bit is automatically cleared
after three clock cycles.
Note: BOD disable is only available for ATmega328P.
Bit 5 – BODSE BOD Sleep Enable
BODSE enables setting of BODS control bit, as explained in BODS bit description. BOD disable is
controlled by a timed sequence.
Note: BOD disable is only available for ATmega328P.
Bit 4 – PUD Pull-up Disable
When this bit is written to one, the pull ups in the I/O ports are disabled even if the DDxn and PORTxn
registers are configured to enable the pull ups ({DDxn, PORTxn} = 0b01).
Bit 1 – IVSEL Interrupt Vector Select
When the IVSEL bit is cleared (zero), the interrupt vectors are placed at the start of the Flash memory.
When this bit is set (one), the interrupt vectors are moved to the beginning of the boot loader section of
the Flash. The actual address of the start of the boot Flash section is determined by the BOOTSZ fuses.
To avoid unintentional changes of interrupt vector tables, a special write procedure must be followed to
change the IVSEL bit:
1. Write the Interrupt Vector Change Enable (IVCE) bit to one.
2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.
Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled in the
same cycle as IVCE is written, and interrupts remain disabled until after the instruction following the write
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to IVSEL. If IVSEL is not written, interrupts remain disabled for four cycles. The I-bit in the Status register
is unaffected by the automatic disabling.
Note: If interrupt vectors are placed in the boot loader section and Boot Lock bit BLB02 is programmed,
interrupts are disabled while executing from the application section. If interrupt vectors are placed in the
application section and Boot Lock bit BLB12 is programmed, interrupts are disabled while executing from
the boot loader section.
Bit 0 – IVCE Interrupt Vector Change Enable
The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by hardware
four cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable interrupts, as
explained in the IVSEL description above. See the code example below.
Assembly Code Example
Move_interrupts:
; Get MCUCR
in r16, MCUCR
mov r17, r16
; Enable change of Interrupt Vectors
ori r16, (1<<IVCE)
out MCUCR, r16
; Move interrupts to Boot Flash section
ori r17, (1<<IVSEL)
out MCUCR, r17
ret
C Code Example
void Move_interrupts(void)
{
uchar temp;
/* GET MCUCR*/
temp = MCUCR;
/* Enable change of Interrupt Vectors */
MCUCR = temp|(1<<IVCE);
/* Move interrupts to Boot Flash section */
MCUCR = temp|(1<<IVSEL);
}
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18.4.2 Port B Data Register
Name: PORTB
Offset: 0x25
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x05
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 0, 1, 2, 3, 4, 5, 6, 7 – PORTB Port B Data
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18.4.3 Port B Data Direction Register
Name: DDRB
Offset: 0x24
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x04
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
DDRB7 DDRB6 DDRB5 DDRB4 DDRB3 DDRB2 DDRB1 DDRB0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 0, 1, 2, 3, 4, 5, 6, 7 – DDRB Port B Data Direction
This bit field selects the data direction for the individual pins in the Port. When a Port is mapped as
virtual, accessing this bit field is identical to accessing the actual DIR register for the Port.
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18.4.4 Port B Input Pins Address
Name: PINB
Offset: 0x23
Reset: N/A
Property: When addressing as I/O Register: address offset is 0x03
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x x
Bits 0, 1, 2, 3, 4, 5, 6, 7 – PINB Port B Input Pins Address
Writing to the pin register provides toggle functionality for I/O. Refer to Toggling the Pin.
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18.4.5 Port C Data Register
Name: PORTC
Offset: 0x28
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x08
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
PORTC6 PORTC5 PORTC4 PORTC3 PORTC2 PORTC1 PORTC0
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bits 0, 1, 2, 3, 4, 5, 6 – PORTC Port C Data
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18.4.6 Port C Data Direction Register
Name: DDRC
Offset: 0x27
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x07
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
DDRC6 DDRC5 DDRC4 DDRC3 DDRC2 DDRC1 DDRC0
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bits 0, 1, 2, 3, 4, 5, 6 – DDRC Port C Data Direction
This bit field selects the data direction for the individual pins in the Port. When a Port is mapped as
virtual, accessing this bit field is identical to accessing the actual DIR register for the Port.
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18.4.7 Port C Input Pins Address
Name: PINC
Offset: 0x26
Reset: N/A
Property: When addressing as I/O Register: address offset is 0x06
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
PINC6 PINC5 PINC4 PINC3 PINC2 PINC1 PINC0
Access R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x
Bits 0, 1, 2, 3, 4, 5, 6 – PINC Port C Input Pins Address
Writing to the pin register provides toggle functionality for I/O. Refer to Toggling the Pin.
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18.4.8 Port D Data Register
Name: PORTD
Offset: 0x2B
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x0B
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 0, 1, 2, 3, 4, 5, 6, 7 – PORTD Port D Data
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18.4.9 Port D Data Direction Register
Name: DDRD
Offset: 0x2A
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x0A
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
DDRD7 DDRD6 DDRD5 DDRD4 DDRD3 DDRD2 DDRD1 DDRD0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 0, 1, 2, 3, 4, 5, 6, 7 – DDRD Port D Data Direction
This bit field selects the data direction for the individual pins in the Port. When a Port is mapped as
virtual, accessing this bit field is identical to accessing the actual DIR register for the Port.
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18.4.10 Port D Input Pins Address
Name: PIND
Offset: 0x29
Reset: N/A
Property: When addressing as I/O Register: address offset is 0x09
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x x
Bits 0, 1, 2, 3, 4, 5, 6, 7 – PIND Port D Input Pins Address
Writing to the pin register provides toggle functionality for I/O. Refer to Toggling the Pin.
ATmega328/P
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19. 8-bit Timer/Counter0 (TC0) with PWM
19.1 Features
• Two Independent Output Compare Units
• Double Buffered Output Compare Registers
• Clear Timer on Compare Match (Auto Reload)
• Glitch Free, Phase Correct Pulse Width Modulator (PWM)
• Variable PWM Period
• Frequency Generator
• Three Independent Interrupt Sources (TOV0, OCF0A, and OCF0B)
19.2 Overview
Timer/Counter0 (TC0) is a general purpose 8-bit timer/counter module, with two independent output
compare units, and PWM support. It allows accurate program execution timing (event management) and
wave generation.
A simplified block diagram of the 8-bit timer/counter is shown below. CPU accessible I/O registers,
including I/O bits and I/O pins, are shown in bold. The device specific I/O register and bit locations are
listed in the register description. For the actual placement of I/O pins, refer to the pinout diagram.
The TC0 is enabled by writing the PRTIM0 bit in ”Minimizing Power Consumption” to '0'.
The TC0 is enabled when the PRTIM0 bit in the Power Reduction Register (PRR.PRTIM0) is written to
'1'.
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 127
Figure 19-1. 8-bit Timer/Counter Block Diagram
Clock Select
Timer/Counter
DATA BUS
OCRnA
OCRnB
=
=
TCNTn
Waveform
Generation
Waveform
Generation
OCnA
OCnB
=
Fixed
TOP
Value
Control Logic
= 0
TOP BOTTOM
Count
Clear
Direction
TOVn
(Int.Req.)
OCnA
(Int.Req.)
OCnB
(Int.Req.)
TCCRnA TCCRnB
Tn
Edge
Detector
( From Prescaler )
clkTn
19.2.1 Definitions
Many register and bit references in this section are written in general form:
• n=0 represents the Timer/Counter number
• x=A,B represents the Output Compare Unit A or B
However, when using the register or bit definitions in a program, the precise form must be used, i.e.,
TCNT0 for accessing timer/counter0 counter value.
The following definitions are used throughout the section:
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Table 19-1. Definitions
Constant Description
BOTTOM The counter reaches the BOTTOM when it becomes zero (0x00 for 8-bit counters, or 0x0000
for 16-bit counters).
MAX The counter reaches its Maximum when it becomes 0xFF (decimal 255, for 8-bit counters) or
0xFFFF (decimal 65535, for 16-bit counters).
TOP The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be the fixed value MAX or the value stored in
the OCR0A Register. The assignment is dependent on the mode of operation.
19.2.2 Registers
The Timer/Counter 0 register (TCNT0) and Output Compare TC0x registers (OCR0x) are 8-bit registers.
Interrupt request (abbreviated to Int.Req. in the block diagram) signals are all visible in the Timer Interrupt
Flag Register 0 (TIFR0). All interrupts are individually masked with the Timer Interrupt Mask Register 0
(TIMSK0). TIFR0 and TIMSK0 are not shown in the figure.
The timer/counter (TC) can be clocked internally, via the prescaler, or by an external clock source on the
T0 pin. The clock select logic block controls which clock source and edge are used by the timer/counter
to increment (or decrement) its value. The TC is inactive when no clock source is selected. The output
from the clock select logic is referred to as the timer clock (clkT0).
The double buffered Output Compare Registers (OCR0A and OCR0B) are compared with the timer/
counter value at all times. The result of the compare can be used by the waveform generator to generate
a PWM or variable frequency output on the Output Compare pins (OC0A and OC0B). See Output
Compare Unit for details. The compare match event will also set the Compare Flag (OCF0A or OCF0B),
which can be used to generate an output compare interrupt request.
Related Links
Timer/Counter 0, 1 Prescalers
19.3 Timer/Counter Clock Sources
The TC can be clocked by an internal or an external clock source. The clock source is selected by writing
to the Clock Select (CS0[2:0]) bits in the Timer/Counter Control Register (TCCR0B).
19.4 Counter Unit
The main part of the 8-bit timer/counter is the programmable bi-directional counter unit. Below is the block
diagram of the counter and its surroundings.
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 129
Figure 19-2. Counter Unit Block Diagram
DATA BUS
TCNTn Control Logic
count
TOVn
(Int.Req.)
Clock Select
top
Tn
Edge
Detector
( From Prescaler )
clkTn
bottom
direction
clear
Note: The “n” in the register and bit names indicates the device number (n = 0 for timer/counter 0), and
the “x” indicates output compare unit (A/B).
Table 19-2. Signal Description (Internal Signals)
Signal Name Description
count Increment or decrement TCNT0 by 1.
direction Select between increment and decrement.
clear Clear TCNT0 (set all bits to zero).
clkTn Timer/counter clock, referred to as clkT0 in the following.
top Signalize that TCNT0 has reached maximum value.
bottom Signalize that TCNT0 has reached minimum value (zero).
Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each
timer clock (clkT0). clkT0 can be generated from an external or internal clock source, selected by the Clock
Select bits (CS0[2:0]). When no clock source is selected (CS0=0x0) the timer is stopped. However, the
TCNT0 value can be accessed by the CPU, regardless of whether clkT0 is present or not. A CPU write
overrides (has priority over) all counter clear or count operations.
The counting sequence is determined by the setting of the WGM01 and WGM00 bits located in the Timer/
Counter Control Register (TCCR0A) and the WGM02 bit located in the Timer/Counter Control Register B
(TCCR0B). There are close connections between how the counter behaves (counts) and how waveforms
are generated on the Output Compare outputs OC0A and OC0B. For more details about advanced
counting sequences and waveform generation, see Modes of Operation.
The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected by the
WGM0[2:0] bits. TOV0 can be used for generating a CPU interrupt.
19.5 Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Registers (OCR0A and
OCR0B). Whenever TCNT0 equals OCR0A or OCR0B, the comparator signals a match. A match will set
the Output Compare Flag (OCF0A or OCF0B) at the next timer clock cycle. If the corresponding interrupt
is enabled, the output compare flag generates an output compare interrupt. The output compare flag is
automatically cleared when the interrupt is executed. Alternatively, the flag can be cleared by software by
writing a '1' to its I/O bit location. The waveform generator uses the match signal to generate an output
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 130
according to operating mode set by the WGM02, WGM01, and WGM00 bits and Compare Output mode
(COM0x[1:0]) bits. The maximum and bottom signals are used by the waveform generator for handling
the special cases of the extreme values in some modes of operation.
Figure 19-3. Output Compare Unit, Block Diagram
OCFnx (Int.Req.)
=(8-bit Comparator )
OCRnx
OCnx
DATA BUS
TCNTn
WGMn[1:0]
Waveform Generator
top
FOCn
COMnx[1:0]
bottom
Note: The “n” in the register and bit names indicates the device number (n = 0 for Timer/Counter 0), and
the “x” indicates output compare unit (A/B).
The OCR0x registers are double buffered when using any of the Pulse Width Modulation (PWM) modes.
When double buffering is enabled, the CPU has access to the OCR0x Buffer register. The double
buffering synchronizes the update of the OCR0x Compare registers to either top or bottom of the counting
sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses,
thereby making the output glitch free.
The double buffering is disabled for the normal and Clear Timer on Compare (CTC) modes of operation,
and the CPU will access the OCR0x directly.
19.5.1 Force Output Compare
In non-PWM Waveform Generation modes, the match output of the comparator can be forced by writing a
'1' to the Force Output Compare (TCCR0C.FOCnx) bit. Forcing compare match will not set the OCFnx
flag or reload/clear the timer, but the OCnx pin will be updated as if a real compare match had occurred
(the TCCRnA.COMnx[1:0] bits define whether the OCnx pin is set, cleared or toggled).
19.5.2 Compare Match Blocking by TCNTn Write
All CPU write operations to the TCNTn register will block any compare match that occurs in the next timer
clock cycle, even when the timer is stopped. This feature allows OCRnx to be initialized to the same
value as TCNTn without triggering an interrupt when the timer/counter clock is enabled.
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 131
19.5.3 Using the Output Compare Unit
Since writing TCNTn in any mode of operation will block all compare matches for one timer clock cycle,
there are risks involved when changing TCNTn when using the output compare unit, independently of
whether the timer/counter is running or not. If the value written to TCNTn equals the OCRnx value, the
compare match will be missed, resulting in incorrect waveform generation. Similarly, do not write the
TCNTn1 value equal to BOTTOM when the counter is counting down.
The setup of the OCnx should be performed before setting the Data Direction register for the port pin to
output. The easiest way of setting the OCnx value is to use the Force Output Compare (FOCnx) strobe
bits in Normal mode. The OCnx registers keep their values even when changing between Waveform
Generation modes.
Be aware that the TCCRnA.COMnx[1:0] bits are not double-buffered together with the compare value.
Changing the TCCRnA.COMnx[1:0] bits will take effect immediately.
19.6 Compare Match Output Unit
The Compare Output mode bits in the Timer/Counter Control Register A (TCCR0A.COM0x) have two
functions:
• The waveform generator uses the COM0x bits for defining the Output Compare (OC0x) register
state at the next compare match.
• The COM0x bits control the OC0x pin output source
The figure below shows a simplified schematic of the logic affected by COM0x. The I/O registers, I/O bits,
and I/O pins in the figure are shown in bold. Only the parts of the general I/O port control registers that
are affected by the COM0x bits are shown, namely PORT and DDR.
On system reset the OC0x register is reset to 0x00.
Note: 'OC0x state' is always referring to internal OC0x registers, not the OC0x pin.
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 132
Figure 19-4. Compare Match Output Unit, Schematic
PORT
DDR
D Q
D Q
OCnx
Pin
OCnx
D Q
Waveform
Generator
COMnx[1]
COMnx[0]
0
1
DATA BUS
FOCnx
clkI/O
Note: The “n” in the register and bit names indicates the device number (n = 0 for Timer/Counter 0), and
the “x” indicates output compare unit (A/B).
The general I/O port function is overridden by the Output Compare (OC0x) from the waveform generator
if either of the COM0x[1:0] bits are set. However, the OC0x pin direction (input or output) is still controlled
by the Data Direction Register (DDR) for the port pin. In the DDR, the bit for the OC1x pin (DDR.OC0x)
must be set as output before the OC0x value is visible on the pin. The port override function is
independent of the Waveform Generation mode.
The design of the output compare pin logic allows initialization of the OC0x register state before the
output is enabled. Some TCCR0A.COM0x[1:0] bit settings are reserved for certain modes of operation.
The TCCR0A.COM0x[1:0] bits have no effect on the input capture unit.
Related Links
Register Description
19.6.1 Compare Output Mode and Waveform Generation
The waveform generator uses the TCCR0A.COM0x[1:0] bits differently in Normal, CTC, and PWM
modes. For all modes, setting the TCCR0A.COM0x[1:0]=0x0 tells the waveform generator that no action
on the OC0x register is to be performed on the next compare match. Refer to the descriptions of the
output modes.
A change of the TCCR0A.COM0x[1:0] bits state will have effect at the first compare match after the bits
are written. For non-PWM modes, the action can be forced to have immediate effect by using the
TCCR0C.FOC0x strobe bits.
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 133
19.7 Modes of Operation
The mode of operation determines the behavior of the timer/counter and the output compare pins. It is
defined by the combination of the Waveform Generation mode bits and Compare Output mode
(TCCR0A.WGM0[2:0]) bits in the Timer/Counter Control Registers A and B (TCCR0A.COM0x[1:0]). The
Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode
bits do. The COM0x[1:0] bits control whether the PWM output generated should be inverted or not
(inverted or non-inverted PWM). For non-PWM modes, the COM0x[1:0] bits control whether the output
should be set, cleared, or toggled at a compare match (see the previous section Compare Match Output
Unit).
For detailed timing information refer to the following section Timer/Counter Timing Diagrams.
Related Links
Compare Match Output Unit
Timer/Counter Timing Diagrams
19.7.1 Normal Mode
The simplest mode of operation is the Normal mode (WGM0[2:0] = 0x0). In this mode, the counting
direction is always up (incrementing), and no counter clear is performed. The counter simply overruns
when it passes its maximum 8-bit value (TOP=0xFF) and then restarts from the bottom (0x00). In Normal
mode operation, the Timer/Counter Overflow flag (TOV0) will be set in the same clock cycle in which the
TCNT0 becomes zero. In this case, the TOV0 flag behaves like a ninth bit, except that it is only set, not
cleared. However, combined with the timer overflow interrupt that automatically clears the TOV0 flag, the
timer resolution can be increased by software. There are no special cases to consider in the Normal
mode, a new counter value can be written any time.
The output compare unit can be used to generate interrupts at some given time. Using the output
compare to generate waveforms in Normal mode is not recommended since this will occupy too much of
the CPU time.
19.7.2 Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare (CTC) mode (WGM0[2:0]=0x2), the OCR0A register is used to manipulate the
counter resolution: the counter is cleared to ZERO when the counter value (TCNT0) matches the
OCR0A. The OCR0A defines the top value for the counter, hence its resolution. This mode allows greater
control of the compare match output frequency. It also simplifies the counting of external events.
The timing diagram for the CTC mode is shown below. The counter value (TCNT0) increases until a
compare match occurs between TCNT0 and OCR0A, and then counter (TCNT0) is cleared.
Figure 19-5. CTC Mode, Timing Diagram
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 134
An interrupt can be generated each time the counter value reaches the TOP value by setting the OCF0A
flag. If the interrupt is enabled, the interrupt handler routine can be used for updating the TOP value.
Note: Changing TOP to a value close to BOTTOM while the counter is running must be done with care,
since the CTC mode does not provide double buffering. If the new value written to OCR0A is lower than
the current value of TCNT0, the counter will miss the compare match. The counter will then count to its
maximum value (0xFF for an 8-bit counter, 0xFFFF for a 16-bit counter) and wrap around starting at 0x00
before the compare match will occur.
For generating a waveform output in CTC mode, the OC0A output can be set to toggle its logical level on
each compare match by writing the two least significant Compare Output mode bits in the Timer/Counter
Control Register A Control to toggle mode (TCCR0A.COM0A[1:0]=0x1). The OC0A value will only be
visible on the port pin unless the data direction for the pin is set to output. The waveform generated will
have a maximum frequency of fOC0 = fclk_I/O/2 when OCR0A is written to 0x00. The waveform frequency
is defined by the following equation:
OCnx =clk_I/O
2 1 + OCRnx
N represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the Timer/Counter Overflow flag TOV0 is set in the same clock
cycle that the counter wraps from MAX to 0x00.
19.7.3 Fast PWM Mode
The Fast Pulse Width Modulation or Fast PWM modes (WGM0[2:0]=0x3 or WGM0[2:0]=0x7) provide a
high-frequency PWM waveform generation option. The Fast PWM modes differ from the other PWM
options by their single-slope operation. The counter counts from BOTTOM to TOP and then restarts from
BOTTOM. TOP is defined as 0xFF when WGM0[2:0]=0x3. TOP is defined as OCR0A when
WGM0[2:0]=0x7.
In non-inverting Compare Output mode, the Output Compare register (OC0x) is cleared on the compare
match between TCNT0 and OCR0x, and set at BOTTOM. In inverting Compare Output mode, the output
is set on compare match and cleared at BOTTOM. Due to the single-slope operation, the operating
frequency of the Fast PWM mode can be twice as high as the phase correct PWM modes, which use
dual-slope operation. This high frequency makes the Fast PWM mode well suited for power regulation,
rectification, and DAC applications. High frequency allows physically small sized external components
(coils, capacitors), and therefore reduces total system cost.
In Fast PWM mode, the counter is incremented until the counter value matches the TOP value. The
counter is then cleared at the following timer clock cycle. The timing diagram for the Fast PWM mode is
shown below. The TCNT0 value is in the timing diagram shown as a histogram for illustrating the single-
slope operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal lines
on the TCNT0 slopes mark compare matches between OCR0x and TCNT0.
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 135
Figure 19-6. Fast PWM Mode, Timing Diagram
TCNTn
OCRnx Update and
TOVn Interrupt Flag Set
1
Period 2 3
OCnx
OCnx
(COMnx[1:0] = 0x2)
(COMnx[1:0] = 0x3)
OCRnx Interrupt Flag Set
4 5 6 7
The Timer/Counter Overflow flag (TOV0) is set each time the counter reaches TOP. If the interrupt is
enabled, the interrupt handler routine can be used for updating the compare value.
In Fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins. Writing
the TCCR0A.COM0x[1:0] bits to 0x2 will produce a non-inverted PWM; TCCR0A.COM0x[1:0]=0x3 will
produce an inverted PWM output. Writing the TCCR0A.COM0A[1:0] bits to 0x1 allows the OC0A pin to
toggle on compare matches if the TCCRnB.WGMn2 bit is set. This option is not available for the OC0B
pin. The actual OC0x value will only be visible on the port pin if the data direction for the port pin is set as
output. The PWM waveform is generated by setting (or clearing) the OC0x register at the compare match
between OCR0x and TCNT0, and clearing (or setting) the OC0x register at the timer clock cycle the
counter is cleared (changes from TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
OCnxPWM =clk_I/O
256
N represents the prescale divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A register represent special cases for PWM waveform output in the
Fast PWM mode: If OCR0A is written equal to BOTTOM, the output will be a narrow spike for each MAX
+1 timer clock cycle. Writing OCR0A=MAX will result in a constantly high or low output (depending on the
polarity of the output set by the COM0A[1:0] bits.)
A frequency waveform output with 50% duty cycle can be achieved in Fast PWM mode by selecting
OC0x to toggle its logical level on each compare match (COM0x[1:0]=0x1). The waveform generated will
have a maximum frequency of fOC0 = fclk_I/O/2 when OCR0A=0x00. This feature is similar to the OC0A
toggle in CTC mode, except double buffering of the output compare unit is enabled in the Fast PWM
mode.
19.7.4 Phase Correct PWM Mode
The Phase Correct PWM mode (WGM0[2:0]=0x1 or WGM0[2:0]=0x5) provides a high resolution, phase
correct PWM waveform generation. The Phase Correct PWM mode is based on dual-slope operation:
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 136
The counter counts repeatedly from BOTTOM to TOP, and then from TOP to BOTTOM. When
WGM0[2:0]=0x1 TOP is defined as 0xFF. When WGM0[2:0]=0x5, TOP is defined as OCR0A. In non-
inverting Compare Output mode, the Output Compare (OC0x) bit is cleared on compare match between
TCNT0 and OCR0x while up-counting and OC0x is set on the compare match while down-counting. In
inverting Output Compare mode, the operation is inverted. The dual-slope operation has a lower
maximum operation frequency than single-slope operation. Due to the symmetric feature of the dual-
slope PWM modes, these modes are preferred for motor control applications.
In Phase Correct PWM mode the counter is incremented until the counter value matches TOP. When the
counter reaches TOP, it changes the count direction. The TCNT0 value will be equal to TOP for one timer
clock cycle. The timing diagram for the Phase Correct PWM mode is shown below. The TCNT0 value is
shown as a histogram for illustrating the dual-slope operation. The diagram includes non-inverted and
inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent compare matches
between OCR0x and TCNT0.
Figure 19-7. Phase Correct PWM Mode, Timing Diagram
TOVn Interrupt Flag Set
OCnx Interrupt Flag Set
1 2 3
TCNTn
Period
OCnx
OCnx
(COMnx[1:0] = 2)
(COMnx[1:0] = 3)
OCRnx Update
Note: The “n” in the register and bit names indicates the device number (n = 0 for Timer/Counter 0), and
the “x” indicates Output Compare unit (A/B).
The Timer/Counter Overflow flag (TOV0) is set each time the counter reaches BOTTOM. The interrupt
flag can be used to generate an interrupt each time the counter reaches the BOTTOM value.
In Phase Correct PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pin.
Writing the COM0x[1:0] bits to 0x2 will produce a non-inverted PWM. An inverted PWM output can be
generated by writing COM0x[1:0]=0x3. Setting the Compare Match Output A Mode bit to '1'
(TCCR0A.COM0A0) allows the OC0A pin to toggle on Compare Matches if the TCCR0B.WGM02 bit is
set. This option is not available for the OC0B pin. The actual OC0x value will only be visible on the port
pin if the data direction for the port pin is set as output. The PWM waveform is generated by clearing (or
setting) the OC0x register at the compare match between OCR0x and TCNT0 when the counter
increments, and setting (or clearing) the OC0x register at compare match between OCR0x and TCNT0
when the counter decrements. The PWM frequency for the output when using Phase Correct PWM can
be calculated by:
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 137
OCnxPCPWM =clk_I/O
510
N represents the prescaler factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A register represent special cases when generating a PWM waveform
output in the Phase Correct PWM mode: If the OCR0A register is written equal to BOTTOM, the output
will be continuously low. If OCR0A is written to MAX, the output will be continuously high for non-inverted
PWM mode. For inverted PWM the output will have the opposite logic values.
At the very start of period 2 in the timing diagram above, OC0x has a transition from high to low even
though there is no compare match. This transition serves to guarantee symmetry around BOTTOM.
There are two cases that give a transition without Compare Match:
• OCR0x changes its value from MAX, as in the timing diagram. When the OCR0A value is MAX, the
OC0 pin value is the same as the result of a down-counting compare match. To ensure symmetry
around BOTTOM the OC0x value at MAX must correspond to the result of an up-counting compare
match.
• The timer starts up-counting from a value higher than the one in OCR0x, and for that reason
misses the compare match and consequently, the OC0x does not undergo the change that would
have happened on the way up.
19.8 Timer/Counter Timing Diagrams
The timer/counter is a synchronous design and the timer clock (clkT0) is therefore shown as a clock
enable signal in the following figures. If the given instance of the TC0 supports an Asynchronous mode,
clkI/O should be replaced by the TC oscillator clock.
The figures include information on when interrupt flags are set. The first figure below illustrates timing
data for basic timer/counter operation close to the MAX value in all modes other than phase correct PWM
mode.
Figure 19-8. Timer/Counter Timing Diagram, no Prescaling
clkTn
(clkI/O/1)
TOVn
clkI/O
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
Note: The “n” in the register and bit names indicates the device number (n = 0 for timer/counter 0), and
the “x” indicates output compare unit (A/B).
The next figure shows the same timing data, but with the prescaler enabled.
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 138
Figure 19-9. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
TOVn
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
clkI/O
clkTn
(clkI/O/8)
Note: The “n” in the register and bit names indicates the device number (n = 0 for timer/counter 0), and
the “x” indicates output compare unit (A/B).
The next figure shows the setting of OCF0B in all modes and OCF0A in all modes (except CTC mode
and PWM mode where OCR0A is TOP).
Figure 19-10. Timer/Counter Timing Diagram, Setting of OCF0x, with Prescaler (fclk_I/O/8)
OCFnx
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
clkI/O
clkTn
(clkI/O/8)
Note: The “n” in the register and bit names indicates the device number (n = 0 for timer/counter 0), and
the “x” indicates output compare unit (A/B).
The next figure shows the setting of OCF0A and the clearing of TCNT0 in CTC mode and fast PWM
mode where OCR0A is TOP.
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 139
Figure 19-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with
Prescaler (fclk_I/O/8)
OCFnx
OCRnx
TCNTn
(CTC)
TOP
TOP - 1 TOP BOTTOM BOTTOM + 1
clkI/O
clkTn
(clkI/O/8)
Note: The “n” in the register and bit names indicates the device number (n = 0 for timer/counter 0), and
the “x” indicates output compare unit (A/B).
19.9 Register Description
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 140
19.9.1 TC0 Control Register A
Name: TCCR0A
Offset: 0x44
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x24
Bit 7 6 5 4 3 2 1 0
COM0A[1:0] COM0B[1:0] WGM0[1:0]
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bits 7:6 – COM0A[1:0] Compare Output Mode for Channel A
These bits control the Output Compare pin (OC0A) behavior. If one or both of the COM0A[1:0] bits are
set, the OC0A output overrides the normal port functionality of the I/O pin it is connected to. However,
note that the Data Direction Register (DDR) bit corresponding to the OC0A pin must be set in order to
enable the output driver.
When OC0A is connected to the pin, the function of the COM0A[1:0] bits depends on the WGM0[2:0] bit
setting. The table below shows the COM0A[1:0] bit functionality when the WGM0[2:0] bits are set to a
normal or CTC mode (non-PWM).
Table 19-3. Compare Output Mode, Non-PWM
COM0A[1] COM0A[0] Description
0 0 Normal port operation, OC0A disconnected.
0 1 Toggle OC0A on compare match.
1 0 Clear OC0A on compare match.
1 1 Set OC0A on compare match.
The table below shows the COM0A[1:0] bit functionality when the WGM0[1:0] bits are set to fast PWM
mode.
Table 19-4. Compare Output Mode, Fast PWM(1)
COM0A[1] COM0A[0] Description
0 0 Normal port operation, OC0A disconnected.
0 1 WGM0[2:0]: Normal port operation, OC0A disconnected.
WGM0[2:1]: Toggle OC0A on compare match.
1 0 Clear OC0A on compare match, set OC0A at BOTTOM (Non-inverting mode).
1 1 Set OC0A on compare match, clear OC0A at BOTTOM (Inverting mode).
Note:
1. A special case occurs when OCR0A equals TOP and COM0A[1] is set. In this case the compare
match is ignored, but the set or clear is done at BOTTOM. Refer to Fast PWM Mode for details.
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 141
The table below shows the COM0A[1:0] bit functionality when the WGM0[2:0] bits are set to phase
correct PWM mode.
Table 19-5. Compare Output Mode, Phase Correct PWM Mode(1)
COM0A[1] COM0A[0] Description
0 0 Normal port operation, OC0A disconnected.
0 1 WGM0[2:0]: Normal port operation, OC0A disconnected.
WGM0[2:1]: Toggle OC0A on compare match.
1 0 Clear OC0A on compare match when up-counting. Set OC0A on compare
match when down-counting.
1 1 Set OC0A on compare match when up-counting. Clear OC0A on compare
match when down-counting.
Note:
1. A special case occurs when OCR0A equals TOP and COM0A[1] is set. In this case, the compare
match is ignored, but the set or clear is done at TOP. Refer to Phase Correct PWM Mode for
details.
Bits 5:4 – COM0B[1:0] Compare Output Mode for Channel B
These bits control the Output Compare pin (OC0B) behavior. If one or both of the COM0B[1:0] bits are
set, the OC0B output overrides the normal port functionality of the I/O pin it is connected to. However,
note that the Data Direction Register (DDR) bit corresponding to the OC0B pin must be set in order to
enable the output driver.
When OC0B is connected to the pin, the function of the COM0B[1:0] bits depends on the WGM0[2:0] bit
setting. The table shows the COM0B[1:0] bit functionality when the WGM0[2:0] bits are set to a normal or
CTC mode (non- PWM).
Table 19-6. Compare Output Mode, Non-PWM
COM0B[1] COM0B[0] Description
0 0 Normal port operation, OC0B disconnected.
0 1 Toggle OC0B on compare match.
1 0 Clear OC0B on compare match.
1 1 Set OC0B on compare match.
The table below shows the COM0B[1:0] bit functionality when the WGM0[2:0] bits are set to fast PWM
mode.
Table 19-7. Compare Output Mode, Fast PWM(1)
COM0B[1] COM0B[0] Description
0 0 Normal port operation, OC0B disconnected.
0 1 Reserved.
ATmega328/P
8-bit Timer/Counter0 (TC0) with PWM
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COM0B[1] COM0B[0] Description
1 0 Clear OC0B on compare match, set OC0B at BOTTOM, (Non-inverting mode).
1 1 Set OC0B on compare match, clear OC0B at BOTTOM, (Inverting mode).
Note:
1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the compare
match is ignored, but the set or clear is done at TOP. Refer to Fast PWM Mode for details.
The table below shows the COM0B[1:0] bit functionality when the WGM0[2:0] bits are set to phase
correct PWM mode.
Table 19-8. Compare Output Mode, Phase Correct PWM Mode(1)
COM0B[1] COM0B[0] Description
0 0 Normal port operation, OC0B disconnected.
0 1 Reserved.
1 0 Clear OC0B on compare match when up-counting. Set OC0B on compare
match when down-counting.
1 1 Set OC0B on compare match when up-counting. Clear OC0B on compare
match when down-counting.
Note:
1. A special case occurs when OCR0B equals TOP and COM0B[1] is set. In this case, the compare
match is ignored, but the set or clear is done at TOP. Refer to Phase Correct PWM Mode for
details.
Bits 1:0 – WGM0[1:0] Waveform Generation Mode
Combined with the WGM02 bit found in the TCCR0B register, these bits control the counting sequence of
the counter, the source for maximum (TOP) counter value, and what type of waveform generation to be
used. Modes of operation supported by the Timer/Counter unit are: Normal mode (counter), Clear Timer
on Compare Match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes (see Modes of
Operation).
Table 19-9. Waveform Generation Mode Bit Description
Mode WGM0[2] WGM0[1] WGM0[0] Timer/Counter Mode of Operation TOP Update of OCR0x at TOV Flag Set on(1)(2)
0 0 0 0 Normal 0xFF Immediate MAX
1 0 0 1 PWM, Phase Correct 0xFF TOP BOTTOM
2 0 1 0 CTC OCR0A Immediate MAX
3 0 1 1 Fast PWM 0xFF BOTTOM MAX
4 1 0 0 Reserved - - -
5 1 0 1 PWM, Phase Correct OCR0A TOP BOTTOM
6 1 1 0 Reserved - - -
7 1 1 1 Fast PWM OCR0A BOTTOM TOP
Note:
1. MAX = 0xFF
2. BOTTOM = 0x00
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8-bit Timer/Counter0 (TC0) with PWM
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19.9.2 TC0 Control Register B
Name: TCCR0B
Offset: 0x45
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x25
Bit 7 6 5 4 3 2 1 0
FOC0A FOC0B WGM02 CS0[2:0]
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bit 7 – FOC0A Force Output Compare A
The FOC0A bit is only active when the WGM bits specify a non-PWM mode.
To ensure compatibility with future devices, this bit must be set to zero when TCCR0B is written when
operating in PWM mode. When writing a logical one to the FOC0A bit, an immediate compare match is
forced on the waveform generation unit. The OC0A output is changed according to its COM0A[1:0] bits
setting. The FOC0A bit is implemented as a strobe. Therefore, it is the value present in the COM0A[1:0]
bits that determines the effect of the forced compare.
A FOC0A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0A as
TOP.
The FOC0A bit is always read as zero.
Bit 6 – FOC0B Force Output Compare B
The FOC0B bit is only active when the WGM bits specify a non-PWM mode.
To ensure compatibility with future devices, this bit must be set to zero when TCCR0B is written when
operating in PWM mode. When writing a logical one to the FOC0B bit, an immediate compare match is
forced on the waveform generation unit. The OC0B output is changed according to its COM0B[1:0] bits
setting. The FOC0B bit is implemented as a strobe. Therefore, it is the value present in the COM0B[1:0]
bits that determines the effect of the forced compare.
A FOC0B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0B as
TOP.
The FOC0B bit is always read as zero.
Bit 3 – WGM02 Waveform Generation Mode
Refer to TCCR0A register.
Bits 2:0 – CS0[2:0] Clock Select 0
The three clock select bits select the clock source to be used by the timer/counter.
Table 19-10. Clock Select Bit Description
CS0[2] CS0[1] CS0[0] Description
0 0 0 No clock source (timer/counter stopped)
0 0 1 clkI/O/1 (no prescaling)
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CS0[2] CS0[1] CS0[0] Description
0 1 0 clkI/O/8 (from prescaler)
0 1 1 clkI/O/64 (from prescaler)
1 0 0 clkI/O/256 (from prescaler)
1 0 1 clkI/O/1024 (from prescaler)
1 1 0 External clock source on T0 pin. Clock on falling edge.
1 1 1 External clock source on T0 pin. Clock on rising edge.
If external pin modes are used for the timer/counter0, transitions on the T0 pin will clock the counter even
if the pin is configured as an output. This feature allows software control of the counting.
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8-bit Timer/Counter0 (TC0) with PWM
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19.9.3 TC0 Interrupt Mask Register
Name: TIMSK0
Offset: 0x6E
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
OCIE0B OCIE0A TOIE0
Access R/W R/W R/W
Reset 0 0 0
Bit 2 – OCIE0B Timer/Counter0, Output Compare B Match Interrupt Enable
When the OCIE0B bit is written to one, and the I-bit in the Status register is set, the timer/counter
compare match B interrupt is enabled. The corresponding interrupt is executed if a compare match in
timer/counter occurs, i.e., when the OCF0B bit is set in TIFR0.
Bit 1 – OCIE0A Timer/Counter0, Output Compare A Match Interrupt Enable
When the OCIE0A bit is written to one, and the I-bit in the Status register is set, the timer/counter0
compare match A interrupt is enabled. The corresponding interrupt is executed if a compare match in
timer/counter0 occurs, i.e., when the OCF0A bit is set in TIFR0.
Bit 0 – TOIE0 Timer/Counter0, Overflow Interrupt Enable
When the TOIE0 bit is written to one, and the I-bit in the Status register is set, the timer/counter0 overflow
interrupt is enabled. The corresponding interrupt is executed if an overflow in timer/counter0 occurs, i.e.,
when the TOV0 bit is set in TIFR0.
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8-bit Timer/Counter0 (TC0) with PWM
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19.9.4 General Timer/Counter Control Register
Name: GTCCR
Offset: 0x43
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x23
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
TSM PSRASY PSRSYNC
Access R/W R/W R/W
Reset 0 0 0
Bit 7 – TSM Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the value
that is written to the PSRASY and PSRSYNC bits is kept, hence keeping the corresponding prescaler
Reset signals asserted. This ensures that the corresponding timer/counters are halted and can be
configured to the same value without the risk of one of them advancing during configuration. When the
TSM bit is written to zero, the PSRASY and PSRSYNC bits are cleared by hardware, and the timer/
counters start counting simultaneously.
Bit 1 – PSRASY Prescaler Reset Timer/Counter2
When this bit is one, the timer/counter2 prescaler will be reset. This bit is normally cleared immediately by
hardware. If the bit is written when timer/counter2 is operating in Asynchronous mode, the bit will remain
one until the prescaler has been Reset. The bit will not be cleared by hardware if the TSM bit is set.
Bit 0 – PSRSYNC Prescaler Reset
When this bit is one, timer/counter 0, 1 prescaler will be Reset. This bit is normally cleared immediately by
hardware, except if the TSM bit is set. Note that timer/counter 0, 1 share the same prescaler and a Reset
of this prescaler will affect the mentioned timers.
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8-bit Timer/Counter0 (TC0) with PWM
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19.9.5 TC0 Counter Value Register
Name: TCNT0
Offset: 0x46
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x26
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
TCNT0[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TCNT0[7:0] TC0 Counter Value
The Timer/Counter register gives direct access, both for read and write operations, to the timer/counter
unit 8-bit counter. Writing to the TCNT0 register blocks (removes) the compare match on the following
timer clock. Modifying the counter (TCNT0) while the counter is running, introduces a risk of missing a
compare match between TCNT0 and the OCR0x registers.
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8-bit Timer/Counter0 (TC0) with PWM
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19.9.6 TC0 Output Compare Register A
Name: OCR0A
Offset: 0x47
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x27
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
OCR0A[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR0A[7:0] Output Compare 0 A
The output compare register A contains an 8-bit value that is continuously compared with the counter
value (TCNT0). A match can be used to generate an output compare interrupt or to generate a waveform
output on the OC0A pin.
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8-bit Timer/Counter0 (TC0) with PWM
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19.9.7 TC0 Output Compare Register B
Name: OCR0B
Offset: 0x48
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x28
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
OCR0B[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR0B[7:0] Output Compare 0 B
The output compare register B contains an 8-bit value that is continuously compared with the counter
value (TCNT0). A match can be used to generate an output compare interrupt or to generate a waveform
output on the OC0B pin.
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8-bit Timer/Counter0 (TC0) with PWM
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19.9.8 TC0 Interrupt Flag Register
Name: TIFR0
Offset: 0x35
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x15
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
OCF0B OCF0A TOV0
Access R/W R/W R/W
Reset 0 0 0
Bit 2 – OCF0B Timer/Counter 0, Output Compare B Match Flag
The OCF0B bit is set when a compare match occurs between the Timer/Counter and the data in OCR0B
– Output Compare Register0 B. OCF0B is cleared by hardware when executing the corresponding
interrupt handling vector. Alternatively, OCF0B is cleared by writing a logic one to the flag. When the I-bit
in SREG, OCIE0B (Timer/Counter Compare B Match Interrupt Enable), and OCF0B are set, the Timer/
Counter Compare Match Interrupt is executed.
Bit 1 – OCF0A Timer/Counter 0, Output Compare A Match Flag
The OCF0A bit is set when a compare match occurs between the Timer/Counter0 and the data in OCR0A
– Output Compare Register0. OCF0A is cleared by hardware when executing the corresponding interrupt
handling vector. Alternatively, OCF0A is cleared by writing a logic one to the flag. When the I-bit in SREG,
OCIE0A (Timer/Counter0 Compare Match Interrupt Enable), and OCF0A are set, the Timer/Counter0
Compare Match Interrupt is executed.
Bit 0 – TOV0 Timer/Counter 0, Overflow Flag
The bit TOV0 is set when an overflow occurs in Timer/Counter0. TOV0 is cleared by hardware when
executing the corresponding interrupt handling vector. Alternatively, TOV0 is cleared by writing a logic
one to the flag. When the SREG I-bit, TOIE0 (Timer/Counter0 Overflow Interrupt Enable), and TOV0 are
set, the Timer/Counter 0 Overflow interrupt is executed.
The setting of this flag is dependent on the WGM0[2:0] bit setting. Refer to bit description of WGM0 in
TCCR0A.
Related Links
TCCR0A
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20. 16-bit Timer/Counter1 (TC1) with PWM
20.1 Overview
The 16-bit timer/counter unit allows accurate program execution timing (event management), wave
generation, and signal timing measurement.
A block diagram of the 16-bit timer/counter is shown below. CPU accessible I/O registers, including I/O
bits and I/O pins, are shown in bold. The device-specific I/O register and bit locations are listed in
Register Description. For the actual placement of I/O pins, refer to the Pin Configurations description.
Related Links
I/O-Ports
20.2 Features
• True 16-bit Design (i.e., allows 16-bit PWM)
• Two Independent Output Compare Units
• Double Buffered Output Compare Registers
• One Input Capture Unit
• Input Capture Noise Canceler
• Clear Timer on Compare Match (Auto Reload)
• Glitch-free, Phase Correct Pulse-Width Modulator (PWM)
• Variable PWM Period
• Frequency Generator
• External Event Counter
• Independent Interrupt Sources (TOV, OCFA, OCFB, and ICF)
20.3 Block Diagram
The Power Reduction TC1 bit in the Power Reduction Register (PRRPRR.PRTIM1) must be written to
zero to enable the TC1 module.
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
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Figure 20-1. 16-bit Timer/Counter Block Diagram
Clock Select
Timer/Counter
DATA BUS
OCRnA
OCRnB
ICRn
=
=
TCNTn
Waveform
Generation
Waveform
Generation
OCnA
OCnB
Noise
Canceler
ICPn
=
Fixed
TOP
Values
Edge
Detector
Control Logic
= 0
TOP BOTTOM
Count
Clear
Direction
TOVn
(Int.Req.)
OCnA
(Int.Req.)
OCnB
(Int.Req.)
ICFn (Int.Req.)
TCCRnA TCCRnB
( From Analog
Comparator Ouput )
Tn
Edge
Detector
( From Prescaler )
clkTn
See the related links for actual pin placement.
20.4 Definitions
Many register and bit references in this section are written in general form:
• n=1 represents the timer/counter number
• x=A,B represents the output compare unit A or B
However, when using the register or bit definitions in a program, the precise form must be used, i.e.,
TCNT1 for accessing timer/counter1 counter value.
The following definitions are used throughout the section:
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16-bit Timer/Counter1 (TC1) with PWM
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Table 20-1. Definitions
Constant Description
BOTTOM The counter reaches the BOTTOM when it becomes zero (0x00 for 8-bit counters, or 0x0000
for 16-bit counters).
MAX The counter reaches its maximum when it becomes 0xFF (decimal 255, for 8-bit counters) or
0xFFFF (decimal 65535, for 16-bit counters).
TOP The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be the fixed value MAX or the value stored in
the OCR1A register. The assignment is dependent on the mode of operation.
20.5 Registers
The Timer/Counter (TCNT1), Output Compare registers (OCR1A/B), and Input Capture Register (ICR1)
are all 16-bit registers. Special procedures must be followed when accessing the 16-bit registers. These
procedures are described in section Accessing 16-bit Timer/Counter Registers.
The Timer/Counter Control Registers (TCCR1A/B/C) are 8-bit registers and have no CPU access
restrictions. Interrupt requests (abbreviated to Int. Req. in the block diagram) signals are all visible in the
Timer Interrupt Flag Register (TIFR1). All interrupts are individually masked with the Timer Interrupt Mask
Register (TIMSK1). TIFR1 and TIMSK1 are not shown in the block diagram.
The timer/counter can be clocked internally, via the prescaler, or by an external clock source on the T1
pin. The clock select logic block controls which clock source and edge the timer/counter uses to
increment (or decrement) its value. The timer/counter is inactive when no clock source is selected. The
output from the clock select logic is referred to as the timer clock (clkT1).
The double buffered Output Compare Registers (OCR1A/B) are compared with the timer/counter value at
all time. The result of the compare can be used by the waveform generator to generate a PWM or
variable frequency output on the Output Compare pin (OC1A/B). See Output Compare Units. The
compare match event will also set the Compare Match Flag (OCF1A/B), which can be used to generate
an output compare interrupt request.
The Input Capture register can capture the timer/counter value at a given external (edge triggered) event
on either the Input Capture pin (ICP1) or on the analog comparator pins. The input capture unit includes a
digital filtering unit (Noise canceler) for reducing the chance of capturing noise spikes.
The TOP value, or maximum timer/counter value, can in some modes of operation be defined by either
the OCR1A register, the ICR1 register, or by a set of fixed values. When using OCR1A as TOP value in a
PWM mode, the OCR1A register cannot be used for generating a PWM output. However, the TOP value
will, in this case, be double buffered allowing the TOP value to be changed in runtime. If a fixed TOP
value is required, the ICR1 register can be used as an alternative, freeing the OCR1A to be used as
PWM output.
20.6 Accessing 16-bit Timer/Counter Registers
The TCNT1, OCR1A/B, and ICR1 are 16-bit registers that can be accessed by the AVR CPU via the 8-bit
data bus. The 16-bit register must be accessed byte-wise, using two read or write operations. Each 16-bit
timer has a single 8-bit TEMP register for temporary storing of the high byte of the 16-bit access. The
same temporary register is shared between all 16-bit registers within each 16-bit timer.
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Accessing the low byte triggers the 16-bit read or write operation: When the low byte of a 16-bit register is
written by the CPU, the high byte that is currently stored in TEMP and the low byte being written are both
copied into the 16-bit register in the same clock cycle. When the low byte of a 16-bit register is read by
the CPU, the high byte of the 16-bit register is copied into the TEMP register in the same clock cycle as
the low byte is read, and must be read subsequently.
Note: To perform a 16-bit write operation, the high byte must be written before the low byte. For a 16-bit
read, the low byte must be read before the high byte.
Not all 16-bit accesses use the temporary register for the high byte. Reading the OCR1A/B 16-bit
registers does not involve using the temporary register.
16-bit Access
The following code examples show how to access the 16-bit timer registers assuming that no interrupts
updates the temporary register. The same principle can be used directly for accessing the OCR1A/B and
ICR1 registers. Note that when using C, the compiler handles the 16-bit access.
Assembly Code Example(1)
...
; Set TCNT1 to 0x01FF
ldi r17,0x01
ldi r16,0xFF
out TCNT1H,r17
out TCNT1L,r16
; Read TCNT1 into r17:r16
in r16,TCNT1L
in r17,TCNT1H
...
The assembly code example returns the TCNT1 value in the r17:r16 register pair.
C Code Example(1)
unsigned int i;
...
/* Set TCNT1 to 0x01FF */
TCNT1 = 0x1FF;
/* Read TCNT1 into i */
i = TCNT1;
...
Note:
1. The example code assumes that the part specific header file is included. For I/O registers located in
extended I/O map, IN, OUT, SBIS, SBIC, CBI, and SBI instructions must be replaced with
instructions that allow access to extended I/O. Typically LDS and STS combined with SBRS, SBRC,
SBR, and CBR.
Atomic Read
It is important to notice that accessing 16-bit registers are atomic operations. If an interrupt occurs
between the two instructions accessing the 16-bit register, and the interrupt code updates the temporary
register by accessing the same or any other of the 16-bit timer registers, then the result of the access
outside the interrupt is corrupted. Therefore, when both the main code and the interrupt code update the
temporary register, the main code must disable the interrupts during the 16-bit access.
The following code examples show how to perform an atomic read of the TCNT1 register contents. The
OCR1A/B or ICR1 registers can be read using the same principle.
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Assembly Code Example(1)
TIM16_ReadTCNT1:
; Save global interrupt flag
in r18,SREG
; Disable interrupts
cli
; Read TCNT1 into r17:r16
in r16,TCNT1L
in r17,TCNT1H
; Restore global interrupt flag
out SREG,r18
ret
The assembly code example returns the TCNT1 value in the r17:r16 register pair.
C Code Example(1)
unsigned int TIM16_ReadTCNT1( void )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
_CLI();
/* Read TCNT1 into i */
i = TCNT1;
/* Restore global interrupt flag */
SREG = sreg;
return i;
}
Note:
1. The example code assumes that the part specific header file is included. For I/O registers located in
extended I/O map, IN, OUT, SBIS, SBIC, CBI, and SBI instructions must be replaced with
instructions that allow access to extended I/O. Typically LDS and STS combined with SBRS, SBRC,
SBR, and CBR.
Atomic Write
The following code examples show how to do an atomic write of the TCNT1 register contents. Writing any
of the OCR1A/B or ICR1 registers can be done using the same principle.
Assembly Code Example(1)
TIM16_WriteTCNT1:
; Save global interrupt flag
in r18,SREG
; Disable interrupts
cli
; Set TCNT1 to r17:r16
out TCNT1H,r17
out TCNT1L,r16
; Restore global interrupt flag
out SREG,r18
ret
The assembly code example requires that the r17:r16 register pair contains the value to
be written to TCNT1.
C Code Example(1)
void TIM16_WriteTCNT1( unsigned int i )
{
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
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unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
_CLI();
/* Set TCNT1 to i */
TCNT1 = i;
/* Restore global interrupt flag */
SREG = sreg;
}
Note:
1. The example code assumes that the part specific header file is included. For I/O
registers located in extended I/O map, IN, OUT, SBIS, SBIC, CBI, and SBI
instructions must be replaced with instructions that allow access to extended I/O.
Typically LDS and STS combined with SBRS, SBRC, SBR, and CBR.
Related Links
About Code Examples
20.6.1 Reusing the Temporary High Byte Register
If writing to more than one 16-bit register where the high byte is the same for all registers written, the high
byte only needs to be written once. However, the same rule of atomic operation described previously also
applies in this case.
20.7 Timer/Counter Clock Sources
The timer/counter can be clocked by an internal or an external clock source. The clock source is selected
by the clock select logic, which is controlled by the clock select bits in the Timer/Counter Control Register
B (TCCR1B.CS[2:0]).
Related Links
Timer/Counter 0, 1 Prescalers
20.8 Counter Unit
The main part of the 16-bit timer/counter is the programmable 16-bit bi-directional counter unit, as shown
in the block diagram:
Figure 20-2. Counter Unit Block Diagram
TEMP (8-bit)
DATA BUS (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit) TCNTnL (8-bit) Control Logic
Count
Clear
Direction
TOVn
(Int.Req.)
Clock Select
TOP BOTTOM
Tn
Edge
Detector
( From Prescaler )
clkTn
Note: The “n” in the register and bit names indicates the device number (n = 1 for timer/counter 1), and
the “x” indicates output compare unit (A/B).
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Table 20-2. Signal Description (Internal Signals)
Signal Name Description
Count Increment or decrement TCNT1 by 1.
Direction Select between increment and decrement.
Clear Clear TCNT1 (set all bits to zero).
clkT1 Timer/counter clock.
TOP Signalize that TCNT1 has reached maximum value.
BOTTOM Signalize that TCNT1 has reached minimum value (zero).
The 16-bit counter is mapped into two 8-bit I/O memory locations: Counter High (TCNT1H) containing the
upper eight bits of the counter, and Counter Low (TCNT1L) containing the lower eight bits. The TCNT1H
register can only be accessed indirectly by the CPU. When the CPU does an access to the TCNT1H I/O
location, the CPU accesses the high byte temporary register (TEMP). The temporary register is updated
with the TCNT1H value when the TCNT1L is read, and TCNT1H is updated with the temporary register
value when TCNT1L is written. This allows the CPU to read or write the entire 16-bit counter value within
one clock cycle via the 8-bit data bus.
Note: That there are special cases when writing to the TCNT1 register while the counter is counting will
give unpredictable results. These special cases are described in the sections where they are of
importance.
Depending on the selected mode of operation, the counter is cleared, incremented, or decremented at
each timer clock (clkT1). The clock clkT1 can be generated from an external or internal clock source, as
selected by the clock select bits in the Timer/Counter1 Control Register B (TCCR1B.CS[2:0]). When no
clock source is selected (CS[2:0]=0x0) the timer is stopped. However, the TCNT1 value can be accessed
by the CPU, independent of whether clkT1 is present or not. A CPU write overrides (i.e., has priority over)
all counter clear or count operations.
The counting sequence is determined by the setting of the Waveform Generation Mode bits in the Timer/
Counter Control Registers A and B (TCCR1B.WGM1[3:2] and TCCR1A.WGM1[1:0]). There are close
connections between how the counter behaves (counts) and how waveforms are generated on the
Output Compare outputs OC0x. For more details about advanced counting sequences and waveform
generation, see Modes of Operation.
The timer/counter overflow flag in the TC1 Interrupt Flag Register (TIFR1.TOV) is set according to the
mode of operation selected by the WGM1[3:0] bits. TOV can be used for generating a CPU interrupt.
20.9 Input Capture Unit
The timer/counter1 incorporates an input capture unit that can capture external events and give them a
time-stamp indicating time of occurrence. The external signal indicating an event, or multiple events, can
be applied via the ICP1 pin or alternatively, via the analog-comparator unit. The time-stamps can then be
used to calculate frequency, duty-cycle and other features of the signal applied. Alternatively, the time-
stamps can be used for creating a log of the events.
The input capture unit is illustrated by the block diagram below. The elements of the block diagram that
are not directly a part of the input capture unit are gray shaded. The lower case “n” in register and bit
names indicates the timer/counter number.
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 158
Figure 20-3. Input Capture Unit Block Diagram for TC1
ICFn (Int.Req.)
Analog
Comparator
WRITE ICRn (16-bit Register)
ICRnH (8-bit)
Noise
Canceler
ICPn
Edge
Detector
TEMP (8-bit)
DATA BUS (8-bit)
ICRnL (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit) TCNTnL (8-bit)
ACIC ICNC ICES
ACO
Note: The “n” in the register and bit names indicates the device number (n = 1 for timer/counter 1), and
the “x” indicates output compare unit (A/B).
When a change of the logic level (an event) occurs on the input capture pin (ICP1), or alternatively on the
Analog Comparator Output (ACO), and this change confirms to the setting of the edge detector, a capture
will be triggered: the 16-bit value of the counter (TCNT1) is written to the Input Capture Register (ICR1).
The Input Capture Flag (ICF) is set at the same system clock cycle as the TCNT1 value is copied into the
ICR1 . If enabled (TIMSK1.ICIE=1), the ICF generates an input capture interrupt. The ICF1 is
automatically cleared when the interrupt is executed. Alternatively, the ICF can be cleared by software by
writing '1' to its I/O bit location.
Reading the 16-bit value in the ICR1 is done by first reading the low byte (ICR1L) and then the high byte
(ICR1H). When the low byte is read form ICR1L, the high byte is copied into the high byte temporary
register (TEMP). When the CPU reads the ICR1H I/O location it will access the TEMP register.
The ICR1 can only be written when using a Waveform Generation mode that utilizes the ICR1 for defining
the counter’s TOP value. In these cases the Waveform Generation Mode bits (WGM1[3:0]) must be set
before the TOP value can be written to the ICR1. When writing the ICR1, the high byte must be written to
the ICR1H I/O location before the low byte is written to ICR1L.
Related Links
Accessing 16-bit Timer/Counter Registers
20.9.1 Input Capture Trigger Source
The main trigger source for the input capture unit is the Input Capture pin (ICP1). Timer/Counter1 can
alternatively use the analog comparator output as trigger source for the input capture unit. The analog
comparator is selected as a trigger source by setting the Analog Comparator Input Capture (ACIC) bit in
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 159
the Analog Comparator Control and Status Register (ACSR). Be aware that changing trigger source can
trigger a capture. The input capture flag must, therefore, be cleared after the change.
Both the Input Capture Pin (ICP1) and the Analog Comparator Output (ACO) inputs are sampled using
the same technique as for the T1 pin. The edge detector is identical. However, when the noise canceler is
enabled, additional logic is inserted before the edge detector, which increases the delay by four system
clock cycles. The input of the noise canceler and edge detector is always enabled unless the Timer/
Counter is set in a Waveform Generation mode that uses ICR1 to define TOP.
An input capture can be triggered by software by controlling the port of the ICP1 pin.
Related Links
Timer/Counter 0, 1 Prescalers
20.9.2 Noise Canceler
The noise canceler improves noise immunity by using a simple digital filtering scheme. The noise
canceler input is monitored over four samples, and all four must be equal for changing the output that in
turn is used by the edge detector.
The noise canceler is enabled by setting the Input Capture Noise Canceler bit in the Timer/Counter
Control Register B (TCCR1B.ICNC). When enabled, the noise canceler introduces an additional delay of
four system clock cycles between a change applied to the input and the update of the ICR1 Register. The
noise canceler uses the system clock and is therefore not affected by the prescaler.
20.9.3 Using the Input Capture Unit
The main challenge when using the input capture unit is to assign enough processor capacity for handling
the incoming events. The time between two events is critical. If the processor has not read the captured
value in the ICR1 before the next event occurs, the ICR1 will be overwritten with a new value. In this case
the result of the capture will be incorrect.
When using the input capture interrupt, the ICR1 should be read as early in the interrupt handler routine
as possible. Even though the input capture interrupt has relatively high priority, the maximum interrupt
response time is dependent on the maximum number of clock cycles it takes to handle any of the other
interrupt requests.
Using the input capture unit in any mode of operation when the TOP value (resolution) is actively
changed during operation, is not recommended.
Measurement of an external signal’s duty cycle requires that the trigger edge is changed after each
capture. Changing the edge sensing must be done as early as possible after the ICR1 has been read.
After a change of the edge, the ICF must be cleared by software (writing a logical one to the I/O bit
location). For measuring frequency only, the clearing of the ICF is not required (if an interrupt handler is
used).
20.10 Output Compare Units
The 16-bit comparator continuously compares TCNT1 with the Output Compare Register (OCR1x). If
TCNT equals OCR1x the comparator signals a match. A match will set the Output Compare Flag
(TIFR1.OCFx) at the next timer clock cycle. If enabled (TIMSK1.OCIEx = 1), the output compare flag
generates an output compare interrupt. The OCFx is automatically cleared when the interrupt is
executed. Alternatively, the OCFx can be cleared by software by writing a logical one to its I/O bit
location. The waveform generator uses the match signal to generate an output according to operating
mode set by the Waveform Generation mode (WGM1[3:0]) bits and Compare Output mode (COM1x[1:0])
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 160
bits. The TOP and BOTTOM signals are used by the waveform generator for handling the special cases
of the extreme values in some modes of operation, see Modes of Operation.
A special feature of output compare unit A allows it to define the Timer/Counter TOP value (i.e., counter
resolution). In addition to the counter resolution, the TOP value defines the period time for waveforms
generated by the waveform generator.
Below is a block diagram of the output compare unit. The elements of the block diagram that are not
directly a part of the output compare unit are gray shaded.
Figure 20-4. Output Compare Unit, Block Diagram
OCFnx (Int.Req.)
= (16-bit Comparator )
OCRnx Buffer (16-bit Register)
OCRnxH Buf. (8-bit)
OCnx
TEMP (8-bit)
DATA BUS (8-bit)
OCRnxL Buf. (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit) TCNTnL (8-bit)
COMnx[1:0]WGMn[3:0]
OCRnx (16-bit Register)
OCRnxH (8-bit) OCRnxL (8-bit)
Waveform Generator
TOP
BOTTOM
Note: The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates output compare unit (A/B).
The OCR1x is double buffered when using any of the twelve Pulse Width Modulation (PWM) modes. For
the Normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The
double buffering synchronizes the update of the OCR1x to either TOP or BOTTOM of the counting
sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses,
thereby making the output glitch-free.
When double buffering is enabled, the CPU has access to the OCR1x Buffer register. When double
buffering is disabled, the CPU will access the OCR1x directly.
The content of the OCR1x (Buffer or Compare) register is only changed by a write operation (the Timer/
Counter does not update this register automatically as the TCNT1 and ICR1). Therefore OCR1x is not
read via the high byte temporary register (TEMP). However, it is good practice to read the low byte first as
when accessing other 16-bit registers. Writing the OCR1x must be done via the TEMP register since the
compare of all 16 bits is done continuously. The high byte (OCR1xH) has to be written first. When the
high byte I/O location is written by the CPU, the TEMP register will be updated by the value written. Then
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 161
when the low byte (OCR1xL) is written to the lower eight bits, the high byte will be copied into the upper
8-bits of either the OCR1x buffer or OCR1x in the same system clock cycle.
Related Links
Accessing 16-bit Timer/Counter Registers
20.10.1 Force Output Compare
In non-PWM Waveform Generation modes, the match output of the comparator can be forced by writing a
one to the Force Output Compare (TCCR1C.FOC1x) bit. Forcing compare match will not set the OCF1x
Flag or reload/clear the timer, but the OC1x pin will be updated as if a real compare match had occurred
(the TCCR1C.COM1x[1:0] bits settings define whether the OC1x pin is set, cleared or toggled).
20.10.2 Compare Match Blocking by TCNT1 Write
All CPU writes to the TCNT1 register will block any compare match that occurs in the next timer clock
cycle, even when the timer is stopped. This feature allows OCR1x to be initialized to the same value as
TCNT1 without triggering an interrupt when the timer/counter clock is enabled.
20.10.3 Using the Output Compare Unit
Since writing TCNT1 in any mode of operation will block all compare matches for one timer clock cycle,
there are risks involved when changing TCNT1 when using any of the output compare channels,
independent of whether the timer/counter is running or not. If the value written to TCNT1 equals the
OCR1x value, the compare match will be missed, resulting in incorrect waveform generation. Do not write
the TCNT1 equal to TOP in PWM modes with variable TOP values. The compare match for the TOP will
be ignored and the counter will continue to 0xFFFF. Similarly, do not write the TCNT1 value equal to
BOTTOM when the counter is down counting.
The setup of the OC1x should be performed before setting the Data Direction register for the port pin to
output. The easiest way of setting the OC1x value is to use the Force Output Compare (FOC1x) strobe
bits in Normal mode. The OC1x register keeps its value even when changing between Waveform
Generation modes.
Be aware that the TCCR1A.COM1x[1:0] bits are not double buffered together with the compare value.
Changing the TCCR1A.COM1x[1:0] will take effect immediately.
20.11 Compare Match Output Unit
The Compare Output mode (TCCR1A.COM1x[1:0]) bits have two functions. The waveform generator
uses the TCCR1A.COM1x[1:0] bits for defining the Output Compare (OC1x) state at the next compare
match. Secondly the TCCR1A.COM1x[1:0] bits control the OC1x pin output source. The figure below
shows a simplified schematic of the logic affected by the TCCR1A.COM1x[1:0] bit setting. The I/O
registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of the general I/O port
control registers (DDR and PORT) that are affected by the TCCR1A.COM1x[1:0] bits are shown. When
referring to the OC1x state, the reference is for the internal OC1x register, not the OC1x pin. If a System
Reset occurs, the OC1x register is reset to “0”.
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 162
Figure 20-5. Compare Match Output Unit, Schematic
PORT
DDR
D Q
D Q
OCnx
Pin
OCnx
D Q
Waveform
Generator
COMnx[1]
COMnx[0]
0
1
DATA BUS
FOCnx
clkI/O
Note: The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates output compare unit (A/B).
The general I/O port function is overridden by the Output Compare (OC1x) from the waveform generator
if either of the TCCR1A.COM1x[1:0] bits are set. However, the OC1x pin direction (input or output) is still
controlled by the Data Direction Register (DDR) for the port pin. The DDR bit for the OC1x pin
(DDR_OC1x) must be set as output before the OC1x value is visible on the pin. The port override
function is generally independent of the waveform generation mode, but there are some exceptions.
The design of the output compare pin logic allows initialization of the OC1x state before the output is
enabled. Note that some TCCR1A.COM1x[1:0] bit settings are reserved for certain modes of operation.
The TCCR1A.COM1x[1:0] bits have no effect on the input capture unit.
20.11.1 Compare Output Mode and Waveform Generation
The waveform generator uses the TCCR1A.COM1x[1:0] bits differently in normal, CTC, and PWM
modes. For all modes, setting the TCCR1A.COM1x[1:0] = 0 tells the waveform generator that no action
on the OC1x register is to be performed on the next compare match. Refer also to the descriptions of the
output modes.
A change of the TCCR1A.COM1x[1:0] bits state will have effect at the first compare match after the bits
are written. For non-PWM modes, the action can be forced to have immediate effect by using the
TCCR1C.FOC1x strobe bits.
20.12 Modes of Operation
The mode of operation, i.e., the behavior of the timer/counter and the output compare pins, is defined by
the combination of the Waveform Generation mode (WGM1[3:0]) and Compare Output mode
(TCCR1A.COM1x[1:0]) bits. The Compare Output mode bits do not affect the counting sequence, while
the Waveform Generation mode bits do. The TCCR1A.COM1x[1:0] bits control whether the PWM output
generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes the
TCCR1A.COM1x[1:0] bits control whether the output should be set, cleared, or toggle at a compare
match.
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 163
Related Links
Timer/Counter Timing Diagrams
Compare Match Output Unit
20.12.1 Normal Mode
The simplest mode of operation is the Normal mode (TCCR1A.WGM1[3:0]=0). In this mode, the counting
direction is always up (incrementing), and no counter clear is performed. The counter simply overruns
when it passes its maximum 16-bit value (MAX=0xFFFF) and then restarts from BOTTOM=0x0000. In
normal operation, the Timer/Counter Overflow Flag (TIFR1.TOVn) will be set in the same timer clock
cycle as the TCNT1 becomes zero. In this case, the TOVn flag behaves like a 17th bit, except that it is
only set, not cleared. However, combined with the timer overflow interrupt that automatically clears the
TOVn flag, the timer resolution can be increased by software. There are no special cases to consider in
the Normal mode, a new counter value can be written any time.
The input capture unit is easy to use in Normal mode. However, observe that the maximum interval
between the external events must not exceed the resolution of the counter. If the interval between events
are too long, the timer overflow interrupt or the prescaler must be used to extend the resolution for the
capture unit.
The output compare units can be used to generate interrupts at some given time. Using the output
compare to generate waveforms in Normal mode is not recommended since this will occupy too much of
the CPU time.
20.12.2 Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare (CTC) modes (mode 4 or 12, WGM1[3:0]=0x4 or 0xC), the OCR1A or ICR1
registers are used to manipulate the counter resolution: the counter is cleared to ZERO when the counter
value (TCNT1) matches either the OCR1A (if WGM1[3:0]=0x4) or the ICR1 (WGM1[3:0]=0xC). The
OCR1A or ICR1 define the top value for the counter, hence also its resolution. This mode allows greater
control of the compare match output frequency. It simplifies the operation of counting external events.
The timing diagram for the CTC mode is shown below. The counter value (TCNT1) increases until a
compare match occurs with either OCR1A or ICR1, and then TCNT1 is cleared.
Figure 20-6. CTC Mode, Timing Diagram
TCNTn
OCnA
(Toggle)
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 4
Period 2 3
(COMnA[1:0] = 0x1)
Note: The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates output compare unit (A/B).
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 164
An interrupt can be generated at each time the counter value reaches the TOP value by either using the
OCF1A or ICF1 flag, depending on the actual CTC mode. If the interrupt is enabled, the interrupt handler
routine can be used for updating the TOP value.
Note: Changing TOP to a value close to BOTTOM while the counter is running must be done with care
since the CTC mode does not provide double buffering. If the new value written to OCR1A is lower than
the current value of TCNT1, the counter will miss the compare match. The counter will then count to its
maximum value (0xFF for an 8-bit counter, 0xFFFF for a 16-bit counter) and wrap around starting at 0x00
before the compare match will occur.
In many cases, this feature is not desirable. An alternative will then be to use the Fast PWM mode using
OCR1A for defining TOP (WGM1[3:0]=0xF), since the OCR1A then will be double buffered.
For generating a waveform output in CTC mode, the OC1A output can be set to toggle its logical level on
each compare match by setting the Compare Output mode bits to toggle mode (COM1A[1:0]=0x1). The
OC1A value will not be visible on the port pin unless the data direction for the pin is set to output
(DDR_OC1A=1). The waveform generated will have a maximum frequency of fOC1A = fclk_I/O/2 when
OCR1A is set to ZERO (0x0000). The waveform frequency is defined by the following equation:
OCnA =clk_I/O
2 1 + OCRnA
Note:
• The “n” indicates the device number (n = 1 for Timer/Counter 1), and the “x” indicates Output
Compare unit (A/B).
•N represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the Timer Counter TOV flag is set in the same timer clock cycle that
the counter counts from MAX to 0x0000.
20.12.3 Fast PWM Mode
The Fast Pulse Width Modulation or Fast PWM modes (modes 5, 6, 7, 14, and 15, WGM1[3:0]= 0x5, 0x6,
0x7, 0xE, 0xF) provide a high frequency PWM waveform generation option. The Fast PWM differs from
the other PWM options by its single-slope operation. The counter counts from BOTTOM to TOP then
restarts from BOTTOM.
In non-inverting Compare Output mode, the Output Compare (OC1x) is cleared on the compare match
between TCNT1 and OCR1x and set at BOTTOM. In inverting Compare Output mode output is set on
compare match and cleared at BOTTOM. Due to the single-slope operation, the operating frequency of
the Fast PWM mode can be twice as high as the phase correct, and phase and frequency correct PWM
modes that use dual-slope operation. This high frequency makes the Fast PWM mode well suited for
power regulation, rectification, and DAC applications. High frequency allows physically small sized
external components (coils, capacitors), hence reduces total system cost.
The PWM resolution for Fast PWM can be fixed to 8-, 9-, or 10-bit, or defined by either ICR1 or OCR1A.
The minimum resolution allowed is 2-bit (ICR1 or OCR1A register set to 0x0003), and the maximum
resolution is 16-bit (ICR1 or OCR1A registers set to MAX). The PWM resolution in bits can be calculated
by using the following equation:
FPWM =log TOP+1
log 2
In Fast PWM mode the counter is incremented until the counter value matches either one of the fixed
values 0x00FF, 0x01FF, or 0x03FF (WGM1[3:0] = 0x5, 0x6, or 0x7), the value in ICR1 (WGM1[3:0]=0xE),
or the value in OCR1A (WGM1[3:0]=0xF). The counter is then cleared at the following timer clock cycle.
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 165
The timing diagram for the Fast PWM mode using OCR1A or ICR1 to define TOP is shown below. The
TCNT1 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. The
diagram includes non-inverted and inverted PWM outputs. The small horizontal lines on the TCNT1
slopes mark compare matches between OCR1x and TCNT1. The OC1x interrupt flag will be set when a
compare match occurs.
Figure 20-7. Fast PWM Mode, Timing Diagram
TCNTn
OCRnx/TOP Update and
TOVn Interrupt Flag Set and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 7
Period 2 3 4 5 6 8
OCnx
OCnx
(COMnx[1:0] = 0x2)
(COMnx[1:0] = 0x3)
Note: The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates output compare unit (A/B).
The Timer/Counter Overflow flag (TOV1) is set each time the counter reaches TOP. In addition, when
either OCR1A or ICR1 is used for defining the TOP value, the OC1A or ICF1 flag is set at the same timer
clock cycle TOV1 is set. If one of the interrupts are enabled, the interrupt handler routine can be used for
updating the TOP and compare values.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the
value of all of the Compare registers. If the TOP value is lower than any of the Compare registers, a
compare match will never occur between the TCNT1 and the OCR1x. Note that when using fixed TOP
values the unused bits are masked to zero when any of the OCR1x registers are written.
The procedure for updating ICR1 differs from updating OCR1A when used for defining the TOP value.
The ICR1 register is not double buffered. This means that if ICR1 is changed to a low value when the
counter is running with none or a low prescaler value, there is a risk that the new ICR1 value written is
lower than the current value of TCNT1. As result, the counter will miss the compare match at the TOP
value. The counter will then have to count to the MAX value (0xFFFF) and wrap around starting at
0x0000 before the compare match can occur. The OCR1A Register, however, is double buffered. This
feature allows the OCR1A I/O location to be written any time. When the OCR1A I/O location is written the
value written will be put into the OCR1A Buffer register. The OCR1A Compare register will then be
updated with the value in the Buffer register at the next timer clock cycle the TCNT1 matches TOP. The
update is performed at the same timer clock cycle as the TCNT1 is cleared and the TOV1 flag is set.
Using the ICR1 register for defining TOP works well when using fixed TOP values. By using ICR1, the
OCR1A is free to be used for generating a PWM output on OC1A. However, if the base PWM frequency
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 166
is actively changed (by changing the TOP value), using the OCR1A as TOP is clearly a better choice due
to its double buffer feature.
In Fast PWM mode, the compare units allow generation of PWM waveforms on the OC1x pins. Writing
the COM1x[1:0] bits to 0x2 will produce an inverted PWM and a non-inverted PWM output can be
generated by writing the COM1x[1:0] to 0x3. The actual OC1x value will only be visible on the port pin if
the data direction for the port pin is set as output (DDR_OC1x). The PWM waveform is generated by
setting (or clearing) the OC1x Register at the compare match between OCR1x and TCNT1, and clearing
(or setting) the OC1x register at the timer clock cycle the counter is cleared (changes from TOP to
BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
OCnxPWM =clk_I/O
1 + TOP
Note:
• The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates output compare unit (A/B).
•N represents the prescale divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x registers represent special cases when generating a PWM waveform
output in the Fast PWM mode. If the OCR1x is set equal to BOTTOM (0x0000) the output will be a narrow
spike for each TOP+1 timer clock cycle. Setting the OCR1x equal to TOP will result in a constant high or
low output (depending on the polarity of the output which is controlled by COM1x[1:0]).
A frequency waveform output with 50% duty cycle can be achieved in Fast PWM mode by selecting
OC1A to toggle its logical level on each compare match (COM1A[1:0]=0x1). This applies only if OCR1A is
used to define the TOP value (WGM1[3:0]=0xF). The waveform generated will have a maximum
frequency of fOC1A = fclk_I/O/2 when OCR1A is set to zero (0x0000). This feature is similar to the OC1A
toggle in CTC mode, except the double buffer feature of the output compare unit is enabled in the Fast
PWM mode.
20.12.4 Phase Correct PWM Mode
The Phase Correct Pulse Width Modulation or Phase Correct PWM modes (WGM1[3:0]= 0x1, 0x2, 0x3,
0xA, and 0xB) provide a high resolution, phase correct PWM waveform generation option. The Phase
Correct PWM mode is, like the phase and frequency correct PWM mode, based on a dual-slope
operation. The counter counts repeatedly from BOTTOM (0x0000) to TOP and then from TOP to
BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC1x) is cleared on the
compare match between TCNT1 and OCR1x while up-counting, and set on the compare match while
down-counting. In inverting Output Compare mode, the operation is inverted. The dual-slope operation
has lower maximum operation frequency than single-slope operation. However, due to the symmetric
feature of the dual-slope PWM modes, these modes are preferred for motor control applications.
The PWM resolution for the Phase Correct PWM mode can be fixed to 8-, 9-, or 10-bit, or defined by
either ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and the
maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM resolution in bits can be calculated
by using the following equation:
PCPWM =log TOP+1
log 2
In Phase Correct PWM mode the counter is incremented until the counter value matches either one of the
fixed values 0x00FF, 0x01FF, or 0x03FF (WGM1[3:0]= 0x1, 0x2, or 0x3), the value in ICR1
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 167
(WGM1[3:0]=0xA), or the value in OCR1A (WGM1[3:0]=0xB). The counter has then reached the TOP and
changes the count direction. The TCNT1 value will be equal to TOP for one timer clock cycle. The timing
diagram for the Phase Correct PWM mode is shown below, using OCR1A or ICR1 to define TOP. The
TCNT1 value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. The
diagram includes non-inverted and inverted PWM outputs. The small horizontal lines on the TCNT1
slopes mark compare matches between OCR1x and TCNT1. The OC1x interrupt flag will be set when a
compare match occurs.
Figure 20-8. Phase Correct PWM Mode, Timing Diagram
OCRnx/TOP Update and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 2 3 4
TOVn Interrupt Flag Set
(Interrupt on Bottom)
TCNTn
Period
OCnx
OCnx
(COMnx[1:0]] = 0x2)
(COMnx[1:0] = 0x3)
Note: The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates output compare unit (A/B).
The Timer/Counter Overflow flag (TOV1) is set each time the counter reaches BOTTOM. When either
OCR1A or ICR1 is used for defining the TOP value, the OC1A or ICF1 Flag is set accordingly at the same
timer clock cycle as the OCR1x registers are updated with the double buffer value (at TOP). The interrupt
flags can be used to generate an interrupt each time the counter reaches the TOP or BOTTOM value.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the
value of all of the compare registers. If the TOP value is lower than any of the compare registers, a
compare match will never occur between the TCNT1 and the OCR1x. Note that when using fixed TOP
values, the unused bits are masked to zero when any of the OCR1x registers is written. As illustrated by
the third period in the timing diagram, changing the TOP actively while the Timer/Counter is running in the
phase correct mode can result in an unsymmetrical output. The reason for this can be found in the time of
update of the OCR1x. Since the OCR1x update occurs at TOP, the PWM period starts and ends at TOP.
This implies that the length of the falling slope is determined by the previous TOP value, while the length
of the rising slope is determined by the new TOP value. When these two values differ the two slopes of
the period will differ in length. The difference in length gives the unsymmetrical result on the output.
It is recommended to use the Phase and Frequency Correct mode instead of the Phase Correct mode
when changing the TOP value while the Timer/Counter is running. When using a static TOP value, there
are practically no differences between the two modes of operation.
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 168
In Phase Correct PWM mode, the compare units allow generation of PWM waveforms on the OC1x pins.
Writing COM1x[1:0] bits to 0x2 will produce a non-inverted PWM. An inverted PWM output can be
generated by writing the COM1x[1:0] to 0x3. The actual OC1x value will only be visible on the port pin if
the data direction for the port pin is set as output (DDR_OC1x). The PWM waveform is generated by
setting (or clearing) the OC1x register at the compare match between OCR1x and TCNT1 when the
counter increments, and clearing (or setting) the OC1x register at compare match between OCR1x and
TCNT1 when the counter decrements. The PWM frequency for the output when using Phase Correct
PWM can be calculated by the following equation:
OCnxPCPWM =clk_I/O
2 TOP
N represents the prescale divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x represent special cases when generating a PWM waveform output in
the Phase Correct PWM mode. If the OCR1x is set equal to BOTTOM the output will be continuously low
and if set equal to TOP the output will be continuously high for non-inverted PWM mode. For inverted
PWM the output will have the opposite logic values. If OCR1A is used to define the TOP value
(WGM1[3:0]=0xB) and COM1A[1:0]=0x1, the OC1A output will toggle with a 50% duty cycle.
20.12.5 Phase and Frequency Correct PWM Mode
The phase and frequency correct Pulse Width Modulation, or phase and frequency correct PWM mode
(WGM1[3:0] = 0x8 or 0x9) provides a high-resolution phase and frequency correct PWM waveform
generation option. The phase and frequency correct PWM mode are, like the phase correct PWM mode,
based on a dual-slope operation. The counter counts repeatedly from BOTTOM (0x0000) to TOP and
then from TOP to BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC1x) is
cleared on the compare match between TCNT1 and OCR1x while up-counting, and set on the compare
match while down-counting. In inverting Compare Output mode, the operation is inverted. The dual-slope
operation gives a lower maximum operation frequency compared to the single-slope operation. However,
due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for motor control
applications.
The main difference between the phase correct, and the phase and frequency correct PWM mode is the
time the OCR1x is updated by the OCR1x Buffer register, (see Figure 20-8 and the Timing Diagram
below).
The PWM resolution for the phase and frequency correct PWM mode can be defined by either ICR1 or
OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and the maximum
resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM resolution in bits can be calculated using the
following equation:
PFCPWM =log TOP+1
log 2
In phase and frequency correct PWM mode the counter is incremented until the counter value matches
either the value in ICR1 (WGM1[3:0]=0x8), or the value in OCR1A (WGM1[3:0]=0x9). The counter has
then reached the TOP and changes the count direction. The TCNT1 value will be equal to TOP for one
timer clock cycle. The timing diagram for the phase correct and frequency correct PWM mode is shown
below. The figure shows phase and frequency correct PWM mode when OCR1A or ICR1 is used to
define TOP. The TCNT1 value is in the timing diagram shown as a histogram for illustrating the dual-slope
operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks
on the TCNT1 slopes represent compare matches between OCR1x and TCNT1. The OC1x interrupt flag
will be set when a compare match occurs.
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 169
Figure 20-9. Phase and Frequency Correct PWM Mode, Timing Diagram
OCRnx/TOP Updateand
TOVn Interrupt Flag Set
(Interrupt on Bottom)
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 2 3 4
TCNTn
Period
OCnx
OCnx
(COMnx[1:0] = 0x2)
(COMnx[1:0] = 0x3)
Note: The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates output compare unit (A/B).
The Timer/Counter Overflow flag (TOV1) is set at the same timer clock cycle as the OCR1x registers are
updated with the double buffer value (at BOTTOM). When either OCR1A or ICR1 is used for defining the
TOP value, the OC1A or ICF1 Flag set when TCNT1 has reached TOP. The interrupt flags can then be
used to generate an interrupt each time the counter reaches the TOP or BOTTOM value.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the
value of all of the Compare registers. If the TOP value is lower than any of the Compare registers, a
compare match will never occur between the TCNT1 and the OCR1x.
As shown in the timing diagram above, the output generated is, in contrast to the phase correct mode,
symmetrical in all periods. Since the OCR1x registers are updated at BOTTOM, the length of the rising
and the falling slopes will always be equal. This gives symmetrical output pulses and is, therefore,
frequency correct.
Using the ICR1 register for defining TOP works well when using fixed TOP values. By using ICR1, the
OCR1A register is free to be used for generating a PWM output on OC1A. However, if the base PWM
frequency is actively changed by changing the TOP value, using the OCR1A as TOP is clearly a better
choice due to its double buffer feature.
In phase and frequency correct PWM mode, the compare units allow generation of PWM waveforms on
the OC1x pins. Setting the COM1x[1:0] bits to 0x2 will produce a non-inverted PWM and an inverted
PWM output can be generated by setting the COM1x[1:0] to 0x3 (see the description of TCCRA.COM1x).
The actual OC1x value will only be visible on the port pin if the data direction for the port pin is set as
output (DDR_OC1x). The PWM waveform is generated by setting (or clearing) the OC1x register at the
compare match between OCR1x and TCNT1 when the counter increments, and clearing (or setting) the
OC1x register at compare match between OCR1x and TCNT1 when the counter decrements. The PWM
frequency for the output when using phase and frequency correct PWM can be calculated by the
following equation:
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 170
OCnxPFCPWM =clk_I/O
2 TOP
Note:
• The “n” in the register and bit names indicates the device number (n = 1 for Timer/Counter 1), and
the “x” indicates output compare unit (A/B).
•N represents the prescale divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x register represent special cases when generating a PWM waveform
output in the phase correct PWM mode. If the OCR1x is set equal to BOTTOM the output will be
continuously low and if set equal to TOP the output will be set to high for non-inverted PWM mode. For
inverted PWM the output will have the opposite logic values. If OCR1A is used to define the TOP value
(WGM1[3:0]=0x9) and COM1A[1:0]=0x1, the OC1A output will toggle with a 50% duty cycle.
20.13 Timer/Counter 0, 1 Prescalers
The 8-bit Timer/Counter0 (TC0) and the 16-bit Timer/Counter1 (TC1) share the same prescaler module,
but the timer/counters can have different prescaler settings. The following description applies to TC0,
TC1.
Related Links
8-bit Timer/Counter0 (TC0) with PWM
16-bit Timer/Counter1 (TC1) with PWM
20.14 Timer/Counter Timing Diagrams
The timer/counter is a synchronous design and the timer clock (clkT1) is therefore shown as a clock
enable signal in the following figures. The figures include information on when interrupt flags are set, and
when the OCR1x is updated with the OCR1x buffer value (only for modes utilizing double buffering). The
first figure shows a timing diagram for the setting of OCF1x.
Figure 20-10. Timer/Counter Timing Diagram, Setting of OCF1x, no Prescaling
clkTn
(clkI/O/1)
OCFnx
clkI/O
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
Note: The “n” in the register and bit names indicates the device number (n = 1 for timer/counter 1), and
the “x” indicates output compare unit (A/B).
The next figure shows the same timing data, but with the prescaler enabled.
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 171
Figure 20-11. Timer/Counter Timing Diagram, Setting of OCF1x, with Prescaler (fclk_I/O/8)
OCFnx
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
clkI/O
clkTn
(clkI/O/8)
Note: The “n” in the register and bit names indicates the device number (n = 1 for timer/counter 1), and
the “x” indicates output compare unit (A/B).
The next figure shows the count sequence close to TOP in various modes. When using phase and
frequency correct PWM mode the OCR1x is updated at BOTTOM. The timing diagrams will be the same,
but TOP should be replaced by BOTTOM, TOP-1 by BOTTOM+1 and so on. The same renaming applies
for modes that set the TOV1 flag at BOTTOM.
Figure 20-12. Timer/Counter Timing Diagram, no Prescaling.
TOVn (FPWM)
and ICFn (if used
as TOP)
OCRnx
(Update at TOP)
TCNTn
(CTC and FPWM)
TCNTn
(PC and PFC PWM) TOP - 1 TOP TOP - 1 TOP - 2
Old OCRnx Value New OCRnx Value
TOP - 1 TOP BOTTOM BOTTOM + 1
clkTn
(clkI/O/1)
clkI/O
Note: The “n” in the register and bit names indicates the device number (n = 1 for timer/counter 1), and
the “x” indicates output compare unit (A/B).
The next figure shows the same timing data, but with the prescaler enabled.
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 172
Figure 20-13. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
TOVn (FPWM)
and ICFn (if used
as TOP)
OCRnx
(Update at TOP)
TCNTn
(CTC and FPWM)
TCNTn
(PC and PFC PWM) TOP - 1 TOP TOP - 1 TOP - 2
Old OCRnx Value New OCRnx Value
TOP - 1 TOP BOTTOM BOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O
/8)
Note: The “n” in the register and bit names indicates the device number (n = 1 for timer/counter 1), and
the “x” indicates output compare unit (A/B).
20.15 Register Description
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 173
20.15.1 TC1 Control Register A
Name: TCCR1A
Offset: 0x80
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
COM1A[1:0] COM1B[1:0] WGM1[1:0]
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bits 4:5, 6:7 – COM1 Compare Output Mode for Channel
The COM1A[1:0] and COM1B[1:0] control the output compare pins (OC1A and OC1B respectively)
behavior. If one or both of the COM1A[1:0] bits are written to one, the OC1A output overrides the normal
port functionality of the I/O pin it is connected to. If one or both of the COM1B[1:0] bit are written to one,
the OC1B output overrides the normal port functionality of the I/O pin it is connected to. However, note
that the Data Direction Register (DDR) bit corresponding to the OC1A or OC1B pin must be set in order
to enable the output driver.
When the OC1A or OC1B is connected to the pin, the function of the COM1x[1:0] bits is dependent on
the WGM1[3:0] bits setting. The table below shows the COM1x[1:0] bit functionality when the WGM1[3:0]
bits are set to a Normal or a CTC mode (non-PWM).
Table 20-3. Compare Output Mode, Non-PWM
COM1A[1]/
COM1B[1]
COM1A[0]/
COM1B[0]
Description
0 0 Normal port operation, OC1A/OC1B disconnected.
0 1 Toggle OC1A/OC1B on compare match.
1 0 Clear OC1A/OC1B on compare match (Set output to low
level).
1 1 Set OC1A/OC1B on compare match (Set output to high
level).
The table below shows the COM1x[1:0] bit functionality when the WGM1[3:0] bits are set to the fast PWM
mode.
Table 20-4. Compare Output Mode, Fast PWM
COM1A[1]/
COM1B[1]
COM1A[0]/
COM1B[0]
Description
0 0 Normal port operation, OC1A/OC1B disconnected.
0 1 WGM1[3:0] = 14 or 15: Toggle OC1A on compare match, OC1B
disconnected (normal port operation). For all other WGM1
settings, normal port operation, OC1A/OC1B disconnected.
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 174
COM1A[1]/
COM1B[1]
COM1A[0]/
COM1B[0]
Description
1 0 Clear OC1A/OC1B on compare match, set OC1A/OC1B at
BOTTOM (Non-inverting mode)
1 1 Set OC1A/OC1B on compare match, clear OC1A/OC1B at
BOTTOM (Inverting mode)
Note:
1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A[1]/COM1B[1] is set. In this
case the compare match is ignored, but the set or clear is done at BOTTOM. Refer to Fast PWM
Mode for details.
The table below shows the COM1x[1:0] bit functionality when the WGM1[3:0] bits are set to the phase
correct or the phase and frequency correct, PWM mode.
Table 20-5. Compare Output Mode, Phase Correct, and Phase and Frequency Correct PWM
COM1A[1]/
COM1B[1]
COM1A[0]/
COM1B[0]
Description
0 0 Normal port operation, OC1A/OC1B disconnected.
0 1 WGM1[3:0] = 9 or 11: Toggle OC1A on compare match, OC1B
disconnected (normal port operation). For all other WGM1
settings, normal port operation, OC1A/OC1B disconnected.
1 0 Clear OC1A/OC1B on compare match when up-counting. Set
OC1A/OC1B on compare match when down-counting.
1 1 Set OC1A/OC1B on compare match when up-counting. Clear
OC1A/OC1B on compare match when down-counting.
Note:
1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A[1]/COM1B[1] is set. Refer to
Phase Correct PWM Mode for details.
Bits 1:0 – WGM1[1:0] Waveform Generation Mode
Combined with the WGM1[3:2] bits found in the TCCR1B register, these bits control the counting
sequence of the counter, the source for maximum (TOP) counter value, and what type of waveform
generation to be used. Modes of operation supported by the timer/counter unit are; Normal mode
(counter), Clear Timer on Compare Match (CTC) mode, and three types of Pulse-Width Modulation
(PWM) modes. (See Modes of Operation).
Table 20-6. Waveform Generation Mode Bit Description
Mode WGM1[3] WGM1[2]
(CTC1)(1)
WGM1[1]
(PWM1[1])(1)
WGM1[0]
(PWM1[0])(1)
Timer/
Counter
Mode of
Operation
TOP Update of
OCR1x at
TOV1 Flag
Set on
0 0 0 0 0 Normal 0xFFFF Immediate MAX
1 0 0 0 1 PWM, Phase
Correct, 8-bit
0x00FF TOP BOTTOM
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 175
Mode WGM1[3] WGM1[2]
(CTC1)(1)
WGM1[1]
(PWM1[1])(1)
WGM1[0]
(PWM1[0])(1)
Timer/
Counter
Mode of
Operation
TOP Update of
OCR1x at
TOV1 Flag
Set on
2 0 0 1 0 PWM, Phase
Correct, 9-bit
0x01FF TOP BOTTOM
3 0 0 1 1 PWM, Phase
Correct, 10-bit
0x03FF TOP BOTTOM
4 0 1 0 0 CTC OCR1A Immediate MAX
5 0 1 0 1 Fast PWM, 8-
bit
0x00FF BOTTOM TOP
6 0 1 1 0 Fast PWM, 9-
bit
0x01FF BOTTOM TOP
7 0 1 1 1 Fast PWM, 10-
bit
0x03FF BOTTOM TOP
8 1 0 0 0 PWM, Phase
and Frequency
Correct
ICR1 BOTTOM BOTTOM
9 1 0 0 1 PWM, Phase
and Frequency
Correct
OCR1A BOTTOM BOTTOM
10 1 0 1 0 PWM, Phase
Correct
ICR1 TOP BOTTOM
11 1 0 1 1 PWM, Phase
Correct
OCR1A TOP BOTTOM
12 1 1 0 0 CTC ICR1 Immediate MAX
13 1 1 0 1 Reserved - - -
14 1 1 1 0 Fast PWM ICR1 BOTTOM TOP
15 1 1 1 1 Fast PWM OCR1A BOTTOM TOP
Note:
1. The CTC1 and PWM1[1:0] bit definition names are obsolete. Use the WGM1[3:0] definitions.
However, the functionality and location of these bits are compatible with previous versions of the
timer.
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 176
20.15.2 TC1 Control Register B
Name: TCCR1B
Offset: 0x81
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
ICNC1 ICES1 WGM13 WGM12 CS1[2:0]
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bit 7 – ICNC1 Input Capture Noise Canceler
Writing this bit to '1' activates the input capture noise canceler. When the noise canceler is activated, the
input from the Input Capture pin (ICP1) is filtered. The filter function requires four successive equal
valued samples of the ICP1 pin for changing its output. The input capture is therefore delayed by four
oscillator cycles when the noise canceler is enabled.
Bit 6 – ICES1 Input Capture Edge Select
This bit selects which edge on the Input Capture pin (ICP1) that is used to trigger a capture event. When
the ICES1 bit is written to zero, a falling (negative) edge is used as a trigger, and when the ICES1 bit is
written to '1', a rising (positive) edge will trigger the capture.
When a capture is triggered according to the ICES1 setting, the counter value is copied into the Input
Capture Register (ICR1). The event will also set the Input Capture Flag (ICF1) and this can be used to
cause an input capture interrupt, if this interrupt is enabled.
When the ICR1 is used as TOP value (see description of the WGM1[3:0] bits located in the TCCR1A and
the TCCR1B register), the ICP1 is disconnected and consequently, the input capture function is disabled.
Bits 3, 4 – WGM1 Waveform Generation Mode
Refer to TCCR1A.
Bits 2:0 – CS1[2:0] Clock Select 1
The three clock select bits select the clock source to be used by the timer/counter. Refer to Figure 20-10
and Figure 20-11.
Table 20-7. Clock Select Bit Description
CS1[2] CS1[1] CS1[0] Description
0 0 0 No clock source (Timer/Counter stopped).
0 0 1 clkI/O/1 (No prescaling)
0 1 0 clkI/O/8 (From prescaler)
0 1 1 clkI/O/64 (From prescaler)
1 0 0 clkI/O/256 (From prescaler)
1 0 1 clkI/O/1024 (From prescaler)
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 177
CS1[2] CS1[1] CS1[0] Description
1 1 0 External clock source on T1 pin. Clock on falling edge.
1 1 1 External clock source on T1 pin. Clock on rising edge.
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 178
20.15.3 TC1 Control Register C
Name: TCCR1C
Offset: 0x82
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
FOC1A FOC1B
Access R/W R/W
Reset 0 0
Bits 6, 7 – FOC1 Force Output Compare for Channel B and A
The FOC1A/FOC1B bits are only active when the WGM1[3:0] bits specifies a non-PWM mode. When
writing a logical one to the FOC1A/FOC1B bit, an immediate compare match is forced on the waveform
generation unit. The OC1A/OC1B output is changed according to its COM1x[1:0] bits setting. Note that
the FOC1A/FOC1B bits are implemented as strobes. Therefore it is the value present in the COM1x[1:0]
bits that determine the effect of the forced compare.
A FOC1A/FOC1B strobe will not generate any interrupt nor will it clear the timer in Clear Timer on
Compare Match (CTC) mode using OCR1A as TOP. The FOC1A/FOC1B bits are always read as zero.
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 179
20.15.4 TC1 Counter Value Low and High byte
Name: TCNT1L and TCNT1H
Offset: 0x84
Reset: 0x00
Property: -
The TCNT1L and TCNT1H register pair represents the 16-bit value, TCNT1. The low byte [7:0] (suffix L)
is accessible at the original offset. The high byte [15:8] (suffix H) can be accessed at offset + 0x01. For
more details on reading and writing 16-bit registers, refer to Accessing 16-bit Timer/Counter Registers.
Bit 15 14 13 12 11 10 9 8
TCNT1[15:8]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 6 5 4 3 2 1 0
TCNT1[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 15:0 – TCNT1[15:0] Timer/Counter 1 Counter Value
The two Timer/Counter I/O locations (TCNT1H and TCNT1L, combined TCNT1) give direct access, both
for read and for write operations, to the timer/counter unit 16-bit counter. To ensure that both the high and
low bytes are read and written simultaneously when the CPU accesses these registers, the access is
performed using an 8-bit temporary high byte register (TEMP). This temporary register is shared by all the
other 16-bit registers. Refer to Accessing 16-bit Timer/Counter Registers for details.
Modifying the counter (TCNT1) while the counter is running introduces a risk of missing a compare match
between TCNT1 and one of the OCR1x registers.
Writing to the TCNT1 register blocks (removes) the compare match on the following timer clock for all
compare units.
Related Links
Accessing 16-bit Timer/Counter Registers
ATmega328/P
16-bit Timer/Counter1 (TC1) with PWM
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 180
20.15.5 Input Capture Register 1 Low and High byte
Name: ICR1L and ICR1H
Offset: 0x86
Reset: 0x00
Property: -
The ICR1L and ICR1H register pair represents the 16-bit value, ICR1. The low byte [7:0] (suffix L) is
accessible at the original offset. The high byte [15:8] (suffix H) can be accessed at offset + 0x01. For
more details on reading and writing 16-bit registers, refer to Accessing 16-bit Timer/Counter Registers.
Bit 15 14 13 12 11 10 9 8
ICR1[15:8]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 6 5 4 3 2 1 0
ICR1[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 15:0 – ICR1[15:0] Input Capture 1
The input capture is updated with the counter (TCNT1) value each time an event occurs on the ICP1 pin
(or optionally on the analog comparator output for Timer/Counter1). The input capture can be used for
defining the counter TOP value.
The Input Capture register is 16-bit in size. To ensure that both the high and low bytes are read
simultaneously when the CPU accesses these registers, the access is performed using an 8-bit
temporary High Byte register (TEMP). This temporary register is shared by all the other 16-bit registers.
Refer to Accessing 16-bit Timer/Counter Registers for details.
Related Links
Accessing 16-bit Timer/Counter Registers
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20.15.6 Output Compare Register 1 A Low and High byte
Name: OCR1AL and OCR1AH
Offset: 0x88
Reset: 0x00
Property: -
The OCR1AL and OCR1AH register pair represents the 16-bit value, OCR1A. The low byte [7:0] (suffix L)
is accessible at the original offset. The high byte [15:8] (suffix H) can be accessed at offset + 0x01. For
more details on reading and writing 16-bit registers, refer to Accessing 16-bit Timer/Counter Registers.
Bit 15 14 13 12 11 10 9 8
OCR1A[15:8]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 6 5 4 3 2 1 0
OCR1A[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 15:0 – OCR1A[15:0] Output Compare 1 A
The Output Compare registers contain a 16-bit value that is continuously compared with the counter
value (TCNT1). A match can be used to generate an output compare interrupt or to generate a waveform
output on the OC1A pin.
The Output Compare registers are 16-bit in size. To ensure that both the high and low bytes are written
simultaneously when the CPU writes to these registers, the access is performed using an 8-bit temporary
High Byte Register (TEMP). This temporary register is shared by all the other 16-bit registers. Refer to
Accessing 16-bit Timer/Counter Registers for details.
Related Links
Accessing 16-bit Timer/Counter Registers
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20.15.7 Output Compare Register 1 B Low and High byte
Name: OCR1BL and OCR1BH
Offset: 0x8A
Reset: 0x00
Property: -
The OCR1BL and OCR1BH register pair represents the 16-bit value, OCR1B. The low byte [7:0] (suffix L)
is accessible at the original offset. The high byte [15:8] (suffix H) can be accessed at offset + 0x01. For
more details on reading and writing 16-bit registers, refer to Accessing 16-bit Timer/Counter Registers.
Bit 15 14 13 12 11 10 9 8
OCR1B[15:8]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 6 5 4 3 2 1 0
OCR1B[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 15:0 – OCR1B[15:0] Output Compare 1 B
The output compare registers contain a 16-bit value that is continuously compared with the counter value
(TCNT1). A match can be used to generate an output compare interrupt or to generate a waveform
output on the OC1B pin.
The output compare registers are 16-bit in size. To ensure that both the high and low bytes are written
simultaneously when the CPU writes to these registers, the access is performed using an 8-bit temporary
high byte register (TEMP). This temporary register is shared by all the other 16-bit registers. Refer to
Accessing 16-bit Timer/Counter Registers for details.
Related Links
Accessing 16-bit Timer/Counter Registers
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20.15.8 Timer/Counter 1 Interrupt Mask Register
Name: TIMSK1
Offset: 0x6F
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
ICIE1 OCIE1B OCIE1A TOIE1
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 5 – ICIE1 Timer/Counter 1, Input Capture Interrupt Enable
When this bit is written to '1', and the I-flag in the Status register is set (interrupts globally enabled), the
timer/counter 1 input capture interrupt is enabled. The corresponding interrupt vector is executed when
the ICF1 flag, located in TIFR1, is set.
Bit 2 – OCIE1B Timer/Counter 1, Output Compare B Match Interrupt Enable
When this bit is written to '1', and the I-flag in the Status register is set (interrupts globally enabled), the
timer/counter 1 output compare B match interrupt is enabled. The corresponding interrupt vector is
executed when the OCF1B flag, located in TIFR1, is set.
Bit 1 – OCIE1A Timer/Counter 1, Output Compare A Match Interrupt Enable
When this bit is written to '1', and the I-flag in the Status register is set (interrupts globally enabled), the
timer/counter 1 output compare A match interrupt is enabled. The corresponding interrupt vector is
executed when the OCF1A flag, located in TIFR1, is set.
Bit 0 – TOIE1 Timer/Counter 1, Overflow Interrupt Enable
When this bit is written to '1', and the I-flag in the Status register is set (interrupts globally enabled), the
timer/counter 1 overflow interrupt is enabled. The corresponding interrupt vector is executed when the
TOV1 flag, located in TIFR1, is set.
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20.15.9 TC1 Interrupt Flag Register
Name: TIFR1
Offset: 0x36
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x16
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
ICF1 OCF1B OCF1A TOV1
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 5 – ICF1 Timer/Counter 1, Input Capture Flag
This flag is set when a capture event occurs on the ICP1 pin. When the Input Capture Register (ICR1) is
set by the WGM1[3:0] to be used as the TOP value, the ICF1 flag is set when the counter reaches the
TOP value.
ICF1 is automatically cleared when the input capture interrupt vector is executed. Alternatively, ICF1 can
be cleared by writing a logic one to its bit location.
Bit 2 – OCF1B Timer/Counter 1, Output Compare B Match Flag
This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output Compare
Register B (OCR1B).
Note that a Forced Output Compare (FOC1B) strobe will not set the OCF1B flag.
OCF1B is automatically cleared when the output compare match B interrupt vector is executed.
Alternatively, OCF1B can be cleared by writing a logic one to its bit location.
Bit 1 – OCF1A Timer/Counter 1, Output Compare A Match Flag
This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output Compare
Register A (OCR1A).
Note that a Forced Output Compare (FOC1A) strobe will not set the OCF1A flag.
OCF1A is automatically cleared when the output compare match A interrupt vector is executed.
Alternatively, OCF1A can be cleared by writing a logic one to its bit location.
Bit 0 – TOV1 Timer/Counter 1, Overflow Flag
The setting of this flag is dependent on the WGM1[3:0] bits setting. In Normal and CTC modes, the TOV1
flag is set when the timer overflows. Refer to the Waveform Generation mode bit description for the TOV1
flag behavior when using another WGM1[3:0] bit setting.
TOV1 is automatically cleared when the timer/counter 1 overflow interrupt vector is executed.
Alternatively, TOV1 can be cleared by writing a logic one to its bit location.
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21. Timer/Counter 0, 1 Prescalers
The 8-bit Timer/Counter0 (TC0) and the 16-bit Timer/Counter1 (TC1) share the same prescaler module,
but the timer/counters can have different prescaler settings. The following description applies to TC0,
TC1.
Related Links
8-bit Timer/Counter0 (TC0) with PWM
16-bit Timer/Counter1 (TC1) with PWM
21.1 Internal Clock Source
The timer/counter can be clocked directly by the system clock (by setting the CSn[2:0]=0x01). This
provides the fastest operation, with a maximum timer/counter clock frequency equal to system clock
frequency (fCLK_I/O). Alternatively, one of four taps from the prescaler can be used as a clock source. The
prescaled clock has a frequency of either fCLK_I/O/8, fCLK_I/O/64, fCLK_I/O/256, or fCLK_I/O/1024.
21.2 Prescaler Reset
The prescaler is free-running, i.e., it operates independently of the clock select logic of the timer/counter,
and it is shared by timer/counter1 and timer/counter0. Since the prescaler is not affected by the timer/
counter’s clock select, the state of the prescaler will have implications for situations where a prescaled
clock is used. One example of prescaling artifacts occurs when the timer is enabled and clocked by the
prescaler (0x06 > CSn[2:0] > 0x01). The number of system clock cycles from when the timer is enabled to
the first count occurs can be from 1 to N+1 system clock cycles, where N equals the prescaler divisor (8,
64, 256, or 1024).
It is possible to use the prescaler Reset for synchronizing the timer/counter to program execution.
However, care must be taken if the other timer/counter that shares the same prescaler also uses
prescaling. A prescaler Reset will affect the prescaler period for all timer/counters it is connected to.
21.3 External Clock Source
An external clock source applied to the T1/T0 pin can be used as timer/counter clock (clkT1/clkT0). The
T1/T0 pin is sampled once every system clock cycle by the pin synchronization logic. The synchronized
(sampled) signal is then passed through the edge detector. See the block diagram of the T1/T0
synchronization and edge detector logic below. The registers are clocked at the positive edge of the
internal system clock (clkI/O). The latch is transparent in the high period of the internal system clock.
The edge detector generates one clkT1/clkT0 pulse for each positive (CSn[2:0]=0x7) or negative
(CSn[2:0]=0x6) edge it detects.
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Figure 21-1. T1/T0 Pin Sampling
Tn_sync
(To Clock
Select Logic)
Edge DetectorSynchronization
D QD Q
LE
D Q
Tn
clkI/O
The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles from an
edge has been applied to the T1/T0 pin to the counter is updated.
Enabling and disabling of the clock input must be done when T1/T0 has been stable for at least one
system clock cycle, otherwise it is a risk that a false timer/counter clock pulse is generated.
Each half period of the external clock applied must be longer than one system clock cycle to ensure
correct sampling. The external clock must be guaranteed to have less than half the system clock
frequency (fTn < fclk_I/O/2) given a 50% duty cycle. Since the edge detector uses sampling, the maximum
frequency of an external clock it can detect is half the sampling frequency (Nyquist sampling theorem).
However, due to variation of the system clock frequency and duty cycle caused by the tolerances of the
oscillator source (crystal, resonator, and capacitors), it is recommended that maximum frequency of an
external clock source is less than fclk_I/O/2.5.
An external clock source cannot be prescaled.
Figure 21-2. Prescaler for Timer/Counter0 and Timer/Counter1(1)
CSn0
CSn1
CSn2
Synchronization
10-BIT T/C PRESCALER
Tn
clkI/O
PSR10
Clear
CK/8
CK/256
CK/64
CK/1024
OFF
TIMER /COUNTERn CLOCK
SOURCE clk Tn
Note: 1. The synchronization logic on the input pins (T1/T0) is shown in the block diagram above.
ATmega328/P
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21.4 Register Description
ATmega328/P
Timer/Counter 0, 1 Prescalers
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21.4.1 General Timer/Counter Control Register
Name: GTCCR
Offset: 0x43
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x23
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
TSM PSRASY PSRSYNC
Access R/W R/W R/W
Reset 0 0 0
Bit 7 – TSM Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the value
that is written to the PSRASY and PSRSYNC bits is kept, hence keeping the corresponding prescaler
Reset signals asserted. This ensures that the corresponding timer/counters are halted and can be
configured to the same value without the risk of one of them advancing during configuration. When the
TSM bit is written to zero, the PSRASY and PSRSYNC bits are cleared by hardware, and the timer/
counters start counting simultaneously.
Bit 1 – PSRASY Prescaler Reset Timer/Counter2
When this bit is one, the timer/counter2 prescaler will be reset. This bit is normally cleared immediately by
hardware. If the bit is written when timer/counter2 is operating in Asynchronous mode, the bit will remain
one until the prescaler has been Reset. The bit will not be cleared by hardware if the TSM bit is set.
Bit 0 – PSRSYNC Prescaler Reset
When this bit is one, timer/counter 0, 1 prescaler will be Reset. This bit is normally cleared immediately by
hardware, except if the TSM bit is set. Note that timer/counter 0, 1 share the same prescaler and a Reset
of this prescaler will affect the mentioned timers.
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22. 8-bit Timer/Counter2 (TC2) with PWM and Asynchronous Operation
22.1 Features
• Channel Counter
• Clear Timer on Compare Match (Auto Reload)
• Glitch-free, Phase Correct Pulse-Width Modulator (PWM)
• Frequency Generator
• 10-bit Clock Prescaler
• Overflow and Compare Match Interrupt Sources (TOV2, OCF2A, and OCF2B)
• Allows Clocking from External 32 kHz Watch Crystal Independent of the I/O Clock
22.2 Overview
Timer/Counter2 (TC2) is a general purpose, channel, 8-bit timer/counter module.
A simplified block diagram of the 8-bit timer/counter is shown below. CPU accessible I/O registers,
including I/O bits and I/O pins, are shown in bold. The device-specific I/O register and bit locations are
listed in the following register description. For the actual placement of I/O pins, refer to the pinout
diagram.
The TC2 is enabled when the PRTIM2 bit in the Power Reduction Register (PRR.PRTIM2) is written to
'1'.
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Figure 22-1. 8-bit Timer/Counter Block Diagram
Clock Select
Timer/Counter
DATA BUS
OCRnA
OCRnB
=
=
TCNTn
Waveform
Generation
Waveform
Generation
OCnA
OCnB
=
Fixed
TOP
Value
Control Logic
= 0
TOP BOTTOM
Count
Clear
Direction
TOVn
(Int.Req.)
OCnA
(Int.Req.)
OCnB
(Int.Req.)
TCCRnA TCCRnB
Tn
Edge
Detector
( From Prescaler )
clkTn
Related Links
Pin Configurations
Pin Descriptions
22.2.1 Definitions
Many register and bit references in this section are written in general form:
• n=2 represents the timer/counter number
• x=A,B represents the output compare Unit A or B
However, when using the register or bit definitions in a program, the precise form must be used, i.e.,
TCNT2 for accessing timer/counter2 counter value.
The following definitions are used throughout the section:
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Table 22-1. Definitions
Constant Description
BOTTOM The counter reaches the BOTTOM when it becomes zero (0x00).
MAX The counter reaches its maximum when it becomes 0xFF (decimal 255).
TOP The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be the fixed value 0xFF (MAX) or the value
stored in the OCR2A Register. The assignment is dependent on the mode of operation.
22.2.2 Registers
The Timer/Counter (TCNT2) and Output Compare Register (OCR2A and OCR2B) are 8-bit registers.
Interrupt request (shorten as Int.Req.) signals are all visible in the Timer Interrupt Flag Register (TIFR2).
All interrupts are individually masked with the Timer Interrupt Mask register (TIMSK2). TIFR2 and
TIMSK2 are not shown in the figure.
The timer/counter can be clocked internally, via the prescaler, or asynchronously clocked from the
TOSC1/2 pins, as detailed later in this section. The asynchronous operation is controlled by the
Asynchronous Status Register (ASSR). The clock select logic block controls which clock source the timer/
counter uses to increment (or decrement) its value. The timer/counter is inactive when no clock source is
selected. The output from the clock select logic is referred to as the timer clock (clkT2).
The double buffered Output Compare Register (OCR2A and OCR2B) are compared with the timer/
counter value at all times. The result of the compare can be used by the waveform generator to generate
a PWM or variable frequency output on the Output Compare pins (OC2A and OC2B). See Output
Compare Unit for details. The compare match event will also set the Compare Flag (OCF2A or OCF2B),
which can be used to generate an output compare interrupt request.
22.3 Timer/Counter Clock Sources
The timer/counter can be clocked by an internal synchronous or an external asynchronous clock source:
The clock source clkT2 is by default equal/synchronous to the MCU clock, clkI/O.
When the Asynchronous TC2 bit in the Asynchronous Status Register (ASSR.AS2) is written to '1', the
clock source is taken from the Timer/Counter Oscillator connected to TOSC1 and TOSC2.
For details on asynchronous operation, see the description of the ASSR. For details on clock sources and
prescaler, see Timer/Counter Prescaler.
22.4 Counter Unit
The main part of the 8-bit timer/counter is the programmable bi-directional counter unit. Below is the block
diagram of the counter and its surroundings.
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Figure 22-2. Counter Unit Block Diagram
DATA BUS
TCNTn Control Logic
TOVn
(Int.Req.)
topbottom
direction
TOSC1
T/C
Oscillator
TOSC2
Prescaler
clkI/O
clk Tn
clear
count
Table 22-2. Signal description (internal signals):
Signal name Description
count Increment or decrement TCNT2 by 1.
direction Selects between increment and decrement.
clear Clear TCNT2 (set all bits to zero).
clkTn Timer/counter clock, referred to as clkT2 in the following.
top Signalizes that TCNT2 has reached maximum value.
bottom Signalizes that TCNT2 has reached minimum value (zero).
Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each
timer clock (clkT2). clkT2 can be generated from an external or internal clock source, selected by the Clock
Select bits (CS2[2:0]). When no clock source is selected (CS2[2:0]=0x0) the timer is stopped. However,
the TCNT2 value can be accessed by the CPU, regardless of whether clkT2 is present or not. A CPU write
overrides (has priority over) all counter clear or count operations.
The counting sequence is determined by the setting of the WGM21 and WGM20 bits located in the Timer/
Counter Control Register (TCCR2A) and the WGM22 bit located in the Timer/Counter Control Register B
(TCCR2B). There are close connections between how the counter behaves (counts) and how waveforms
are generated on the Output Compare outputs OC2A and OC2B. For more details about advanced
counting sequences and waveform generation, see Modes of Operation.
The Timer/Counter Overflow Flag (TOV2) is set according to the mode of operation selected by the
TCC2B.WGM2[2:0] bits. TOV2 can be used for generating a CPU interrupt.
22.5 Output Compare Unit
The 8-bit comparator continuously compares TCNT2 with the Output Compare Register (OCR2A and
OCR2B). Whenever TCNT2 equals OCR2A or OCR2B, the comparator signals a match. A match will set
the Output Compare Flag (OCF2A or OCF2B) at the next timer clock cycle. If the corresponding interrupt
is enabled, the output compare flag generates an output compare interrupt. The output compare flag is
automatically cleared when the interrupt is executed. Alternatively, the output compare flag can be
cleared by software by writing a logical one to its I/O bit location. The waveform generator uses the match
signal to generate an output according to operating mode set by the WGM2[2:0] bits and Compare
Output mode (COM2x[1:0]) bits. The max and bottom signals are used by the waveform generator for
handling the special cases of the extreme values in some modes of operation (See Modes of Operation).
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The following figure shows a block diagram of the output compare unit.
Figure 22-3. Output Compare Unit, Block Diagram
OCFnx (Int.Req.)
=(8-bit Comparator )
OCRnx
OCnx
DATA BUS
TCNTn
WGMn[1:0]
Waveform Generator
top
FOCn
COMnx[1:0]
bottom
The OCR2x is double buffered when using any of the Pulse Width Modulation (PWM) modes. For the
Normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The
double buffering synchronizes the update of the OCR2x to either top or bottom of the counting sequence.
The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby
making the output glitch-free.
The OCR2x access may seem complex, but this is not the case. When the double buffering is enabled,
the CPU has access to the OCR2x buffer register, and if double buffering is disabled the CPU will access
the OCR2x directly.
Related Links
Modes of Operation
22.5.1 Force Output Compare
In non-PWM Waveform Generation modes, the match output of the comparator can be forced by writing a
one to the Force Output Compare (FOC2x) bit. Forcing compare match will not set the OCF2x flag or
reload/clear the timer, but the OC2x pin will be updated as if a real compare match had occurred (the
COM2x[1:0] bits settings define whether the OC2x pin is set, cleared or toggled).
22.5.2 Compare Match Blocking by TCNT2 Write
All CPU write operations to the TCNT2 register will block any compare match that occurs in the next timer
clock cycle, even when the timer is stopped. This feature allows OCR2x to be initialized to the same
value as TCNT2 without triggering an interrupt when the timer/counter clock is enabled.
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22.5.3 Using the Output Compare Unit
Since writing TCNT2 in any mode of operation will block all compare matches for one timer clock cycle,
there are risks involved when changing TCNT2 when using the output compare channel, independently of
whether the Timer/Counter is running or not. If the value written to TCNT2 equals the OCR2x value, the
compare match will be missed, resulting in incorrect waveform generation. Similarly, do not write the
TCNT2 value equal to BOTTOM when the counter is counting down.
The setup of the OC2x should be performed before setting the data direction register for the port pin to
output. The easiest way of setting the OC2x value is to use the Force Output Compare (FOC2x) strobe
bit in Normal mode. The OC2x register keeps its value even when changing between Waveform
Generation modes.
Be aware that the COM2x[1:0] bits are not double buffered together with the compare value. Changing
the COM2x[1:0] bits will take effect immediately.
22.6 Compare Match Output Unit
The Compare Output mode (COM2x[1:0]) bits have two functions. The waveform generator uses the
COM2x[1:0] bits for defining the Output Compare (OC2x) state at the next compare match. Also, the
COM2x[1:0] bits control the OC2x pin output source. The following figure shows a simplified schematic of
the logic affected by the COM2x[1:0] bit setting. The I/O registers, I/O bits, and I/O pins in the figure are
shown in bold. Only the parts of the general I/O Port Control registers (DDR and PORT) that are affected
by the COM2x[1:0] bits are shown. When referring to the OC2x state, the reference is for the internal
OC2x register, not the OC2x pin.
Figure 22-4. Compare Match Output Unit, Schematic
PORT
DDR
D Q
D Q
OCnx
Pin
OCnx
D Q
Waveform
Generator
COMnx[1]
COMnx[0]
0
1
DATA BUS
FOCnx
clkI/O
The general I/O port function is overridden by the Output Compare (OC2x) from the waveform generator
if either of the COM2x1:0 bits are set. However, the OC2x pin direction (input or output) is still controlled
by the Data Direction Register (DDR) for the port pin. The DDR bit for the OC2x pin (DDR_OC2x) must
be set as output before the OC2x value is visible on the pin. The port override function is independent of
the Waveform Generation mode.
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The design of the output compare pin logic allows initialization of the OC2x state before the output is
enabled. Note that some COM2x[1:0] bit settings are reserved for certain modes of operation. See
Register Description.
Related Links
Modes of Operation
22.6.1 Compare Output Mode and Waveform Generation
The waveform generator uses the COM2x[1:0] bits differently in normal, CTC, and PWM modes. For all
modes, setting the COM2x[1:0] = 0 tells the waveform generator that no action on the OC2x register is to
be performed on the next compare match. Refer also to the descriptions of the output modes.
A change of the COM2x[1:0] bits state will have effect at the first compare match after the bits are written.
For non-PWM modes, the action can be forced to have an immediate effect by using the FOC2x strobe
bits.
22.7 Modes of Operation
The mode of operation, i.e., the behavior of the timer/counter and the output compare pins, is defined by
the combination of the Waveform Generation mode (WGM2[2:0]) and Compare Output mode
(COM2x[1:0]) bits. The Compare Output mode bits do not affect the counting sequence, while the
Waveform Generation mode bits do. The COM2x[1:0] bits control whether the PWM output generated
should be inverted or not (inverted or non-inverted PWM). For non-PWM modes, the COM2x[1:0] bits
control whether the output should be set, cleared, or toggled at a compare match (See Compare Match
Output Unit).
For detailed timing information refer to Timer/Counter Timing Diagrams.
22.7.1 Normal Mode
The simplest mode of operation is the Normal mode (WGM2[2:0] = 0). In this mode, the counting direction
is always up (incrementing), and no counter clear is performed. The counter simply overruns when it
passes its maximum 8-bit value (TOP = 0xFF) and then restarts from the bottom (0x00). In normal
operation, the Timer/Counter Overflow Flag (TOV2) will be set in the same timer clock cycle as the
TCNT2 becomes zero. The TOV2 flag, in this case, behaves like a ninth bit, except that it is only set, not
cleared. However, combined with the timer overflow interrupt that automatically clears the TOV2 flag, the
timer resolution can be increased by software. There are no special cases to consider in the Normal
mode, a new counter value can be written anytime.
The output compare unit can be used to generate interrupts at some given time. Using the output
compare to generate waveforms in Normal mode is not recommended since this will occupy too much of
the CPU time.
22.7.2 Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM2[2:0] = 2), the OCR2A Register is used to manipulate
the counter resolution. In CTC mode the counter is cleared to zero when the counter value (TCNT2)
matches the OCR2A. The OCR2A defines the top value for the counter, hence also its resolution. This
mode allows greater control of the compare match output frequency. It also simplifies the operation of
counting external events.
The timing diagram for the CTC mode is as follows. The counter value (TCNT2) increases until a
compare match occurs between TCNT2 and OCR2A, and then counter (TCNT2) is cleared.
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Figure 22-5. CTC Mode, Timing Diagram
An interrupt can be generated each time the counter value reaches the TOP value by using the OCF2A
Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating the TOP value.
However, changing TOP to a value close to BOTTOM when the counter is running with none or a low
prescaler value must be done with care since the CTC mode does not have the double buffering feature.
If the new value written to OCR2A is lower than the current value of TCNT2, the counter will miss the
compare match. The counter will then have to count to its maximum value (0xFF) and wrap around
starting at 0x00 before the compare match can occur.
For generating a waveform output in CTC mode, the OC2A output can be set to toggle its logical level on
each compare match by setting the Compare Output mode bits to toggle mode (COM2A[1:0] = 1). The
OC2A value will not be visible on the port pin unless the data direction for the pin is set to output. The
waveform generated will have a maximum frequency of fOC2A = fclk_I/O/2 when OCR2A is set to zero
(0x00). The waveform frequency is defined by the following equation:
OCnx =clk_I/O
2 1 + OCRnx
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
As for the Normal mode of operation, the TOV2 Flag is set in the same timer clock cycle that the counter
counts from MAX to 0x00.
22.7.3 Fast PWM Mode
The fast Pulse-Width Modulation (fast PWM) mode (WGM2[2:0] = 0x3 or 0x7) provides a high frequency
PWM waveform generation option. The fast PWM differs from the other PWM option by its single-slope
operation. The counter counts from BOTTOM to TOP then restarts from BOTTOM. TOP is defined as
0xFF when WGM2[2:0] = 0x3, and OCR2A when WGM2[2:0] = 0x7. In non-inverting Compare Output
mode, the Output Compare (OC2x) is cleared on the compare match between TCNT2 and OCR2x and
set at BOTTOM. In inverting Compare Output mode, the output is set on compare match and cleared at
BOTTOM. Due to the single-slope operation, the operating frequency of the fast PWM mode can be twice
as high as the phase correct PWM mode that uses dual-slope operation. This high frequency makes the
fast PWM mode well suited for power regulation, rectification, and DAC applications. High frequency
allows physically small sized external components (coils, capacitors), and therefore reduces total system
cost.
In fast PWM mode, the counter is incremented until the counter value matches the TOP value. The
counter is then cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is
depicted in the following figure. The TCNT2 value is in the timing diagram shown as a histogram for
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illustrating the single-slope operation. The diagram includes non-inverted and inverted PWM outputs. The
small horizontal line marks on the TCNT2 slopes represent compare matches between OCR2x and
TCNT2.
Figure 22-6. Fast PWM Mode, Timing Diagram
TCNTn
OCRnx Update and
TOVn Interrupt Flag Set
1
Period 2 3
OCnx
OCnx
(COMnx[1:0] = 0x2)
(COMnx[1:0] = 0x3)
OCRnx Interrupt Flag Set
4 5 6 7
The Timer/Counter Overflow flag (TOV2) is set each time the counter reaches TOP. If the interrupt is
enabled, the interrupt handler routine can be used for updating the compare value.
In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC2x pin. Setting the
COM2x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated
by setting the COM2x[1:0] to three. TOP is defined as 0xFF when WGM2[2:0] = 0x3, and OCR2A when
MGM2[2:0] = 0x7. The actual OC2x value will only be visible on the port pin if the data direction for the
port pin is set as output. The PWM waveform is generated by setting (or clearing) the OC2x register at
the compare match between OCR2x and TCNT2, and clearing (or setting) the OC2x register at the timer
clock cycle the counter is cleared (changes from TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
OCnxPWM =clk_I/O
256
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2A register represent special cases when generating a PWM waveform
output in the fast PWM mode. If the OCR2A is set equal to BOTTOM, the output will be a narrow spike for
each MAX+1 timer clock cycle. Setting the OCR2A equal to MAX will result in a constantly high or low
output (depending on the polarity of the output set by the COM2A[1:0] bits).
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC2x
to toggle its logical level on each compare match (COM2x[1:0] = 1). The waveform generated will have a
maximum frequency of foc2 = fclk_I/O/2 when OCR2A is set to zero. This feature is similar to the OC2A
toggle in CTC mode, except the double buffer feature of the output compare unit is enabled in the fast
PWM mode.
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22.7.4 Phase Correct PWM Mode
The phase correct PWM mode (WGM2[2:0] = 0x1 or 0x5) provides a high resolution phase correct PWM
waveform generation option. The phase correct PWM mode is based on a dual-slope operation. The
counter counts repeatedly from BOTTOM to TOP and then from TOP to BOTTOM. TOP is defined as
0xFF when WGM2[2:0] = 0x3, and OCR2A when MGM2[2:0] = 7. In non-inverting Compare Output
mode, the Output Compare (OC2x) is cleared on the compare match between TCNT2 and OCR2x while
counting up, and set on the compare match while down-counting. In inverting Output Compare mode, the
operation is inverted. The dual-slope operation has lower maximum operation frequency than single-
slope operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are
preferred for motor control applications.
In phase correct PWM mode the counter is incremented until the counter value matches TOP. When the
counter reaches TOP, it changes the count direction. The TCNT2 value will be equal to TOP for one timer
clock cycle. The timing diagram for the phase correct PWM mode is shown in Figure 22-7. The TCNT2
value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. The diagram
includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT2 slopes
represent compare matches between OCR2x and TCNT2.
Figure 22-7. Phase Correct PWM Mode, Timing Diagram
TOVn Interrupt Flag Set
OCnx Interrupt Flag Set
1 2 3
TCNTn
Period
OCnx
OCnx
(COMnx[1:0] = 2)
(COMnx[1:0] = 3)
OCRnx Update
The Timer/Counter Overflow flag (TOV2) is set each time the counter reaches BOTTOM. The interrupt
flag can be used to generate an interrupt each time the counter reaches the BOTTOM value.
In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the OC2x pin.
Setting the COM2x[1:0] bits to two will produce a non-inverted PWM. An inverted PWM output can be
generated by setting the COM2x[1:0] to three. TOP is defined as 0xFF when WGM2[2:0] = 0x3, and
OCR2A when WGM2[2:0] = 7. The actual OC2x value will only be visible on the port pin if the data
direction for the port pin is set as output. The PWM waveform is generated by clearing (or setting) the
OC2x Register at the compare match between OCR2x and TCNT2 when the counter increments, and
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setting (or clearing) the OC2x register at compare match between OCR2x and TCNT2 when the counter
decrements. The PWM frequency for the output when using phase correct PWM can be calculated by the
following equation:
OCnxPCPWM =clk_I/O
510
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2A represent special cases when generating a PWM waveform output in
the phase correct PWM mode. If the OCR2A is set equal to BOTTOM, the output will be continuously low
and if set equal to MAX the output will be continuously high for non-inverted PWM mode. For inverted
PWM the output will have the opposite logic values.
At the very start of period 2 in the above figure OC2x has a transition from high to low even though there
is no compare match. The point of this transition is to guarantee symmetry around BOTTOM. There are
two cases that give a transition without compare match.
• OCR2A changes its value from MAX, as shown in the preceding figure. When the OCR2A value is
MAX the OC2 pin value is the same as the result of a down-counting compare match. To ensure
symmetry around BOTTOM the OC2 value at MAX must correspond to the result of an up-counting
Compare Match.
• The timer starts counting from a value higher than the one in OCR2A, and for that reason misses
the compare match and hence the OC2 change that would have happened on the way up.
22.8 Timer/Counter Timing Diagrams
The following figures show the timer/counter in Synchronous mode, and the timer clock (clkT2) is
therefore shown as a clock enable signal. In Asynchronous mode, clkI/O should be replaced by the timer/
counter oscillator clock. The figures include information on when interrupt flags are set. The following
figure contains timing data for basic timer/counter operation. The figure shows the count sequence close
to the MAX value in all modes other than phase correct PWM mode.
Figure 22-8. Timer/Counter Timing Diagram, no Prescaling
clkTn
(clkI/O/1)
TOVn
clkI/O
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
The following figure shows the same timing data, but with the prescaler enabled.
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Figure 22-9. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
TOVn
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
clkI/O
clkTn
(clkI/O/8)
The following figure shows the setting of OCF2A in all modes except CTC mode.
Figure 22-10. Timer/Counter Timing Diagram, Setting of OCF2A, with Prescaler (fclk_I/O/8)
OCFnx
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
clkI/O
clkTn
(clkI/O/8)
The following figure shows the setting of OCF2A and the clearing of TCNT2 in CTC mode.
Figure 22-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with
Prescaler (fclk_I/O/8)
OCFnx
OCRnx
TCNTn
(CTC)
TOP
TOP - 1 TOP BOTTOM BOTTOM + 1
clkI/O
clkTn
(clkI/O/8)
22.9 Asynchronous Operation of Timer/Counter2
When TC2 operates asynchronously, some considerations must be taken:
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• When switching between asynchronous and synchronous clocking of TC2, the registers TCNT2,
OCR2x, and TCCR2x might be corrupted. A safe procedure for switching clock source is:
1. Disable the TC2 interrupts by clearing OCIE2x and TOIE2.
2. Select clock source by setting AS2 as appropriate.
3. Write new values to TCNT2, OCR2x, and TCCR2x.
4. To switch to asynchronous operation: Wait for TCN2xUB, OCR2xUB, and TCR2xUB.
5. Clear the TC2 interrupt flags.
6. Enable interrupts, if needed.
• The CPU main clock frequency must be more than four times the oscillator frequency.
• When writing to one of the registers TCNT2, OCR2x, or TCCR2x, the value is transferred to a
temporary register and latched after two positive edges on TOSC1. The user should not write a
new value before the contents of the temporary register have been transferred to its destination.
Each of the five mentioned registers has its individual temporary register, which means that e.g.
writing to TCNT2 does not disturb an OCR2x write in progress. The Asynchronous Status Register
(ASSR) indicates that a transfer to the destination register has taken place.
• When entering Power-Save or ADC Noise Reduction mode after having written to TCNT2, OCR2x,
or TCCR2x, the user must wait until the written register has been updated if TC2 is used to wake
up the device. Otherwise, the MCU will enter sleep mode before the changes are effective. This is
particularly important if any of the Output Compare2 interrupts is used to wake up the device, since
the Output Compare function is disabled during writing to OCR2x or TCNT2. If the write cycle is not
finished, and the MCU enters sleep mode before the corresponding OCR2xUB bit returns to zero,
the device will never receive a compare match interrupt, and the MCU will not wake up.
• If TC2 is used to wake the device up from Power-Save or ADC Noise Reduction mode, precautions
must be taken if the user wants to re-enter one of these modes: If re-entering sleep mode within the
TOSC1 cycle, the interrupt will immediately occur and the device wakes up again. The result is
multiple interrupts and wake-ups within one TOSC1 cycle from the first interrupt. If the user is in
doubt whether the time before re-entering Power-save or ADC Noise Reduction mode is sufficient,
the following algorithm can be used to ensure that one TOSC1 cycle has elapsed:
1. Write a value to TCCR2x, TCNT2, or OCR2x.
2. Wait until the corresponding update busy flag in ASSR returns to zero.
3. Enter Power-Save or ADC Noise Reduction mode.
• When the asynchronous operation is selected, the 32.768 kHz oscillator for TC2 is always running,
except in Power-Down and Standby modes. After a Power-up Reset or wake-up from Power-Down
or Standby mode, the user should be aware of the fact that this oscillator might take as long as one
second to stabilize. The user is advised to wait for at least one second before using TC2 after
power-up or wake-up from Power-Down or Standby mode. The contents of all TC2 registers must
be considered lost after a wake-up from Power-Down or Standby mode due to unstable clock signal
upon start-up, no matter whether the oscillator is in use or a clock signal is applied to the TOSC1
pin.
• Description of wake up from Power-Save or ADC Noise Reduction mode when the timer is clocked
asynchronously: When the interrupt condition is met, the wake up process is started on the
following cycle of the timer clock, that is, the timer is always advanced by at least one before the
processor can read the counter value. After wake-up, the MCU is halted for four cycles, it executes
the interrupt routine, and resumes execution from the instruction following SLEEP.
• Reading of the TCNT2 register shortly after wake-up from Power-Save may give an incorrect result.
Since TCNT2 is clocked on the asynchronous TOSC clock, reading TCNT2 must be done through
a register synchronized to the internal I/O clock domain. Synchronization takes place for every
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rising TOSC1 edge. When waking up from Power-Save mode, and the I/O clock (clkI/O) again
becomes active, TCNT2 will read as the previous value (before entering sleep) until the next rising
TOSC1 edge. The phase of the TOSC clock after waking up from Power-Save mode is essentially
unpredictable, as it depends on the wake-up time. The recommended procedure for reading
TCNT2 is thus as follows:
8.1. Wait for the corresponding update busy flag to be cleared.
8.2. Read TCNT2.
• During asynchronous operation, the synchronization of the interrupt flags for the asynchronous
timer takes three processor cycles plus one timer cycle. The timer is therefore advanced by at least
one before the processor can read the timer value causing the setting of the interrupt flag. The
output compare pin is changed on the timer clock and is not synchronized to the processor clock.
22.10 Timer/Counter Prescaler
Figure 22-12. Prescaler for TC2
10-BIT T/C PRESCALER
TIMER/COUNTER2 CLOCK SOURCE
I/O clkT2S
TOSC1
clkT2S
/8
clkT2S
/64
clkT2S
/128
clkT2S
/1024
clkT2S
/256
clkT2S
/32
0
PSRASY
T2
AS2
CS20
CS21
CS22
Clear
clk
clk
The clock source for TC2 is named clkT2S. It is by default connected to the main system I/O clock clkI/O.
By writing a '1' to the Asynchronous TC2 bit in the Asynchronous Status Register (ASSR.AS2), TC2 is
asynchronously clocked from the TOSC1 pin. This enables the use of TC2 as a Real Time Counter
(RTC). When AS2 is set, pins TOSC1 and TOSC2 are disconnected from Port B. A crystal can then be
connected between the TOSC1 and TOSC2 pins to serve as an independent clock source for TC2. The
oscillator is optimized for use with a 32.768 kHz crystal.
For TC2, the possible prescaled selections are: clkT2S/8, clkT2S/32, clkT2S/64, clkT2S/128, clkT2S/256, and
clkT2S/1024. Additionally, clkT2S, as well as 0 (stop), may be selected. The prescaler is reset by writing a
'1' to the Prescaler Reset TC2 bit in the General TC2 Control Register (GTCCR.PSRASY). This allows
the user to operate with a defined prescaler.
22.11 Register Description
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22.11.1 TC2 Control Register A
Name: TCCR2A
Offset: 0xB0
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
COM2A[1:0] COM2B[1:0] WGM2[1:0]
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bits 7:6 – COM2A[1:0] Compare Output Mode for Channel A
These bits control the Output Compare pin (OC2A) behavior. If one or both of the COM2A[1:0] bits are
set, the OC2A output overrides the normal port functionality of the I/O pin it is connected to. However,
note that the Data Direction Register (DDR) bit corresponding to the OC2A pin must be set in order to
enable the output driver.
When OC2A is connected to the pin, the function of the COM2A[1:0] bits depends on the WGM2[2:0] bit
setting. The table below shows the COM2A[1:0] bit functionality when the WGM2[2:0] bits are set to a
normal or CTC mode (non-PWM).
Table 22-3. Compare Output Mode, Non-PWM
COM2A[1] COM2A[0] Description
0 0 Normal port operation, OC2A disconnected.
0 1 Toggle OC2A on compare match.
1 0 Clear OC2A on compare match.
1 1 Set OC2A on compare match .
The table below shows the COM2A[1:0] bit functionality when the WGM2[1:0] bits are set to fast PWM
mode.
Table 22-4. Compare Output Mode, Fast PWM(1)
COM2A[1] COM2A[0] Description
0 0 Normal port operation, OC2A disconnected.
0 1 WGM2[2:0]: Normal port operation, OC2A disconnected
WGM2[2:1]: Toggle OC2A on compare match
1 0 Clear OC2A on compare match, set OC2A at BOTTOM (non-inverting mode)
1 1 Set OC2A on compare match, clear OC2A at BOTTOM (inverting mode)
Note:
1. A special case occurs when OCR2A equals TOP and COM2A[1] is set. In this case the compare
match is ignored, but the set or clear is done at BOTTOM. Refer to Fast PWM Mode for details.
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The table below shows the COM2A[1:0] bit functionality when the WGM2[2:0] bits are set to phase
correct PWM mode.
Table 22-5. Compare Output Mode, Phase Correct PWM Mode(1)
COM2A[1] COM2A[0] Description
0 0 Normal port operation, OC2A disconnected.
0 1 WGM2[2 :0]: Normal port operation, OC2A disconnected.
WGM2[2:1]: Toggle OC2A on compare match.
1 0 Clear OC2A on compare match when up-counting. Set OC2A on compare
match when down-counting.
1 1 Set OC2A on compare match when up-counting. Clear OC2A on compare
match when down-counting.
Note:
1. A special case occurs when OCR2A equals TOP and COM2A1 is set. In this case, the compare
match is ignored, but the set or clear is done at TOP. Refer to Phase Correct PWM Mode for
details.
Bits 5:4 – COM2B[1:0] Compare Output Mode for Channel B
These bits control the Output Compare pin (OC2B) behavior. If one or both of the COM2B[1:0] bits are
set, the OC2B output overrides the normal port functionality of the I/O pin it is connected to. However,
note that the Data Direction Register (DDR) bit corresponding to the OC2B pin must be set in order to
enable the output driver.
When OC2B is connected to the pin, the function of the COM2B[1:0] bits depends on the WGM2[2:0] bit
setting. The table shows the COM2B[1:0] bit functionality when the WGM2[2:0] bits are set to a normal or
CTC mode (non- PWM).
Table 22-6. Compare Output Mode, Non-PWM
COM2B[1] COM2B[0] Description
0 0 Normal port operation, OC2B disconnected.
0 1 Toggle OC2B on compare match.
1 0 Clear OC2B on compare match.
1 1 Set OC2B on compare match.
The table below shows the COM0B[1:0] bit functionality when the WGM0[2:0] bits are set to fast PWM
mode.
Table 22-7. Compare Output Mode, Fast PWM(1)
COM2B[1] COM2B[0] Description
0 0 Normal port operation, OC0B disconnected.
0 1 Reserved
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COM2B[1] COM2B[0] Description
1 0 Clear OC0B on compare match, set OC0B at BOTTOM, (non-inverting mode)
1 1 Set OC0B on compare match, clear OC0B at BOTTOM, (inverting mode)
Note:
1. A special case occurs when OCR2B equals TOP and COM2B[1] is set. In this case, the compare
match is ignored, but the set or clear is done at TOP. Refer to Fast PWM Mode for details.
The table below shows the COM2B[1:0] bit functionality when the WGM2[2:0] bits are set to phase
correct PWM mode.
Table 22-8. Compare Output Mode, Phase Correct PWM Mode(1)
COM2B[1] COM2B[0] Description
0 0 Normal port operation, OC2B disconnected.
0 1 Reserved
1 0 Clear OC2B on compare match when up-counting. Set OC2B on compare
match when down-counting.
1 1 Set OC2B on compare match when up-counting. Clear OC2B on compare
match when down-counting.
Note:
1. A special case occurs when OCR2B equals TOP and COM2B[1] is set. In this case, the compare
match is ignored, but the set or clear is done at TOP. Refer to Phase Correct PWM Mode for
details.
Bits 1:0 – WGM2[1:0] Waveform Generation Mode
Combined with the WGM2[2] bit found in the TCCR2B register, these bits control the counting sequence
of the counter, the source for maximum (TOP) counter value, and what type of waveform generation to be
used. Modes of operation supported by the Timer/Counter unit are: Normal mode (counter), Clear Timer
on Compare Match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes (see Modes of
Operation).
Table 22-9. Waveform Generation Mode Bit Description
Mode WGM2[2] WGM2[1] WGM2[0] Timer/Counter
Mode of
Operation
TOP Update of
OCR0x at
TOV Flag Set
on(1)
0 0 0 0 Normal 0xFF Immediate MAX
1 0 0 1 PWM, Phase
Correct
0xFF TOP BOTTOM
2 0 1 0 CTC OCR2A Immediate MAX
3 0 1 1 Fast PWM 0xFF BOTTOM MAX
4 1 0 0 Reserved - - -
5 1 0 1 PWM, Phase
Correct
OCR2A TOP BOTTOM
6 1 1 0 Reserved - - -
7 1 1 1 Fast PWM OCR2A BOTTOM TOP
Note:
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1. MAX = 0xFF
2. BOTTOM = 0x00
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22.11.2 TC2 Control Register B
Name: TCCR2B
Offset: 0xB1
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
FOC2A FOC2B WGM22 CS2[2:0]
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bit 7 – FOC2A Force Output Compare A
The FOC2A bit is only active when the WGM bits specify a non-PWM mode.
To ensure compatibility with future devices, this bit must be set to zero when TCCR2B is written when
operating in PWM mode. When writing a logical one to the FOC2A bit, an immediate compare match is
forced on the waveform generation unit. The OC2A output is changed according to its COM2A[1:0] bits
setting. Note that the FOC2A bit is implemented as a strobe. Therefore it is the value present in the
COM2A[1:0] bits that determines the effect of the forced compare.
A FOC2A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR2A as
TOP.
The FOC2A bit is always read as zero.
Bit 6 – FOC2B Force Output Compare B
The FOC2B bit is only active when the WGM bits specify a non-PWM mode.
To ensure compatibility with future devices, this bit must be set to zero when TCCR2B is written when
operating in PWM mode. When writing a logical one to the FOC2B bit, an immediate compare match is
forced on the waveform generation unit. The OC2B output is changed according to its COM2B[1:0] bits
setting. Note that the FOC2B bit is implemented as a strobe. Therefore it is the value present in the
COM2B[1:0] bits that determines the effect of the forced compare.
A FOC2B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR2B as
TOP.
The FOC2B bit is always read as zero.
Bit 3 – WGM22 Waveform Generation Mode
Refer to TCCR2A.
Bits 2:0 – CS2[2:0] Clock Select 2 [n = 0..2]
The three Clock Select bits select the clock source to be used by the timer/counter.
Table 22-10. Clock Select Bit Description
CS22 CS21 CS20 Description
0 0 0 No clock source (Timer/counter stopped).
0 0 1 clkI/O/1 (No prescaling)
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CS22 CS21 CS20 Description
0 1 0 clkI/O/8 (From prescaler)
0 1 1 clkI/O/32 (From prescaler)
1 0 0 clkI/O/64 (From prescaler)
1 0 1 clkI/O/128 (From prescaler)
1 1 0 clkI/O/256 (From prescaler)
1 1 1 clkI/O/1024 (From prescaler)
If external pin modes are used for the timer/counter0, transitions on the T0 pin will clock the counter even
if the pin is configured as an output. This feature allows software control of the counting.
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22.11.3 TC2 Counter Value Register
Name: TCNT2
Offset: 0xB2
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
TCNT2[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TCNT2[7:0] Timer/Counter 2 Counter Value
The Timer/Counter register gives direct access, both for read and write operations, to the Timer/Counter
unit 8-bit counter. Writing to the TCNT2 register blocks (removes) the compare match on the following
timer clock. Modifying the counter (TCNT2) while the counter is running, introduces a risk of missing a
compare match between TCNT2 and the OCR2x registers.
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22.11.4 TC2 Output Compare Register A
Name: OCR2A
Offset: 0xB3
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
OCR2A[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR2A[7:0] Output Compare 2 A
The output compare register A contains an 8-bit value that is continuously compared with the counter
value (TCNT2). A match can be used to generate an output compare interrupt or to generate a waveform
output on the OC2A pin.
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8-bit Timer/Counter2 (TC2) with PWM and A...
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22.11.5 TC2 Output Compare Register B
Name: OCR2B
Offset: 0xB4
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
OCR2B[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR2B[7:0] Output Compare 2 B
The output compare register B contains an 8-bit value that is continuously compared with the counter
value (TCNT2). A match can be used to generate an output compare interrupt or to generate a waveform
output on the OC2B pin.
Related Links
Timer/Counter Oscillator
ATmega328/P
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22.11.6 TC2 Interrupt Mask Register
Name: TIMSK2
Offset: 0x70
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
OCIE2B OCIE2A TOIE2
Access R/W R/W R/W
Reset 0 0 0
Bit 2 – OCIE2B Timer/Counter 2, Output Compare B Match Interrupt Enable
When the OCIE2B bit is written to '1' and the I-bit in the Status register is set (one), the Timer/Counter2
Compare Match B interrupt is enabled. The corresponding interrupt is executed if a compare match in
Timer/Counter 2 occurs, i.e., when the OCF2B bit is set in TIFR2.
Bit 1 – OCIE2A Timer/Counter 2, Output Compare A Match Interrupt Enable
When the OCIE2A bit is written to '1' and the I-bit in the Status register is set (one), the Timer/Counter2
Compare Match A interrupt is enabled. The corresponding interrupt is executed if a compare match in
Timer/Counter 2 occurs, i.e., when the OCF2A bit is set in TIFR2.
Bit 0 – TOIE2 Timer/Counter 2, Overflow Interrupt Enable
When the TOIE2 bit is written to '1' and the I-bit in the Status register is set (one), the Timer/Counter2
Overflow interrupt is enabled. The corresponding interrupt is executed if an overflow in Timer/Counter 2
occurs, i.e., when the TOV2 bit is set in TIFR2.
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22.11.7 TC2 Interrupt Flag Register
Name: TIFR2
Offset: 0x37
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x17
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
OCF2B OCF2A TOV2
Access R/W R/W R/W
Reset 0 0 0
Bit 2 – OCF2B Timer/Counter 2, Output Compare B Match Flag
The OCF2B bit is set (one) when a compare match occurs between the timer/counter2 and the data in
Output Compare Register2 (OCR2B). OCF2B is cleared by hardware when executing the corresponding
interrupt handling vector. Alternatively, OCF2B is cleared by writing a logic one to the flag. When the I-bit
in SREG, OCIE2B (timer/counter2 compare match interrupt enable), and OCF2B are set (one), the timer/
counter2 compare match interrupt is executed.
Bit 1 – OCF2A Timer/Counter 2, Output Compare A Match Flag
The OCF2A bit is set (one) when a compare match occurs between the timer/counter2 and the data in
Output Compare Register2 (OCRA). OCF2A is cleared by hardware when executing the corresponding
interrupt handling vector. Alternatively, OCF2A is cleared by writing a logic one to the flag. When the I-bit
in SREG, OCIE2A (timer/counter2 compare match interrupt enable), and OCF2A are set (one), the timer/
counter 2 compare match interrupt is executed.
Bit 0 – TOV2 Timer/Counter 2, Overflow Flag
The TOV2 bit is set (one) when an overflow occurs in Timer/Counter 2. TOV2 is cleared by hardware
when executing the corresponding interrupt handling vector. Alternatively, TOV2 is cleared by writing a
logic one to the flag. When the SREG I-bit, TOIE2A (timer/counter 2 overflow interrupt enable), and TOV2
are set (one), the timer/counter 2 overflow interrupt is executed. In PWM mode, this bit is set when timer/
counter 2 changes counting direction at 0x00.
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22.11.8 Asynchronous Status Register
Name: ASSR
Offset: 0xB6
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
EXCLK AS2 TCN2UB OCR2AUB OCR2BUB TCR2AUB TCR2BUB
Access R/W R/W R R R R R
Reset 0 0 0 0 0 0 0
Bit 6 – EXCLK Enable External Clock Input
When EXCLK is written to one, and asynchronous clock is selected, the external clock input buffer is
enabled and an external clock can be input on Timer Oscillator 1 (TOSC1) pin instead of a 32 kHz crystal.
Writing to EXCLK should be done before asynchronous operation is selected. Note that the crystal
oscillator will run only when this bit is zero.
Bit 5 – AS2 Asynchronous Timer/Counter2
When AS2 is written to zero, timer/counter2 is clocked from the I/O clock, clkI/O. When AS2 is written to
one, timer/counter2 is clocked from a crystal oscillator connected to the timer oscillator 1 (TOSC1) pin.
When the value of AS2 is changed, the contents of TCNT2, OCR2A, OCR2B, TCCR2A, and TCCR2B
might be corrupted.
Bit 4 – TCN2UB Timer/Counter2 Update Busy
When timer/counter2 operates asynchronously and TCNT2 is written, this bit becomes set. When TCNT2
has been updated from the temporary storage register, this bit is cleared by hardware. A logical zero in
this bit indicates that TCNT2 is ready to be updated with a new value.
Bit 3 – OCR2AUB Output Compare Register2A Update Busy
When timer/counter2 operates asynchronously and OCR2A is written, this bit becomes set. When
OCR2A has been updated from the temporary storage register, this bit is cleared by hardware. A logical
zero in this bit indicates that OCR2A is ready to be updated with a new value.
Bit 2 – OCR2BUB Output Compare Register2B Update Busy
When timer/counter2 operates asynchronously and OCR2B is written, this bit becomes set. When
OCR2B has been updated from the temporary storage register, this bit is cleared by hardware. A logical
zero in this bit indicates that OCR2B is ready to be updated with a new value.
Bit 1 – TCR2AUB Timer/Counter Control Register2 Update Busy
When timer/counter2 operates asynchronously and TCCR2A is written, this bit becomes set. When
TCCR2A has been updated from the temporary storage register, this bit is cleared by hardware. A logical
zero in this bit indicates that TCCR2A is ready to be updated with a new value.
Bit 0 – TCR2BUB Timer/Counter Control Register2 Update Busy
When timer/counter2 operates asynchronously and TCCR2B is written, this bit becomes set. When
TCCR2B has been updated from the temporary storage register, this bit is cleared by hardware. A logical
zero in this bit indicates that TCCR2B is ready to be updated with a new value.
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If a write is performed to any of the five timer/counter2 registers while its update busy flag is set, the
updated value might get corrupted and cause an unintentional interrupt to occur.
Related Links
Timer/Counter Oscillator
ATmega328/P
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22.11.9 General Timer/Counter Control Register
Name: GTCCR
Offset: 0x43
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x23
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
TSM PSRASY PSRSYNC
Access R/W R/W R/W
Reset 0 0 0
Bit 7 – TSM Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the value
that is written to the PSRASY and PSRSYNC bits is kept, hence keeping the corresponding prescaler
Reset signals asserted. This ensures that the corresponding timer/counters are halted and can be
configured to the same value without the risk of one of them advancing during configuration. When the
TSM bit is written to zero, the PSRASY and PSRSYNC bits are cleared by hardware, and the timer/
counters start counting simultaneously.
Bit 1 – PSRASY Prescaler Reset Timer/Counter2
When this bit is one, the timer/counter2 prescaler will be reset. This bit is normally cleared immediately by
hardware. If the bit is written when timer/counter2 is operating in Asynchronous mode, the bit will remain
one until the prescaler has been Reset. The bit will not be cleared by hardware if the TSM bit is set.
Bit 0 – PSRSYNC Prescaler Reset
When this bit is one, timer/counter 0, 1 prescaler will be Reset. This bit is normally cleared immediately by
hardware, except if the TSM bit is set. Note that timer/counter 0, 1 share the same prescaler and a Reset
of this prescaler will affect the mentioned timers.
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23. Serial Peripheral Interface (SPI)
23.1 Features
• Full-duplex, Three-wire Synchronous Data Transfer
• Master or Slave Operation
• LSB First or MSB First Data Transfer
• Seven Programmable Bit Rates
• End of Transmission Interrupt Flag
• Write Collision Flag Protection
• Wake-up from Idle Mode
• Double Speed (CK/2) Master SPI Mode
23.2 Overview
The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer between the device
and peripheral units, or between several AVR devices.
The USART can be used in Master SPI mode, refer to chapter USART in SPI Mode.
To enable the SPI module, Power Reduction Serial Peripheral Interface bit in the Power Reduction
Register (PRR.PRSPI0) must be written to '0'.
ATmega328/P
Serial Peripheral Interface (SPI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 218
Figure 23-1. SPI Block Diagram
SPI2X
SPI2X
DIVIDER
/2/4/8/16/32/64/128
Note: Refer to the pin-out description and the I/O Port description for SPI pin placement.
The interconnection between master and slave CPUs with SPI is shown in the figure below. The system
consists of two shift registers and a master clock generator. The SPI Master initiates the communication
cycle when pulling low the Slave Select SS pin of the desired slave. Master and slave prepare the data to
be sent in their respective shift registers, and the master generates the required clock pulses on the SCK
line to interchange data. Data is always shifted from master to slave on the Master Out – Slave In (MOSI)
line, and from slave to master on the Master In – Slave Out (MISO) line. After each data packet, the
master will synchronize the slave by pulling high the Slave Select, SS, line.
When configured as a master, the SPI interface has no automatic control of the SS line. This must be
handled by user software before communication can start. When this is done, writing a byte to the SPI
Data register starts the SPI clock generator, and the hardware shifts the eight bits into the slave. After
shifting one byte, the SPI clock generator stops, setting the end of Transmission Flag (SPIF). If the SPI
Interrupt Enable bit (SPIE) in the SPCR register is set, an interrupt is requested. The master may
continue to shift the next byte by writing it into SPDR, or signal the end of packet by pulling high the Slave
Select, SS line. The last incoming byte will be kept in the Buffer register for later use.
When configured as a slave, the SPI interface will remain sleeping with MISO tri-stated as long as the SS
pin is driven high. In this state, the software may update the contents of the SPDR, but the data will not
be shifted out by incoming clock pulses on the SCK pin until the SS pin is driven low. As one byte has
been completely shifted, the end of transmission flag, SPIF is set. If the SPIE in the SPCR register is set,
ATmega328/P
Serial Peripheral Interface (SPI)
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an interrupt is requested. The slave may continue to place new data to be sent to SPDR before reading
the incoming data. The last incoming byte will be kept in the Buffer register for later use.
Figure 23-2. SPI Master-Slave Interconnection
SHIFT
ENABLE
The system is single buffered in the transmit direction and double buffered in the receive direction. This
means that bytes to be transmitted cannot be written to the SPI Data register before the entire shift cycle
is completed. When receiving data, however, a received character must be read from the SPI Data
register before the next character has been completely shifted in. Otherwise, the first byte is lost.
In SPI Slave mode, the control logic will sample the incoming signal of the SCK pin. To ensure correct
sampling of the clock signal, the minimum low and high periods should be longer than two CPU clock
cycles.
When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden
according to the table below. For more details on automatic port overrides, refer to the I/O Ports
description.
Table 23-1. SPI Pin Overrides
Pin Direction, Master SPI Direction, Slave SPI
MOSI User Defined Input
MISO Input User Defined
SCK User Defined Input
SS User Defined Input
Note: 1. See the I/O Ports description for how to define the SPI pin directions.
The following code examples show how to initialize the SPI as a master and how to perform a simple
transmission. DDR_SPI in the examples must be replaced by the actual Data Direction register controlling
the SPI pins. DD_MOSI, DD_MISO, and DD_SCK must be replaced by the actual data direction bits for
these pins, for example, if MOSI is placed on pin PB5, replace DD_MOSI with DDB5 and DDR_SPI with
DDRB.
Assembly Code Example
SPI_MasterInit:
; Set MOSI and SCK output, all others input
ldi r17,(1<<DD_MOSI)|(1<<DD_SCK)
out DDR_SPI,r17
; Enable SPI, Master, set clock rate fck/16
ldi r17,(1<<SPE)|(1<<MSTR)|(1<<SPR0)
out SPCR,r17
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Serial Peripheral Interface (SPI)
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ret
SPI_MasterTransmit:
; Start transmission of data (r16)
out SPDR,r16
Wait_Transmit:
; Wait for transmission complete
in r16, SPSR
sbrs r16, SPIF
rjmp Wait_Transmit
ret
C Code Example
void SPI_MasterInit(void)
{
/* Set MOSI and SCK output, all others input */
DDR_SPI = (1<<DD_MOSI)|(1<<DD_SCK);
/* Enable SPI, Master, set clock rate fck/16 */
SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0);
}
void SPI_MasterTransmit(char cData)
{
/* Start transmission */
SPDR = cData;
/* Wait for transmission complete */
while(!(SPSR & (1<<SPIF)))
;
}
The following code examples show how to initialize the SPI as a slave and how to
perform a simple reception.
Assembly Code Example
SPI_SlaveInit:
; Set MISO output, all others input
ldi r17,(1<<DD_MISO)
out DDR_SPI,r17
; Enable SPI
ldi r17,(1<<SPE)
out SPCR,r17
ret
SPI_SlaveReceive:
; Wait for reception complete
in r16, SPSR
sbrs r16, SPIF
rjmp SPI_SlaveReceive
; Read received data and return
in r16,SPDR
ret
C Code Example
void SPI_SlaveInit(void)
{
/* Set MISO output, all others input */
DDR_SPI = (1<<DD_MISO);
/* Enable SPI */
SPCR = (1<<SPE);
}
char SPI_SlaveReceive(void)
{
/* Wait for reception complete */
while(!(SPSR & (1<<SPIF)))
;
/* Return Data Register */
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Serial Peripheral Interface (SPI)
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return SPDR;
}
Related Links
Pin Descriptions
USART in SPI (USARTSPI) Mode
Power Management and Sleep Modes
I/O-Ports
About Code Examples
23.3 SS Pin Functionality
23.3.1 Slave Mode
When the SPI is configured as a slave, the Slave Select (SS) pin is always input. When SS is held low,
the SPI is activated, and MISO becomes an output if configured so by the user. All other pins are inputs.
When SS is driven high, all pins are inputs, and the SPI is passive, which means that it will not receive
incoming data. The SPI logic will be reset once the SS pin is driven high.
The SS pin is useful for packet/byte synchronization to keep the slave bit counter synchronous with the
master clock generator. When the SS pin is driven high, the SPI slave will immediately reset the send and
receive logic, and drop any partially received data in the Shift register.
23.3.2 Master Mode
When the SPI is configured as a Master (MSTR in SPCR is set), the user can determine the direction of
the SS pin.
If SS is configured as an output, the pin is a general output pin that does not affect the SPI system.
Typically, the pin will be driving the SS pin of the SPI slave.
If SS is configured as an input, it must be held high to ensure master SPI operation. If the SS pin is driven
low by peripheral circuitry when the SPI is configured as a master with the SS pin defined as an input, the
SPI system interprets this as another master selecting the SPI as a slave and starting to send data to it.
To avoid bus contention, the SPI system takes the following actions:
1. The MSTR bit in SPCR is cleared and the SPI system becomes a slave. As a result of the SPI
becoming a slave, the MOSI and SCK pins become inputs.
2. The SPIF flag in SPSR is set, and if the SPI interrupt is enabled, and the I-bit in SREG is set, the
interrupt routine will be executed.
Thus, when interrupt-driven SPI transmission is used in Master mode, and there exists a possibility that
SS is driven low, the interrupt should always check that the MSTR bit is still set. If the MSTR bit has been
cleared by a slave select, it must be set by the user to re-enable SPI Master mode.
23.4 Data Modes
There are four combinations of SCK phase and polarity with respect to serial data, which are determined
by control bits CPHA and CPOL. Data bits are shifted out and latched in on opposite edges of the SCK
signal, ensuring sufficient time for data signals to stabilize. The following table summarizes SPCR.CPOL
and SPCR.CPHA settings.
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Serial Peripheral Interface (SPI)
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Table 23-2. SPI Modes
SPI Mode Conditions Leading Edge Trailing Edge
0 CPOL=0, CPHA=0 Sample (Rising) Setup (Falling)
1 CPOL=0, CPHA=1 Setup (Rising) Sample (Falling)
2 CPOL=1, CPHA=0 Sample (Falling) Setup (Rising)
3 CPOL=1, CPHA=1 Setup (Falling) Sample (Rising)
The SPI data transfer formats are shown in the following figure.
Figure 23-3. SPI Transfer Format with CPHA = 0
Bit 1
Bit 6
LSB
MSB
SCK (CPOL = 0)
mode 0
SAMPLE I
MOSI/MISO
CHANGE 0
MOSI PIN
CHANGE 0
MISO PIN
SCK (CPOL = 1)
mode 2
SS
MSB
LSB
Bit 6
Bit 1
Bit 5
Bit 2
Bit 4
Bit 3
Bit 3
Bit 4
Bit 2
Bit 5
MSB first (DORD = 0)
LSB first (DORD = 1)
Figure 23-4. SPI Transfer Format with CPHA = 1
SCK (CPOL = 0)
mode 1
SAMPLE I
MOSI/MISO
CHANGE 0
MOSI PIN
CHANGE 0
MISO PIN
SCK (CPOL = 1)
mode 3
SS
MSB
LSB
Bit 6
Bit 1
Bit 5
Bit 2
Bit 4
Bit 3
Bit 3
Bit 4
Bit 2
Bit 5
Bit 1
Bit 6
LSB
MSB
MSB first (DORD = 0)
LSB first (DORD = 1)
23.5 Register Description
ATmega328/P
Serial Peripheral Interface (SPI)
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23.5.1 SPI Control Register 0
Name: SPCR0
Offset: 0x4C [ID-000004d0]
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x2C
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
SPIE0 SPE0 DORD0 MSTR0 CPOL0 CPHA0 SPR0[1:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – SPIE0 SPI0 Interrupt Enable
This bit causes the SPI interrupt to be executed if the SPIF bit in the SPSR register is set and if the global
interrupt enable bit in SREG is set.
Bit 6 – SPE0 SPI0 Enable
When the SPE bit is written to one, the SPI is enabled. This bit must be set to enable any SPI operations.
Bit 5 – DORD0 Data0 Order
When the DORD bit is written to one, the LSB of the data word is transmitted first.
When the DORD bit is written to zero, the MSB of the data word is transmitted first.
Bit 4 – MSTR0 Master/Slave0 Select
This bit selects the Master SPI mode when written to one, and the Slave SPI mode when written logic
zero. If SS is configured as an input and is driven low while MSTR is set, MSTR will be cleared and SPIF
in SPSR will become set. The user will then have to set MSTR to re-enable Master SPI mode.
Bit 3 – CPOL0 Clock0 Polarity
When this bit is written to one, SCK is high when idle. When CPOL is written to zero, SCK is low when
idle. Refer to Figure 23-3 and Figure 23-4 for an example. The CPOL functionality is summarized below:
Table 23-3. CPOL0 Functionality
CPOL0 Leading Edge Trailing Edge
0 Rising Falling
1 Falling Rising
Bit 2 – CPHA0 Clock0 Phase
The settings of the Clock Phase bit (CPHA) determine if data is sampled on the leading (first) or trailing
(last) edge of SCK. Refer to Figure 23-3 and Figure 23-4 for an example. The CPHA functionality is
summarized below:
ATmega328/P
Serial Peripheral Interface (SPI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 224
Table 23-4. CPHA0 Functionality
CPHA0 Leading Edge Trailing Edge
0 Sample Setup
1 Setup Sample
Bits 1:0 – SPR0[1:0] SPI0 Clock Rate Select
These two bits control the SCK rate of the device configured as a master. SPR1 and SPR0 have no effect
on the slave. The relationship between SCK and the Oscillator Clock frequency fosc is shown in the table
below.
Table 23-5. Relationship Between SCK and Oscillator Frequency
SPI2X SPR0[1] SPR0[0] SCK Frequency
0 0 0 fosc/4
0 0 1 fosc/16
0 1 0 fosc/64
0 1 1 fosc/128
1 0 0 fosc/2
1 0 1 fosc/8
1 1 0 fosc/32
1 1 1 fosc/64
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Serial Peripheral Interface (SPI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 225
23.5.2 SPI Status Register 0
Name: SPSR0
Offset: 0x4D [ID-000004d0]
Reset: 0x00
Property: When addressing as I/O Register: address offset is 0x2D
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
SPIF0 WCOL0 SPI2X0
Access R R R/W
Reset 0 0 0
Bit 7 – SPIF0 SPI Interrupt Flag
When a serial transfer is complete, the SPIF Flag is set. An interrupt is generated if SPIE in SPCR is set
and global interrupts are enabled. If SS is an input and is driven low when the SPI is in Master mode, this
will also set the SPIF Flag. SPIF is cleared by hardware when executing the corresponding interrupt
handling vector. Alternatively, the SPIF bit is cleared by first reading the SPI Status Register with SPIF
set, then accessing the SPI Data Register (SPDR).
Bit 6 – WCOL0 Write Collision Flag
The WCOL bit is set if the SPI Data Register (SPDR) is written during a data transfer. The WCOL bit (and
the SPIF bit) are cleared by first reading the SPI Status Register with WCOL set, and then accessing the
SPI Data Register.
Bit 0 – SPI2X0 Double SPI Speed Bit
When this bit is written logic one the SPI speed (SCK Frequency) will be doubled when the SPI is in
Master mode (refer to Table 23-5). This means that the minimum SCK period will be two CPU clock
periods. When the SPI is configured as Slave, the SPI is only guaranteed to work at fosc/4 or lower.
The SPI interface is also used for program memory and EEPROM downloading or uploading. See Serial
Downloading for serial programming and verification.
Related Links
Serial Downloading
ATmega328/P
Serial Peripheral Interface (SPI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 226
23.5.3 SPI Data Register 0
Name: SPDR0
Offset: 0x4E [ID-000004d0]
Reset: 0xXX
Property: When addressing as I/O Register: address offset is 0x2E
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
SPID[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x x
Bits 7:0 – SPID[7:0] SPI Data
The SPI Data register is a read/write register used for data transfer between the register file and the SPI
Shift register. Writing to the register initiates data transmission. Reading the register causes the Shift
register receive buffer to be read.
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24. Universal Synchronous Asynchronous Receiver Transceiver (USART)
24.1 Features
• Full Duplex Operation (Independent Serial Receive and Transmit Registers)
• Asynchronous or Synchronous Operation
• Master or Slave Clocked Synchronous Operation
• High-Resolution Baud Rate Generator
• Supports Serial Frames with 5, 6, 7, 8, or 9 data bits and 1 or 2 stop bits
• Odd or Even Parity Generation and Parity Check Supported by Hardware
• Data OverRun Detection
• Framing Error Detection
• Noise Filtering Includes False Start Bit Detection and Digital Low Pass Filter
• Three Separate Interrupts on TX Complete, TX Data Register Empty, and RX Complete
• Multi-processor Communication Mode
• Double Speed Asynchronous Communication Mode
24.2 Overview
The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a highly
flexible serial communication device.
The USART can also be used in Master SPI mode. The Power Reduction USART bit in the Power
Reduction Register (PRR.PRUSARTn) must be written to '0' in order to enable USARTn.
Related Links
USART in SPI (USARTSPI) Mode
I/O-Ports
PRR
24.3 Block Diagram
In the USART block diagram, the CPU accessible I/O registers and I/O pins are shown in bold. The
dashed boxes in the block diagram separate the three main parts of the USART (listed from the top):
Clock generator, transmitter, and receiver. Control registers are shared by all units. The clock generation
logic consists of synchronization logic for external clock input used by synchronous slave operation, and
the baud rate generator. The XCKn (Transfer clock) pin is only used by synchronous transfer mode. The
transmitter consists of a single write buffer, a serial Shift register, parity generator, and control logic for
handling different serial frame formats. The write buffer allows a continuous transfer of data without any
delay between frames. The receiver is the most complex part of the USART module due to its clock and
data recovery units. The recovery units are used for asynchronous data reception. In addition to the
recovery units, the receiver includes a parity checker, control logic, a Shift register, and a two-level
receive buffer (UDRn). The receiver supports the same frame formats as the transmitter and can detect
frame error, data overrun, and parity errors.
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Figure 24-1. USART Block Diagram
PARITY
GENERATOR
UBRRn [H:L]
UDRn(Transmit)
UCSRnA UCSRnB UCSRnC
BAUD RATE GENERATOR
TRANSMIT SHIFT REGISTER
RECEIVE SHIFT REGISTER RxDn
TxDn
PIN
CONTROL
UDRn (Receive)
PIN
CONTROL
XCKn
DATA
RECOVERY
CLOCK
RECOVERY
PIN
CONTROL
TX
CONTROL
RX
CONTROL
PARITY
CHECKER
DATA BUS
OSC
SYNC LOGIC
Clock Generator
Transmitter
Receiver
Note: Refer to the Pin Configurations and the I/O-Ports description for USART pin placement.
24.4 Clock Generation
The clock generation logic generates the base clock for the transmitter and receiver. The USART
supports four modes of clock operation: Normal asynchronous, Double Speed asynchronous, Master
synchronous, and Slave synchronous mode. The USART mode select bit 0 in the USART Control and
Status Register n C (UCSRnC.UMSELn0) selects between asynchronous and synchronous operation.
Double speed (asynchronous mode only) is controlled by the U2Xn found in the UCSRnA register. When
using synchronous mode (UMSELn0=1), the data direction register for the XCKn pin (DDR_XCKn)
controls whether the clock source is internal (Master mode) or external (Slave mode). The XCKn pin is
only active when using Synchronous mode.
Below is a block diagram of the clock generation logic.
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Figure 24-2. Clock Generation Logic, Block Diagram
Prescaling
Down-Counter /2
UBRRn
/4 /2
fosc
UBRRn+1
Sync
Register
OSC
XCKn
Pin
txclk
U2Xn
UMSELn
DDR_XCKn
0
1
0
1
xcki
xcko
DDR_XCKn rxclk
0
1
1
0
Edge
Detector
UCPOLn
Signal description:
• txclk: Transmitter clock (internal signal).
• rxclk: Receiver base clock (internal signal).
• xcki: Input from XCKn pin (internal signal). Used for synchronous slave operation.
• xcko: Clock output to XCKn pin (internal signal). Used for synchronous master operation.
• fosc: System clock frequency.
24.4.1 Internal Clock Generation – The Baud Rate Generator
Internal clock generation is used for the Asynchronous and the Synchronous Master modes of operation.
The description in this section refers to the clock generation logic block diagram in the previous section.
The USART Baud Rate Register (UBRRn) and the down-counter connected to it function as a
programmable prescaler or baud rate generator. The down-counter, running at system clock (fosc), is
loaded with the UBRRn value each time the counter has counted down to zero or when the UBRRnL
register is written. A clock is generated each time the counter reaches zero. This clock is the baud rate
generator clock output (= fosc/(UBRRn+1)). The transmitter divides the baud rate generator clock output
by 2, 8, or 16 depending on the mode. The baud rate generator output is used directly by the receiver’s
clock and data recovery units. However, the recovery units use a state machine that uses 2, 8, or 16
states depending on the mode set by the state of the UMSEL, U2Xn and DDR_XCK bits.
The table below contains equations for calculating the baud rate (in bits per second) and for calculating
the UBRRn value for each mode of operation using an internally generated clock source.
Table 24-1. Equations for Calculating Baud Rate Register Setting
Operating Mode Equation for Calculating Baud
Rate(1)
Equation for Calculating UBRRn
Value
Asynchronous Normal mode
(U2Xn = 0) BAUD = OSC
16 + 1 =OSC
16BAUD 1
Asynchronous Double Speed
mode (U2Xn = 1) BAUD = OSC
8 + 1 =OSC
8BAUD 1
Synchronous Master mode BAUD = OSC
2 + 1 =OSC
2BAUD 1
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Note: 1. The baud rate is defined to be the transfer rate in bits per second (bps)
BAUD Baud rate (in bits per second, bps)
fOSC System oscillator clock frequency
UBRRn Contents of the UBRRnH and UBRRnL registers, (0-4095).
Some examples of UBRRn values for some system clock frequencies are found in Examples
of Baud Rate Settings.
24.4.2 Double Speed Operation (U2Xn)
The transfer rate can be doubled by setting the U2Xn bit in UCSRnA. Setting this bit only has effect on
the asynchronous operation. Set this bit to zero when using synchronous operation.
Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling the transfer
rate for asynchronous communication. However, in this case, the Receiver will only use half the number
of samples (reduced from 16 to 8) for data sampling and clock recovery, and therefore a more accurate
baud rate setting and system clock are required when this mode is used.
For the transmitter, there are no downsides.
24.4.3 External Clock
External clocking is used by the synchronous slave modes of operation. The description in this section
refers to the clock generation logic block diagram in the previous section.
External clock input from the XCKn pin is sampled by a synchronization register to minimize the chance
of meta-stability. The output from the synchronization register must then pass through an edge detector
before it can be used by the transmitter and receiver. This process introduces a two CPU clock period
delay and therefore the maximum external XCKn clock frequency is limited by the following equation:
XCKn <OSC
4
The value of fosc depends on the stability of the system clock source. It is therefore recommended to add
some margin to avoid possible loss of data due to frequency variations.
24.4.4 Synchronous Clock Operation
When synchronous mode is used (UMSEL = 1), the XCKn pin will be used as either clock input (slave) or
clock output (master). The dependency between the clock edges and data sampling or data change is the
same. The basic principle is that data input (on RxDn) is sampled at the opposite XCKn clock edge of the
edge the data output (TxDn) is changed.
Figure 24-3. Synchronous Mode XCKn Timing
RxDn / TxDn
XCKn
RxDn / TxDn
XCKn UCPOL = 0
UCPOL = 1
Sample
Sample
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The UCPOL bit UCRSC selects which XCKn clock edge is used for data sampling and which is used for
data change. As the above timing diagram shows, when UCPOL is zero, the data will be changed at
rising XCKn edge and sampled at falling XCKn edge. If UCPOL is set, the data will be changed at falling
XCKn edge and sampled at rising XCKn edge.
24.5 Frame Formats
A serial frame is defined to be one character of data bits with synchronization bits (start and stop bits),
and optionally a parity bit for error checking. The USART accepts all 30 combinations of the following as
valid frame formats:
• 1 start bit
• 5, 6, 7, 8, or 9 data bits
• no, even or odd parity bit
• 1 or 2 stop bits
A frame starts with the start bit, followed by the data bits (from five up to nine data bits in total): first the
least significant data bit, then the next data bits ending with the most significant bit. If enabled, the parity
bit is inserted after the data bits, before the one or two stop bits. When a complete frame is transmitted, it
can be directly followed by a new frame, or the communication line can be set to an idle (high) state. the
figure below illustrates the possible combinations of the frame formats. Bits inside brackets are optional.
Figure 24-4. Frame Formats
10 2 3 4 [5] [6] [7] [8] [P]St Sp (St / IDLE)(IDLE)
FRAME
St Start bit, always low.
(n) Data bits (0 to 8).
P Parity bit. Can be odd or even.
Sp Stop bit, always high.
IDLE No transfers on the communication line (RxDn or TxDn). An IDLE line must be high.
The frame format used by the USART is set by:
• Character Size bits (UCSRnC.UCSZn[2:0]) select the number of data bits in the frame.
• Parity Mode bits (UCSRnC.UPMn[1:0]) enable and set the type of parity bit.
• Stop Bit Select bit (UCSRnC.USBSn) select the number of stop bits. The Receiver ignores the
second stop bit.
The receiver and transmitter use the same setting. Note that changing the setting of any of these bits will
corrupt all ongoing communication for both the receiver and transmitter. An FE (Frame Error) will only be
detected in cases where the first stop bit is zero.
24.5.1 Parity Bit Calculation
The parity bit is calculated by doing an exclusive-or of all the data bits. If odd parity is used, the result of
the exclusive or is inverted. The relation between the parity bit and data bits is as follows:
even =1… 3 2 1 00
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odd = 1… 3 2 1 01
Peven Parity bit using even parity
Podd Parity bit using odd parity
dnData bit n of the character
If used, the parity bit is located between the last data bit and first stop bit of a serial frame.
24.6 USART Initialization
The USART has to be initialized before any communication can take place. The initialization process
normally consists of setting the baud rate, setting frame format and enabling the transmitter or the
receiver depending on the usage. For interrupt driven USART operation, the global interrupt flag should
be cleared (and interrupts globally disabled) when doing the initialization.
Before doing a re-initialization with changed baud rate or frame format, be sure that there are no ongoing
transmissions during the period the registers are changed. The TXC flag (UCSRnA.TXC) can be used to
check that the transmitter has completed all transfers, and the RXC flag can be used to check that there
are no unread data in the receive buffer. The UCSRnA.TXC must be cleared before each transmission
(before UDRn is written) if it is used for this purpose.
The following simple USART initialization code examples show one assembly and one C function that are
equal in functionality. The examples assume asynchronous operation using polling (no interrupts enabled)
and a fixed frame format. The baud rate is given as a function parameter. For the assembly code, the
baud rate parameter is assumed to be stored in the r17, r16 registers.
Assembly Code Example
USART_Init:
; Set baud rate to UBRR0
out UBRR0H, r17
out UBRR0L, r16
; Enable receiver and transmitter
ldi r16, (1<<RXEN0)|(1<<TXEN0)
out UCSR0B,r16
; Set frame format: 8data, 2stop bit
ldi r16, (1<<USBS0)|(3<<UCSZ00)
out UCSR0C,r16
ret
C Code Example
#define FOSC 1843200 // Clock Speed
#define BAUD 9600
#define MYUBRR FOSC/16/BAUD-1
void main( void )
{
...
USART_Init(MYUBRR)
...
}
void USART_Init( unsigned int ubrr)
{
/*Set baud rate */
UBRR0H = (unsigned char)(ubrr>>8);
UBRR0L = (unsigned char)ubrr;
Enable receiver and transmitter */
UCSR0B = (1<<RXEN0)|(1<<TXEN0);
/* Set frame format: 8data, 2stop bit */
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UCSR0C = (1<<USBS0)|(3<<UCSZ00);
}
More advanced initialization routines can be written to include frame format as parameters, disable
interrupts, and so on. However, many applications use a fixed setting of the baud and control registers,
and for these types of applications the initialization code can be placed directly in the main routine, or be
combined with initialization code for other I/O modules.
Related Links
About Code Examples
24.7 Data Transmission – The USART Transmitter
The USART transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRnB register.
When the transmitter is enabled, the normal port operation of the TxDn pin is overridden by the USART
and given the function as the transmitter’s serial output. The baud rate, mode of operation and frame
format must be set up once before doing any transmissions. If synchronous operation is used, the clock
on the XCKn pin will be overridden and used as transmission clock.
24.7.1 Sending Frames with 5 to 8 Data Bits
A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The CPU
can load the transmit buffer by writing to the UDRn I/O location. The buffered data in the transmit buffer
will be moved to the Shift register when the Shift register is ready to send a new frame. The Shift register
is loaded with new data if it is in an idle state (no ongoing transmission) or immediately after the last stop
bit of the previous frame is transmitted. When the Shift register is loaded with new data, it will transfer one
complete frame at the rate given by the Baud register, U2Xn bit or by XCKn depending on the mode of
operation.
The following code examples show a simple USART transmit function based on polling of the Data
Register Empty (UDRE) Flag. When using frames with less than eight bits, the most significant bits
written to the UDR0 are ignored. The USART 0 has to be initialized before the function can be used. For
the assembly code, the data to be sent is assumed to be stored in Register R17.
Assembly Code Example
USART_Transmit:
; Wait for empty transmit buffer
in r17, UCSR0A
sbrs r17, UDRE
rjmp USART_Transmit
; Put data (r16) into buffer, sends the data
out UDR0,r16
ret
C Code Example
void USART_Transmit( unsigned char data )
{
/* Wait for empty transmit buffer */
while ( !( UCSR0A & (1<<UDRE)) )
;
/* Put data into buffer, sends the data */
UDR0 = data;
}
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The function simply waits for the transmit buffer to be empty by checking the UDRE flag, before loading it
with new data to be transmitted. If the data register empty interrupt is utilized, the interrupt routine writes
the data into the buffer.
Related Links
About Code Examples
24.7.2 Sending Frames with 9 Data Bits
If 9-bit characters are used (UCSZn = 7), the ninth bit must be written to the TXB8 bit in UCSRnB before
the low byte of the character is written to UDRn.
The ninth bit can be used for indicating an address frame when using Multiprocessor Communication
mode or for another protocol handling as for example synchronization.
The following code examples show a transmit function that handles 9-bit characters. For the assembly
code, the data to be sent is assumed to be stored in registers R17:R16.
Assembly Code Example
USART_Transmit:
; Wait for empty transmit buffer
in r18, UCSR0A
sbrs r18, UDRE
rjmp USART_Transmit
; Copy 9th bit from r17 to TXB8
cbi UCSR0B,TXB8
sbrc r17,0
sbi UCSR0B,TXB8
; Put LSB data (r16) into buffer, sends the data
out UDR0,r16
ret
C Code Example
void USART_Transmit( unsigned int data )
{
/* Wait for empty transmit buffer */
while ( !( UCSR0A & (1<<UDRE))) )
;
/* Copy 9th bit to TXB8 */
UCSR0B &= ~(1<<TXB8);
if ( data & 0x0100 )
UCSR0B |= (1<<TXB8);
/* Put data into buffer, sends the data */
UDR0 = data;
}
Note: These transmit functions are written to be general functions. They can be optimized if the contents
of the UCSRnB is static. For example, only the TXB8 bit of the UCSRnB register is used after
initialization.
Related Links
About Code Examples
24.7.3 Transmitter Flags and Interrupts
The USART transmitter has two flags that indicate its state: USART Data Register Empty (UDRE) and
Transmit Complete (TXC). Both flags can be used for generating interrupts.
The Data Register Empty (UDRE) flag indicates whether the transmit buffer is ready to receive new data.
This bit is set when the transmit buffer is empty and cleared when the transmit buffer contains data to be
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transmitted that has not yet been moved into the Shift register. For compatibility with future devices,
always write this bit to zero when writing the UCSRnA register.
When the Data Register Empty Interrupt Enable (UDRIE) bit in UCSRnB is written to '1', the USART data
register empty interrupt will be executed as long as UDRE is set (provided that global interrupts are
enabled). UDRE is cleared by writing UDRn. When interrupt-driven data transmission is used, the data
register empty interrupt routine must either write new data to UDRn in order to clear UDRE or disable the
data register empty interrupt - otherwise, a new interrupt will occur once the interrupt routine terminates.
The Transmit Complete (TXC) flag bit is set when the entire frame in the Transmit Shift register has been
shifted out and there are no new data currently present in the transmit buffer. The TXC flag bit is either
automatically cleared when a transmit complete interrupt is executed, or it can be cleared by writing a '1'
to its bit location. The TXC flag is useful in half-duplex communication interfaces (like the RS-485
standard), where a transmitting application must enter Receive mode and free the communication bus
immediately after completing the transmission.
When the Transmit Complete Interrupt Enable (TXCIE) bit in UCSRnB is written to '1', the USART
transmit complete interrupt will be executed when the TXC flag becomes set (provided that global
interrupts are enabled). When the transmit complete interrupt is used, the interrupt handling routine does
not have to clear the TXC flag, this is done automatically when the interrupt is executed.
24.7.4 Parity Generator
The parity generator calculates the Parity bit for the serial frame data. When Parity bit is enabled
(UCSRnC.UPM[1]=1), the transmitter control logic inserts the Parity bit between the last data bit and the
first stop bit of the frame that is sent.
24.7.5 Disabling the Transmitter
When writing the TX Enable bit in the USART Control and Status Register n B (UCSRnB.TXEN) to zero,
the disabling of the transmitter will not become effective until ongoing and pending transmissions are
completed, i.e., when the Transmit Shift register and Transmit Buffer register do not contain data to be
transmitted. When disabled, the transmitter will no longer override the TxDn pin.
24.8 Data Reception – The USART Receiver
The USART receiver is enabled by writing the Receive Enable (RXEN) bit in the UCSRnB Register to '1'.
When the receiver is enabled, the normal pin operation of the RxDn pin is overridden by the USART and
given the function as the receiver’s serial input. The baud rate, mode of operation and frame format must
be set up once before any serial reception can be done. If synchronous operation is used, the clock on
the XCKn pin will be used as transfer clock.
24.8.1 Receiving Frames with 5 to 8 Data Bits
The receiver starts data reception when it detects a valid start bit. Each bit that follows the start bit will be
sampled at the baud rate or XCKn clock, and shifted into the Receive Shift register until the first stop bit
of a frame is received. A second stop bit will be ignored by the receiver. When the first stop bit is
received, i.e., a complete serial frame is present in the Receive Shift register, the contents of the Shift
register will be moved into the receive buffer. The receive buffer can then be read by reading the UDRn
I/O location.
The following code example shows a simple USART receive function based on polling of the Receive
Complete (RXC) flag. When using frames with less than eight bits the most significant bits of the data
read from the UDR0 will be masked to zero. The USART 0 has to be initialized before the function can be
used. For the assembly code, the received data will be stored in R16 after the code completes.
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Assembly Code Example
USART_Receive:
; Wait for data to be received
in r17, UCSR0A
sbrs r17, RXC
rjmp USART_Receive
; Get and return received data from buffer
in r16, UDR0
ret
C Code Example
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSR0A & (1<<RXC)) )
;
/* Get and return received data from buffer */
return UDR0;
}
For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions
must be replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS”
combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.
The function simply waits for data to be present in the receive buffer by checking the RXC flag, before
reading the buffer and returning the value.
Related Links
About Code Examples
24.8.2 Receiving Frames with 9 Data Bits
If 9-bit characters are used (UCSZn=7) the ninth bit must be read from the RXB8 bit in UCSRnB before
reading the low bits from the UDRn. This rule applies to the FE, DOR and UPE Status flags as well. Read
status from UCSRnA, then data from UDRn. Reading the UDRn I/O location will change the state of the
receive buffer FIFO and consequently the TXB8, FE, DOR and UPE bits, which all are stored in the FIFO,
will change.
The following code example shows a simple receive function for USART0 that handles both nine-bit
characters and the status bits. For the assembly code, the received data will be stored in R17:R16 after
the code completes.
Assembly Code Example
USART_Receive:
; Wait for data to be received
in r16, UCSR0A
sbrs r16, RXC
rjmp USART_Receive
; Get status and 9th bit, then data from buffer
in r18, UCSR0A
in r17, UCSR0B
in r16, UDR0
; If error, return -1
andi r18,(1<<FE)|(1<<DOR)|(1<<UPE)
breq USART_ReceiveNoError
ldi r17, HIGH(-1)
ldi r16, LOW(-1)
USART_ReceiveNoError:
; Filter the 9th bit, then return
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lsr r17
andi r17, 0x01
ret
C Code Example
unsigned int USART_Receive( void )
{
unsigned char status, resh, resl;
/* Wait for data to be received */
while ( !(UCSR0A & (1<<RXC)) )
;
/* Get status and 9th bit, then data */
/* from buffer */
status = UCSR0A;
resh = UCSR0B;
resl = UDR0;
/* If error, return -1 */
if ( status & (1<<FE)|(1<<DOR)|(1<<UPE) )
return -1;
/* Filter the 9th bit, then return */
resh = (resh >> 1) & 0x01;
return ((resh << 8) | resl);
}
The receive function example reads all the I/O registers into the register file before any computation is
done. This gives an optimal receive buffer utilization since the buffer location read will be free to accept
new data as early as possible.
Related Links
About Code Examples
24.8.3 Receive Compete Flag and Interrupt
The USART Receiver has one flag that indicates the Receiver state.
The Receive Complete (RXC) Flag indicates if there are unread data present in the receive buffer. This
flag is one when unread data exist in the receive buffer, and zero when the receive buffer is empty (i.e.,
does not contain any unread data). If the Receiver is disabled (RXEN = 0), the receive buffer will be
flushed and consequently, the RXCn bit will become zero.
When the Receive Complete Interrupt Enable (RXCIE) in UCSRnB is set, the USART Receive Complete
interrupt will be executed as long as the RXC Flag is set (provided that global interrupts are enabled).
When interrupt-driven data reception is used, the receive complete routine must read the received data
from UDR in order to clear the RXC Flag, otherwise, a new interrupt will occur once the interrupt routine
terminates.
24.8.4 Receiver Error Flags
The USART receiver has three error flags: Frame Error (FE), Data OverRun (DOR) and Parity Error
(UPE). All can be accessed by reading UCSRnA. Common for the error flags is that they are located in
the receive buffer together with the frame for which they indicate the error status. Due to the buffering of
the error flags, the UCSRnA must be read before the receive buffer (UDRn), since reading the UDRn I/O
location changes the buffer read location. Another equality for the error flags is that they cannot be
altered by software doing a write to the flag location. However, all flags must be set to zero when the
UCSRnA is written for upward compatibility of future USART implementations. None of the error flags can
generate interrupts.
The FE flag indicates the state of the first stop bit of the next readable frame stored in the receive buffer.
The FE flag is zero when the stop bit was correctly read as '1', and the FE flag will be one when the stop
bit was incorrect (zero). This flag can be used for detecting out-of-sync conditions, detecting break
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conditions and protocol handling. The FE flag is not affected by the setting of the USBS bit in UCSRnC
since the receiver ignores all, except for the first, stop bits. For compatibility with future devices, always
set this bit to zero when writing to UCSRnA.
The DOR flag indicates data loss due to a receiver buffer full condition. A DOR occurs when the receive
buffer is full (two characters), a new character is waiting in the Receive Shift register, and a new start bit
is detected. If the DOR flag is set, one or more serial frames were lost between the last frame read from
UDR, and the next frame read from UDR. For compatibility with future devices, always write this bit to
zero when writing to UCSRnA. The DOR flag is cleared when the frame received was successfully moved
from the Shift register to the receive buffer.
The Parity Error (UPE) flag indicates that the next frame in the receive buffer had a UPE when received.
If Parity Check is not enabled the UPE bit will always read '0'. For compatibility with future devices,
always set this bit to zero when writing to UCSRnA. For more details see Parity Bit Calculation and 'Parity
Checker' below.
24.8.5 Parity Checker
The parity checker is active when the high USART Parity Mode bit 1 in the USART Control and Status
Register n C (UCSRnC.UPM[1]) is written to '1'. The type of parity check to be performed (odd or even) is
selected by the UCSRnC.UPM[0] bit. When enabled, the parity checker calculates the parity of the data
bits in incoming frames and compares the result with the Parity bit from the serial frame. The result of the
check is stored in the receive buffer together with the received data and stop bits. The USART parity error
flag in the USART Control and Status Register n A (UCSRnA.UPE) can then be read by software to
check if the frame had a parity error.
The UPEn bit is set if the next character that can be read from the receive buffer had a parity error when
received and the parity checking was enabled at that point (UPM[1] = 1). This bit is valid until the receive
buffer (UDRn) is read.
24.8.6 Disabling the Receiver
In contrast to the transmitter, disabling of the receiver will be immediate. Data from ongoing receptions
will, therefore, be lost. When disabled (i.e., UCSRnB.RXEN is written to zero) the receiver will no longer
override the normal function of the RxDn port pin. The receiver buffer FIFO will be flushed when the
receiver is disabled. Remaining data in the buffer will be lost.
24.8.7 Flushing the Receive Buffer
The receiver buffer FIFO will be flushed when the receiver is disabled, i.e., the buffer will be emptied of its
contents. Unread data will be lost. If the buffer has to be flushed during normal operation, due to for
instance an error condition, read the UDRn I/O location until the RXCn flag is cleared.
The following code shows how to flush the receive buffer of USART0.
Assembly Code Example
USART_Flush:
in r16, UCSR0A
sbrs r16, RXC
ret
in r16, UDR0
rjmp USART_Flush
C Code Example
void USART_Flush( void )
{
ATmega328/P
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unsigned char dummy;
while ( UCSR0A & (1<<RXC) ) dummy = UDR0;
}
Related Links
About Code Examples
24.9 Asynchronous Data Reception
The USART includes a clock recovery and a data recovery unit for handling asynchronous data reception.
The clock recovery logic is used for synchronizing the internally generated baud rate clock to the
incoming asynchronous serial frames at the RxDn pin. The data recovery logic samples and low pass
filters each incoming bit, thereby improving the noise immunity of the receiver. The asynchronous
reception operational range depends on the accuracy of the internal baud rate clock, the rate of the
incoming frames, and the frame size in a number of bits.
24.9.1 Asynchronous Clock Recovery
The clock recovery logic synchronizes the internal clock to the incoming serial frames. The figure below
illustrates the sampling process of the start bit of an incoming frame. The sample rate is 16-times the
baud rate for Normal mode and eight times the baud rate for Double Speed mode. The horizontal arrows
illustrate the synchronization variation due to the sampling process. Note the larger time variation when
using the Double Speed mode (UCSRnA.U2Xn=1) of operation. Samples denoted '0' are samples taken
while the RxDn line is idle (i.e., no communication activity).
Figure 24-5. Start Bit Sampling
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2
STARTIDLE
00
BIT 0
3
1 2 3 4 5 6 7 8 1 20
RxDn
Sample
(U2X = 0)
Sample
(U2X = 1)
When the clock recovery logic detects a high (idle) to low (start) transition on the RxDn line, the start bit
detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in the figure. The
clock recovery logic then uses samples 8, 9, and 10 for Normal mode, and samples 4, 5, and 6 for
Double Speed mode (indicated with sample numbers inside boxes on the figure), to decide if a valid start
bit is received. If two or more of these three samples have logical high levels (the majority wins), the start
bit is rejected as a noise spike and the receiver starts looking for the next high to low-transition on RxDn.
If however, a valid start bit is detected, the clock recovery logic is synchronized and the data recovery can
begin. The synchronization process is repeated for each start bit.
24.9.2 Asynchronous Data Recovery
When the receiver clock is synchronized to the start bit, the data recovery can begin. The data recovery
unit uses a state machine that has 16 states for each bit in Normal mode and eight states for each bit in
Double Speed mode. The figure below shows the sampling of the data bits and the parity bit. Each of the
samples is given a number that is equal to the state of the recovery unit.
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Figure 24-6. Sampling of Data and Parity Bit
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1
BIT n
1 2 3 4 5 6 7 8 1
RxDn
Sample
(U2X = 0)
Sample
(U2X = 1)
The decision of the logic level of the received bit is taken by doing a majority voting of the logic value to
the three samples in the center of the received bit: If two or all three center samples (those marked by
their sample number inside boxes) have high levels, the received bit is registered to be a logic '1'. If two
or all three samples have low levels, the received bit is registered to be a logic '0'. This majority voting
process acts as a low pass filter for the incoming signal on the RxDn pin. The recovery process is then
repeated until a complete frame is received, including the first stop bit. The receiver only uses the first
stop bit of a frame.
The following figure shows the sampling of the stop bit and the earliest possible beginning of the start bit
of the next frame.
Figure 24-7. Stop Bit Sampling and Next Start Bit Sampling
1 2 3 4 5 6 7 8 9 10 0/1 0/1 0/1
STOP 1
1 2 3 4 5 6 0/1
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
(A) (B) (C)
The same majority voting is done to the stop bit as done for the other bits in the frame. If the stop bit is
registered to have a logic '0' value, the Frame Error (UCSRnA.FE) flag will be set.
A new high to low transition indicating the start bit of a new frame can come right after the last of the bits
used for majority voting. For Normal Speed mode, the first low level sample can be taken at the point
marked (A) in the figure above. For Double Speed mode, the first low level must be delayed to (B). (C)
marks a stop bit of full length. The early start bit detection influences the operational range of the receiver.
24.9.3 Asynchronous Operational Range
The operational range of the receiver is dependent on the mismatch between the received bit rate and
the internally generated baud rate. If the transmitter is sending frames at too fast or too slow bit rates or
the internally generated baud rate of the receiver does not have a similar base frequency (see
recommendations below), the receiver will not be able to synchronize the frames to the start bit.
The following equations can be used to calculate the ratio of the incoming data rate and internal receiver
baud rate.
slow =+ 1
1+ +
fast =+ 2
+ 1 +
•D: Sum of character size and parity size (D = 5 to 10 bit).
•S: Samples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed mode.
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•SF: First sample number used for majority voting. SF = 8 for normal speed and SF = 4
for Double Speed mode.
•SM: Middle sample number used for majority voting. SM = 9 for normal speed and
SM = 5 for Double Speed mode.
•Rslow : is the ratio of the slowest incoming data rate that can be accepted in relation to the
receiver baud rate. Rfast is the ratio of the fastest incoming data rate that can be
accepted in relation to the receiver baud rate.
The following tables list the maximum receiver baud rate error that can be tolerated. Note that Normal
Speed mode has higher toleration of baud rate variations.
Table 24-2. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2Xn = 0)
D
# (Data+Parity Bit)
Rslow [%] Rfast [%] Max. Total Error [%] Recommended Max. Receiver Error [%]
5 93.20 106.67 +6.67/-6.8 ±3.0
6 94.12 105.79 +5.79/-5.88 ±2.5
7 94.81 105.11 +5.11/-5.19 ±2.0
8 95.36 104.58 +4.58/-4.54 ±2.0
9 95.81 104.14 +4.14/-4.19 ±1.5
10 96.17 103.78 +3.78/-3.83 ±1.5
Table 24-3. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2Xn = 1)
D
# (Data+Parity Bit)
Rslow [%] Rfast [%] Max Total Error [%] Recommended Max Receiver Error [%]
5 94.12 105.66 +5.66/-5.88 ±2.5
6 94.92 104.92 +4.92/-5.08 ±2.0
7 95.52 104,35 +4.35/-4.48 ±1.5
8 96.00 103.90 +3.90/-4.00 ±1.5
9 96.39 103.53 +3.53/-3.61 ±1.5
10 96.70 103.23 +3.23/-3.30 ±1.0
The recommendations of the maximum receiver baud rate error was made under the assumption that the
receiver and transmitter equally divide the maximum total error.
There are two possible sources for the receivers baud rate error. The receiver’s System Clock (EXTCLK)
will always have some minor instability over the supply voltage range and the temperature range. When
using a crystal to generate the system clock, this is rarely a problem, but for a resonator, the system clock
may differ more than 2% depending on the resonator's tolerance. The second source for the error is more
controllable. The baud rate generator cannot always do an exact division of the system frequency to get
the baud rate wanted. In this case, an UBRRn value that gives an acceptable low error can be used if
possible.
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24.10 Multi-Processor Communication Mode
Setting the Multi-Processor Communication mode (MPCMn) bit in UCSRnA enables a filtering function of
incoming frames received by the USART receiver. Frames that do not contain address information will be
ignored and not put into the receive buffer. This effectively reduces the number of incoming frames that
have to be handled by the CPU, in a system with multiple MCUs that communicate via the same serial
bus. The transmitter is unaffected by the MPCMn setting but has to be used differently when it is a part of
a system utilizing the Multi-processor Communication mode.
If the receiver is set up to receive frames that contain five to eight data bits, then the first stop bit indicates
if the frame contains data or address information. If the receiver is set up for frames with 9 data bits, then
the ninth bit (RXB8) is used for identifying address and data frames. When the frame type bit (the first
stop or the ninth bit) is '1', the frame contains an address. When the frame type bit is '0', the frame is a
data frame.
The Multi-Processor Communication mode enables several slave MCUs to receive data from a master
MCU. This is done by first decoding an address frame to find out which MCU has been addressed. If a
particular slave MCU has been addressed, it will receive the following data frames as normal, while the
other slave MCUs will ignore the received frames until another address frame is received.
24.10.1 Using MPCMn
For an MCU to act as a master MCU, it can use a 9-bit character frame format (UCSZ1=7). The ninth bit
(TXB8) must be set when an address frame (TXB8=1) is being transmitted or cleared when a data frame
(TXB=0) is being transmitted. The slave MCUs must, in this case, be set to use a 9-bit character frame
format.
The following procedure should be used to exchange data in Multi-Processor Communication mode:
1. All slave MCUs are in Multi-Processor Communication mode (MPCM in UCSRnA is set).
2. The master MCU sends an address frame, and all slaves receive and read this frame. In the slave
MCUs, the RXC flag in UCSRnA will be set as normal.
3. Each slave MCU reads the UDRn register and determines if it has been selected. If so, it clears the
MPCM bit in UCSRnA, otherwise, it waits for the next address byte and keeps the MPCM setting.
4. The addressed MCU will receive all data frames until a new address frame is received. The other
slave MCUs, which still have the MPCM bit set, will ignore the data frames.
5. When the last data frame is received by the addressed MCU, the addressed MCU sets the MPCM
bit and waits for a new address frame from the master. The process then repeats from step 2.
Using any of the 5- to 8-bit character frame formats is possible, but impractical since the receiver must
change between using n and n+1 character frame formats. This makes full-duplex operation difficult since
the transmitter and receiver use the same character size setting. If 5- to 8-bit character frames are used,
the transmitter must be set to use two stop bit (USBS = 1) since the first stop bit is used for indicating the
frame type.
Do not use Read-Modify-Write instructions (SBI and CBI) to set or clear the MPCM bit. The MPCM bit
shares the same I/O location as the TXC flag and this might accidentally be cleared when using SBI or
CBI instructions.
24.11 Examples of Baud Rate Setting
For standard crystal and resonator frequencies, the most commonly used baud rates for asynchronous
operation can be generated by using the UBRRn settings as listed in the table below.
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UBRRn values, which yield an actual baud rate differing less than 0.5% from the target baud rate, are
bold in the table. Higher error ratings are acceptable, but the receiver will have less noise resistance
when the error ratings are high, especially for large serial frames (see also section Asynchronous
Operational Range). The error values are calculated using the following equation:
% = BaudRateClosest Match
BaudRate 1
2
100 %
Table 24-4. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
Baud
Rate
[bps]
fosc = 1.0000 MHz fosc = 1.8432 MHz fosc = 2.0000 MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 25 0.2% 51 0.2% 47 0.0% 95 0.0% 51 0.2% 103 0.2%
4800 12 0.2% 25 0.2% 23 0.0% 47 0.0% 25 0.2% 51 0.2%
9600 6 -7.0% 12 0.2% 11 0.0% 23 0.0% 12 0.2% 25 0.2%
14.4k 3 8.5% 8 -3.5% 7 0.0% 15 0.0% 8 -3.5% 16 2.1%
19.2k 2 8.5% 6 -7.0% 5 0.0% 11 0.0% 6 -7.0% 12 0.2%
28.8k 1 8.5% 3 8.5% 3 0.0% 7 0.0% 3 8.5% 8 -3.5%
38.4k 1 -18.6% 2 8.5% 2 0.0% 5 0.0% 2 8.5% 6 -7.0%
57.6k 0 8.5% 1 8.5% 1 0.0% 3 0.0% 1 8.5% 3 8.5%
76.8k – – 1 -18.6% 1 -25.0% 2 0.0% 1 -18.6% 2 8.5%
115.2k – – 0 8.5% 0 0.0% 1 0.0% 0 8.5% 1 8.5%
230.4k – – – – – – 0 0.0% – – – –
250k – – – – – – – – – – 0 0.0%
Max.(1) 62.5 kbps 125 kbps 115.2 kbps 230.4 kbps 125 kbps 250 kbps
Note: 1. UBRRn = 0, Error = 0.0%
Table 24-5. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
Baud Rate
[bps]
fosc = 3.6864 MHz fosc = 4.0000 MHz fosc = 7.3728 MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 95 0.0% 191 0.0% 103 0.2% 207 0.2% 191 0.0% 383 0.0%
4800 47 0.0% 95 0.0% 51 0.2% 103 0.2% 95 0.0% 191 0.0%
9600 23 0.0% 47 0.0% 25 0.2% 51 0.2% 47 0.0% 95 0.0%
14.4k 15 0.0% 31 0.0% 16 2.1% 34 -0.8% 31 0.0% 63 0.0%
19.2k 11 0.0% 23 0.0% 12 0.2% 25 0.2% 23 0.0% 47 0.0%
28.8k 7 0.0% 15 0.0% 8 -3.5% 16 2.1% 15 0.0% 31 0.0%
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Baud Rate
[bps]
fosc = 3.6864 MHz fosc = 4.0000 MHz fosc = 7.3728 MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
38.4k 5 0.0% 11 0.0% 6 -7.0% 12 0.2% 11 0.0% 23 0.0%
57.6k 3 0.0% 7 0.0% 3 8.5% 8 -3.5% 7 0.0% 15 0.0%
76.8k 2 0.0% 5 0.0% 2 8.5% 6 -7.0% 5 0.0% 11 0.0%
115.2k 1 0.0% 3 0.0% 1 8.5% 3 8.5% 3 0.0% 7 0.0%
230.4k 0 0.0% 1 0.0% 0 8.5% 1 8.5% 1 0.0% 3 0.0%
250k 0 -7.8% 1 -7.8% 0 0.0% 1 0.0% 1 -7.8% 3 -7.8%
0.5M – – 0 -7.8% – – 0 0.0% 0 -7.8% 1 -7.8%
1M – – – – – – – – – – 0 -7.8%
Max.(1) 230.4 kbps 460.8 kbps 250 kbps 0.5 Mbps 460.8 kbps 921.6 kbps
(1) UBRRn = 0, Error = 0.0%
Table 24-6. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
Baud Rate
[bps]
fosc = 8.0000 MHz fosc = 11.0592 MHz fosc = 14.7456 MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 207 0.2% 416 -0.1% 287 0.0% 575 0.0% 383 0.0% 767 0.0%
4800 103 0.2% 207 0.2% 143 0.0% 287 0.0% 191 0.0% 383 0.0%
9600 51 0.2% 103 0.2% 71 0.0% 143 0.0% 95 0.0% 191 0.0%
14.4k 34 -0.8% 68 0.6% 47 0.0% 95 0.0% 63 0.0% 127 0.0%
19.2k 25 0.2% 51 0.2% 35 0.0% 71 0.0% 47 0.0% 95 0.0%
28.8k 16 2.1% 34 -0.8% 23 0.0% 47 0.0% 31 0.0% 63 0.0%
38.4k 12 0.2% 25 0.2% 17 0.0% 35 0.0% 23 0.0% 47 0.0%
57.6k 8 -3.5% 16 2.1% 11 0.0% 23 0.0% 15 0.0% 31 0.0%
76.8k 6 -7.0% 12 0.2% 8 0.0% 17 0.0% 11 0.0% 23 0.0%
115.2k 3 8.5% 8 -3.5% 5 0.0% 11 0.0% 7 0.0% 15 0.0%
230.4k 1 8.5% 3 8.5% 2 0.0% 5 0.0% 3 0.0% 7 0.0%
250k 1 0.0% 3 0.0% 2 -7.8% 5 -7.8% 3 -7.8% 6 5.3%
0.5M 0 0.0% 1 0.0% – – 2 -7.8% 1 -7.8% 3 -7.8%
1M – – 0 0.0% – – – – 0 -7.8% 1 -7.8%
Max.(1) 0.5 Mbps 1 Mbps 691.2 kbps 1.3824 Mbps 921.6 kbps 1.8432 Mbps
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(1) UBRRn = 0, Error = 0.0%
Table 24-7. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
Baud Rate
[bps]
fosc = 16.0000 MHz fosc = 18.4320 MHz fosc = 20.0000 MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 416 -0.1% 832 0.0% 479 0.0% 959 0.0% 520 0.0% 1041 0.0%
4800 207 0.2% 416 -0.1% 239 0.0% 479 0.0% 259 0.2% 520 0.0%
9600 103 0.2% 207 0.2% 119 0.0% 239 0.0% 129 0.2% 259 0.2%
14.4k 68 0.6% 138 -0.1% 79 0.0% 159 0.0% 86 -0.2% 173 -0.2%
19.2k 51 0.2% 103 0.2% 59 0.0% 119 0.0% 64 0.2% 129 0.2%
28.8k 34 -0.8% 68 0.6% 39 0.0% 79 0.0% 42 0.9% 86 -0.2%
38.4k 25 0.2% 51 0.2% 29 0.0% 59 0.0% 32 -1.4% 64 0.2%
57.6k 16 2.1% 34 -0.8% 19 0.0% 39 0.0% 21 -1.4% 42 0.9%
76.8k 12 0.2% 25 0.2% 14 0.0% 29 0.0% 15 1.7% 32 -1.4%
115.2k 8 -3.5% 16 2.1% 9 0.0% 19 0.0% 10 -1.4% 21 -1.4%
230.4k 3 8.5% 8 -3.5% 4 0.0% 9 0.0% 4 8.5% 10 -1.4%
250k 3 0.0% 7 0.0% 4 -7.8% 8 2.4% 4 0.0% 9 0.0%
0.5M 1 0.0% 3 0.0% – – 4 -7.8% – – 4 0.0%
1M 0 0.0% 1 0.0% – – – – – – – –
Max.(1) 1 Mbps 2 Mbps 1.152 Mbps 2.304 Mbps 1.25 Mbps 2.5 Mbps
(1) UBRRn = 0, Error = 0.0%
Related Links
Asynchronous Operational Range
24.12 Register Description
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24.12.1 USART I/O Data Register 0
Name: UDR0
Offset: 0xC6
Reset: 0x00
Property: -
The USART transmit data buffer register and USART receive data buffer registers share the same I/O
address referred to as USART Data Register (UDR0). The Transmit Data Buffer register (TXB) will be the
destination for data written to the UDR0 location. Reading the UDR0 location will return the contents of
the Receive Data Buffer register (RXB).
For 5-, 6-, or 7-bit characters the upper unused bits will be ignored by the transmitter and set to zero by
the receiver.
The transmit buffer can only be written when the UDRE0 Flag in the UCSR0A register is set. Data written
to UDR0 when the UDRE0 flag is not set, will be ignored by the USART transmitter. When data is written
to the transmit buffer, and the transmitter is enabled, the transmitter will load the data into the Transmit
Shift register when the Shift register is empty. Then the data will be serially transmitted on the TxD0 pin.
The receive buffer consists of a two level FIFO. The FIFO will change its state whenever the receive
buffer is accessed. Due to this behavior of the receive buffer, do not use Read-Modify-Write instructions
(SBI and CBI) on this location. Be careful when using bit test instructions (SBIC and SBIS), since these
also will change the state of the FIFO.
Bit 7 6 5 4 3 2 1 0
TXB / RXB[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TXB / RXB[7:0] USART Transmit / Receive Data Buffer
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24.12.2 USART Control and Status Register 0 A
Name: UCSR0A
Offset: 0xC0
Reset: 0x20
Property: -
Bit 7 6 5 4 3 2 1 0
RXC0 TXC0 UDRE0 FE0 DOR0 UPE0 U2X0 MPCM0
Access R R/W R R R R R/W R/W
Reset 0 0 1 0 0 0 0 0
Bit 7 – RXC0 USART Receive Complete
This flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer is
empty (i.e., does not contain any unread data). If the receiver is disabled, the receive buffer will be
flushed and consequently the RXC0 bit will become zero. The RXC0 flag can be used to generate a
receive complete interrupt (see description of the RXCIE0 bit).
Bit 6 – TXC0 USART Transmit Complete
This flag bit is set when the entire frame in the transmit shift register has been shifted out and there are
no new data currently present in the transmit buffer (UDR0). The TXC0 flag bit is automatically cleared
when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location.
The TXC0 flag can generate a transmit complete interrupt (see description of the TXCIE0 bit).
Bit 5 – UDRE0 USART Data Register Empty
The UDRE0 flag indicates if the transmit buffer (UDR0) is ready to receive new data. If UDRE0 is one, the
buffer is empty, and therefore ready to be written. The UDRE0 flag can generate a data register empty
interrupt (see description of the UDRIE0 bit). UDRE0 is set after a reset to indicate that the transmitter is
ready.
Bit 4 – FE0 Frame Error
This bit is set if the next character in the receive buffer had a frame error when received. I.e., when the
first stop bit of the next character in the receive buffer is zero. This bit is valid until the receive buffer
(UDR0) is read. The FEn bit is zero when the stop bit of received data is one. Always set this bit to zero
when writing to UCSR0A.
This bit is reserved in Master SPI Mode (MSPIM).
Bit 3 – DOR0 Data OverRun
This bit is set if a data overrun condition is detected. A data overrun occurs when the receive buffer is full
(two characters), it is a new character waiting in the receive shift register, and a new start bit is detected.
This bit is valid until the receive buffer (UDR0) is read. Always set this bit to zero when writing to
UCSR0A.
This bit is reserved in Master SPI Mode (MSPIM).
Bit 2 – UPE0 USART Parity Error
This bit is set if the next character in the receive buffer had a parity error when received and the parity
checking was enabled at that point (UPM01 = 1). This bit is valid until the receive buffer (UDR0) is read.
Always set this bit to zero when writing to UCSR0A.
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This bit is reserved in MSPIM.
Bit 1 – U2X0 Double the USART Transmission Speed
This bit only has effect for the asynchronous operation. Write this bit to zero when using synchronous
operation.
Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effectively doubling the
transfer rate for asynchronous communication.
This bit is reserved in MSPIM.
Bit 0 – MPCM0 Multi-processor Communication Mode
This bit enables the Multi-processor Communication mode. When the MPCMn bit is written to one, all the
incoming frames received by the USART receiver that do not contain address information will be ignored.
The transmitter is unaffected by the MPCM0 setting. Refer to Multi-Processor Communication Mode for
details.
This bit is reserved in MSPIM.
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24.12.3 USART Control and Status Register 0 B
Name: UCSR0B
Offset: 0xC1
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
RXCIE0 TXCIE0 UDRIE0 RXEN0 TXEN0 UCSZ02 RXB80 TXB80
Access R/W R/W R/W R/W R/W R/W R R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – RXCIE0 RX Complete Interrupt Enable 0
Writing this bit to one enables interrupt on the RXC0 flag. A USART receive complete interrupt will be
generated only if the RXCIE0 bit is written to one, the global interrupt flag in SREG is written to one and
the RXC0 bit in UCSR0A is set.
Bit 6 – TXCIE0 TX Complete Interrupt Enable 0
Writing this bit to one enables interrupt on the TXC0 flag. A USART transmit complete interrupt will be
generated only if the TXCIE0 bit is written to one, the global interrupt flag in SREG is written to one and
the TXC0 bit in UCSR0A is set.
Bit 5 – UDRIE0 USART Data Register Empty Interrupt Enable 0
Writing this bit to one enables interrupt on the UDRE0 Flag. A data register empty interrupt will be
generated only if the UDRIE0 bit is written to one, the global interrupt flag in SREG is written to one and
the UDRE0 bit in UCSR0A is set.
Bit 4 – RXEN0 Receiver Enable 0
Writing this bit to one enables the USART Receiver. The receiver will override normal port operation for
the RxDn pin when enabled. Disabling the receiver will flush the receive buffer invalidating the FE0,
DOR0, and UPE0 flags.
Bit 3 – TXEN0 Transmitter Enable 0
Writing this bit to one enables the USART transmitter. The transmitter will override normal port operation
for the TxD0 pin when enabled. The disabling of the transmitter (writing TXEN0 to zero) will not become
effective until ongoing and pending transmissions are completed, i.e., when the transmit shift register and
transmit buffer register do not contain data to be transmitted. When disabled, the transmitter will no longer
override the TxD0 port.
Bit 2 – UCSZ02 Character Size 0
The UCSZ02 bits combined with the UCSZ0[1:0] bit in UCSR0C sets the number of data bits (Character
Size) in a frame the receiver and transmitter use.
This bit is reserved in Master SPI Mode (MSPIM).
Bit 1 – RXB80 Receive Data Bit 8 0
RXB80 is the ninth data bit of the received character when operating with serial frames with nine data
bits. Must be read before reading the low bits from UDR0.
This bit is reserved in MSPIM.
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Bit 0 – TXB80 Transmit Data Bit 8 0
TXB80 is the ninth data bit in the character to be transmitted when operating with serial frames with nine
data bits. Must be written before writing the low bits to UDR0.
This bit is reserved in MSPIM.
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24.12.4 USART Control and Status Register 0 C
Name: UCSR0C
Offset: 0xC2
Reset: 0x06
Property: -
Bit 7 6 5 4 3 2 1 0
UMSEL0 [1:0] UPM0 [1:0] USBS0 UCSZ01 /
UDORD0
UCSZ00 /
UCPHA0
UCPOL0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 1 1 0
Bits 7:6 – UMSEL0 [1:0] USART Mode Select 0
These bits select the mode of operation of the USART0
Table 24-8. USART Mode Selection
UMSEL0[1:0] Mode
00 Asynchronous USART
01 Synchronous USART
10 Reserved
11 Master SPI (MSPIM)(1)
Note:
1. The UDORD0, UCPHA0, and UCPOL0 can be set in the same write operation where the MSPIM is
enabled.
Bits 5:4 – UPM0 [1:0] USART Parity Mode 0
These bits enable and set type of parity generation and check. If enabled, the transmitter will
automatically generate and send the parity of the transmitted data bits within each frame. The receiver
will generate a parity value for the incoming data and compare it to the UPM0 setting. If a mismatch is
detected, the UPE0 Flag in UCSR0A will be set.
Table 24-9. USART Mode Selection
UPM0[1:0] ParityMode
00 Disabled
01 Reserved
10 Enabled, Even Parity
11 Enabled, Odd Parity
These bits are reserved in Master SPI Mode (MSPIM).
Bit 3 – USBS0 USART Stop Bit Select 0
This bit selects the number of stop bits to be inserted by the transmitter. The receiver ignores this setting.
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Table 24-10. Stop Bit Settings
USBS0 Stop Bit(s)
0 1-bit
1 2-bit
This bit is reserved in Master SPI Mode (MSPIM).
Bit 2 – UCSZ01 / UDORD0 USART Character Size / Data Order
UCSZ0[1:0]: USART Modes: The UCSZ0[1:0] bits combined with the UCSZ02 bit in UCSR0B sets the
number of data bits (Character Size) in a frame the receiver and transmitter use.
Table 24-11. Character Size Settings
UCSZ0[2:0] Character Size
000 5-bit
001 6-bit
010 7-bit
011 8-bit
100 Reserved
101 Reserved
110 Reserved
111 9-bit
UDPRD0: Master SPI Mode: When set to one the LSB of the data word is transmitted first. When set to
zero the MSB of the data word is transmitted first. Refer to the USART in SPI Mode - Frame Formats for
details.
Bit 1 – UCSZ00 / UCPHA0 USART Character Size / Clock Phase
UCSZ00: USART Modes: Refer to UCSZ01.
UCPHA0: Master SPI Mode: The UCPHA0 bit setting determine if data is sampled on the leasing edge
(first) or tailing (last) edge of XCK0. Refer to the SPI Data Modes and Timing for details.
Bit 0 – UCPOL0 Clock Polarity 0
USART0 Modes: This bit is used for synchronous mode only. Write this bit to zero when Asynchronous
mode is used. The UCPOL0 bit sets the relationship between data output change and data input sample,
and the Synchronous Clock (XCK0).
Table 24-12. USART Clock Polarity Settings
UCPOL0 Transmitted Data Changed (Output of TxD0
Pin)
Received Data Sampled (Input on RxD0
Pin)
0 Rising XCK0 Edge Falling XCK0 Edge
1 Falling XCK0 Edge Rising XCK0 Edge
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Master SPI Mode: The UCPOL0 bit sets the polarity of the XCK0 clock. The combination of the UCPOL0
and UCPHA0 bit settings determine the timing of the data transfer. Refer to the SPI Data Modes and
Timing for details.
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24.12.5 USART Baud Rate 0 Register Low and High byte
Name: UBRR0L and UBRR0H
Offset: 0xC4
Reset: 0x00
Property: -
The UBRR0L and UBRR0H register pair represents the 16-bit value, UBRR0. The low byte [7:0] (suffix L)
is accessible at the original offset. The high byte [15:8] (suffix H) can be accessed at offset + 0x01. For
more details on reading and writing 16-bit registers, refer to Accessing 16-bit Timer/Counter Registers.
Bit 15 14 13 12 11 10 9 8
UBRR0[11:8]
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 7 6 5 4 3 2 1 0
UBRR0[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 11:0 – UBRR0[11:0] USART Baud Rate
This is a 12-bit register which contains the USART baud rate. The UBRR0H contains the four most
significant bits and the UBRR0L contains the eight least significant bits of the USART 0 baud rate.
Ongoing transmissions by the transmitter and receiver will be corrupted if the baud rate is changed.
Writing UBRR0L will trigger an immediate update of the baud rate prescaler.
Related Links
Accessing 16-bit Timer/Counter Registers
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Universal Synchronous Asynchronous Receiver ...
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25. USART in SPI (USARTSPI) Mode
25.1 Features
• Full Duplex, Three-wire Synchronous Data Transfer
• Master Operation
• Supports all four SPI Modes of Operation (Mode 0, 1, 2, and 3)
• LSB First or MSB First Data Transfer (Configurable Data Order)
• Queued Operation (Double Buffered)
• High-Resolution Baud Rate Generator
• High Speed Operation (fXCKmax = fCK/2)
• Flexible Interrupt Generation
25.2 Overview
The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) can be set to a
Master SPI Compliant mode of operation.
Setting both UMSELn[1:0] bits to one enables the USART in MSPIM logic. In this mode of operation the
SPI master control logic takes direct control over the USART resources. These resources include the
transmitter and receiver shift register and buffers, and the baud rate generator. The parity generator and
checker, the data and clock recovery logic, and the RX and TX control logic is disabled. The USART RX
and TX control logic is replaced by a common SPI transfer control logic. However, the pin control logic
and interrupt generation logic is identical in both modes of operation.
The I/O register locations are the same in both modes. However, some of the functionality of the control
registers changes when using MSPIM.
25.3 Clock Generation
The clock generation logic generates the base clock for the transmitter and receiver. For USART MSPIM
mode of operation only internal clock generation (i.e., master operation) is supported. The Data Direction
register for the XCKn pin (DDR_XCKn) must therefore be set to one (i.e., as output) for the USART in
MSPIM to operate correctly. Preferably the DDR_XCKn should be set up before the USART in MSPIM is
enabled (i.e., TXENn and RXENn bit set to one).
The internal clock generation used in MSPIM mode is identical to the USART Synchronous Master mode.
The table below contains the equations for calculating the baud rate or UBRRn setting for Synchronous
Master mode.
Table 25-1. Equations for Calculating Baud Rate Register Setting
Operating Mode Equation for Calculating Baud
Rate(1)
Equation for Calculating UBRRn
Value
Synchronous Master
mode BAUD = OSC
2 + 1 =OSC
2BAUD 1
Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps)
ATmega328/P
USART in SPI (USARTSPI) Mode
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BAUD Baud rate (in bits per second, bps)
fOSC System oscillator clock frequency
UBRRn Contents of the UBRRnH and UBRRnL Registers, (0-4095)
25.4 SPI Data Modes and Timing
There are four combinations of XCKn (SCK) phase and polarity with respect to serial data, which are
determined by control bits UCPHAn and UCPOLn. The data transfer timing diagrams are shown in the
following figure. Data bits are shifted out and latched in on opposite edges of the XCKn signal, ensuring
sufficient time for data signals to stabilize. The UCPOLn and UCPHAn functionality is summarized in the
following table. Note that changing the setting of any of these bits will corrupt all ongoing communication
for both the receiver and transmitter.
Table 25-2. UCPOLn and UCPHAn Functionality
UCPOLn UCPHAn SPI Mode Leading Edge Trailing Edge
0 0 0 Sample (Rising) Setup (Falling)
0 1 1 Setup (Rising) Sample (Falling)
1 0 2 Sample (Falling) Setup (Rising)
1 1 3 Setup (Falling) Sample (Rising)
Figure 25-1. UCPHAn and UCPOLn Data Transfer Timing Diagrams
XCK
Data setup (TXD)
Data sample (RXD)
XCK
Data setup (TXD)
Data sample (RXD)
XCK
Data setup (TXD)
Data sample (RXD)
XCK
Data setup (TXD)
Data sample (RXD)
UCPOL=0 UCPOL=1
UCPHA=0 UCPHA=1
25.5 Frame Formats
A serial frame for the MSPIM is defined to be one character of eight data bits. The USART in MSPIM
mode has two valid frame formats:
• 8-bit data with MSB first
• 8-bit data with LSB first
ATmega328/P
USART in SPI (USARTSPI) Mode
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A frame starts with the least or most significant data bit. Then the next data bits, up to a total of eight, are
succeeding, ending with the most or least significant bit accordingly. When a complete frame is
transmitted, a new frame can directly follow it, or the communication line can be set to an idle (high) state.
The UDORDn bit in UCSRnC sets the frame format used by the USART in MSPIM mode. The receiver
and transmitter use the same setting. Note that changing the setting of any of these bits will corrupt all
ongoing communication for both the receiver and transmitter.
16-bit data transfer can be achieved by writing two data bytes to UDRn. A UART transmit complete
interrupt will then signal that the 16-bit value has been shifted out.
25.5.1 USART MSPIM Initialization
The USART in MSPIM mode has to be initialized before any communication can take place. The
initialization process normally consists of setting the baud rate, setting Master mode of operation (by
setting DDR_XCKn to one), setting frame format and enabling the transmitter and the receiver. Only the
transmitter can operate independently. For interrupt driven USART operation, the global interrupt flag
should be cleared (and thus interrupts globally disabled) when doing the initialization.
Note: To ensure immediate initialization of the XCKn output the Baud-Rate Register (UBRRn) must be
zero at the time the transmitter is enabled. Contrary to the normal mode USART operation the UBRRn
must then be written to the desired value after the transmitter is enabled, but before the first transmission
is started. Setting UBRRn to zero before enabling the transmitter is not necessary if the initialization is
done immediately after a Reset since UBRRn is reset to zero.
Before doing a re-initialization with changed baud rate, Data mode, or frame format, be sure that there
are no ongoing transmissions during the period the registers are changed. The TXCn flag can be used to
check that the transmitter has completed all transfers, and the RXCn flag can be used to check that there
are no unread data in the receive buffer. Note that the TXCn flag must be cleared before each
transmission (before UDRn is written) if it is used for this purpose.
The following simple USART initialization code examples show one assembly and one C function that are
equal in functionality. The examples assume polling (no interrupts enabled). The baud rate is given as a
function parameter. For the assembly code, the baud rate parameter is assumed to be stored in the
r17:r16 registers.
Assembly Code Example
clr r18
out UBRRnH,r18
out UBRRnL,r18
; Setting the XCKn port pin as output, enables master mode.
sbi XCKn_DDR, XCKn
; Set MSPI mode of operation and SPI data mode 0.
ldi r18, (1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn)
out UCSRnC,r18
; Enable receiver and transmitter.
ldi r18, (1<<RXENn)|(1<<TXENn)
out UCSRnB,r18
; Set baud rate.
; IMPORTANT: The Baud Rate must be set after the transmitter is enabled!
out UBRRnH, r17
out UBRRnL, r18
ret
C Code Example
{
UBRRn = 0;
ATmega328/P
USART in SPI (USARTSPI) Mode
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/* Setting the XCKn port pin as output, enables master mode. */
XCKn_DDR |= (1<<XCKn);
/* Set MSPI mode of operation and SPI data mode 0. */
UCSRnC = (1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn);
/* Enable receiver and transmitter. */
UCSRnB = (1<<RXENn)|(1<<TXENn);
/* Set baud rate. */
/* IMPORTANT: The Baud Rate must be set after the transmitter is enabled */
UBRRn = baud;
}
Related Links
About Code Examples
25.6 Data Transfer
Using the USART in MSPI mode requires the transmitter to be enabled, i.e., the TXENn bit in the
UCSRnB register is set to one. When the transmitter is enabled, the normal port operation of the TxDn
pin is overridden and given the function as the transmitter's serial output. Enabling the receiver is optional
and is done by setting the RXENn bit in the UCSRnB register to one. When the receiver is enabled, the
normal pin operation of the RxDn pin is overridden and given the function as the receiver's serial input.
The XCKn will in both cases be used as the transfer clock.
After initialization, the USART is ready for doing data transfers. A data transfer is initiated by writing to the
UDRn I/O location. This is the case for both sending and receiving data since the transmitter controls the
transfer clock. The data written to UDRn is moved from the transmit buffer to the shift register when the
shift register is ready to send a new frame.
Note: To keep the input buffer in sync with the number of data bytes transmitted, the UDRn register
must be read once for each byte transmitted. The input buffer operation is identical to normal USART
mode, i.e., if an overflow occurs the character last received will be lost, not the first data in the buffer. This
means that if four bytes are transferred, byte 1 first, then byte 2, 3, and 4, and the UDRn is not read
before all transfers are completed, then byte 3 to be received will be lost, and not byte 1.
The following code examples show a simple USART in MSPIM mode transfer function based on polling of
the Data Register Empty (UDREn) flag and the Receive Complete (RXCn) flag. The USART has to be
initialized before the function can be used. For the assembly code, the data to be sent is assumed to be
stored in register R16 and the data received will be available in the same register (R16) after the function
returns.
The function simply waits for the transmit buffer to be empty by checking the UDREn flag before loading it
with new data to be transmitted. The function then waits for data to be present in the receive buffer by
checking the RXCn flag before reading the buffer and returning the value.
Assembly Code Example
USART_MSPIM_Transfer:
; Wait for empty transmit buffer
in r16, UCSRnA
sbrs r16, UDREn
rjmp USART_MSPIM_Transfer
; Put data (r16) into buffer, sends the data
out UDRn,r16
; Wait for data to be received
USART_MSPIM_Wait_RXCn:
in r16, UCSRnA
sbrs r16, RXCn
rjmp USART_MSPIM_Wait_RXCn
; Get and return received data from buffer
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in r16, UDRn
ret
C Code Example
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn)) );
/* Put data into buffer, sends the data */
UDRn = data;
/* Wait for data to be received */
while ( !(UCSRnA & (1<<RXCn)) );
/* Get and return received data from buffer */
return UDRn;
}
Related Links
About Code Examples
25.6.1 Transmitter and Receiver Flags and Interrupts
The RXCn, TXCn, and UDREn flags and corresponding interrupts in USART in MSPIM mode are
identical in function to the normal USART operation. However, the receiver error status flags (FE, DOR,
and PE) are not in use and is always read as zero.
25.6.2 Disabling the Transmitter or Receiver
The disabling of the transmitter or receiver in USART in MSPIM mode is identical in function to the normal
USART operation.
25.7 AVR USART MSPIM vs. AVR SPI
The USART in MSPIM mode is fully compatible with the AVR SPI regarding:
• Master mode timing diagram
• The UCPOLn bit functionality is identical to the SPI CPOL bit
• The UCPHAn bit functionality is identical to the SPI CPHA bit
• The UDORDn bit functionality is identical to the SPI DORD bit
However, since the USART in MSPIM mode reuses the USART resources, the use of the USART in
MSPIM mode is somewhat different compared to the SPI. There are differences in the Control Register
bits and only the master operation is supported by the USART in MSPIM mode. Additionally, the following
features differ between the two modules:
• The USART in MSPIM mode includes (double) buffering of the transmitter. The SPI has no buffer.
• The USART in MSPIM mode receiver includes an additional buffer level
• The SPI WCOL (Write Collision) bit is not included in USART in MSPIM mode
• The SPI double speed mode (SPI2X) bit is not included. However, the same effect is achieved by
setting UBRRn accordingly.
• Interrupt timing is not compatible
• Pin control differs due to the master only operation of the USART in MSPIM mode
A comparison of the USART in MSPIM mode and the SPI pins is shown in the table below.
ATmega328/P
USART in SPI (USARTSPI) Mode
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Table 25-3. Comparison of USART in MSPIM Mode and SPI Pins
USART_MSPIM SPI Comments
TxDn MOSI Master Out only
RxDn MISO Master In only
XCKn SCK (Functionally identical)
(N/A) SS Not supported by USART in MSPIM
25.8 Register Description
Refer to the USART register description.
Related Links
Register Description
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USART in SPI (USARTSPI) Mode
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26. Two-Wire Serial Interface (TWI)
26.1 Features
• Simple, yet Powerful and Flexible Communication Interface, only two Bus Lines Needed
• Both Master and Slave Operation Supported
• Device can Operate as Transmitter or Receiver
• 7-bit Address Space Allows up to 128 Different Slave Addresses
• Multi-master Arbitration Support
• Up to 400 kHz Data Transfer Speed
• Slew-rate Limited Output Drivers
• Noise Suppression Circuitry Rejects Spikes on Bus Lines
• Fully Programmable Slave Address with General Call Support
• Address Recognition Causes Wake-up When AVR is in Sleep Mode
• Compatible with Philips’ I2C protocol
26.2 Two-Wire Serial Interface Bus Definition
The Two-Wire Serial Interface (TWI) is ideally suited for typical microcontroller applications. The TWI
protocol allows the systems designer to interconnect up to 128 different devices using only two bi-
directional bus lines: one for clock (SCL) and one for data (SDA). The only external hardware needed to
implement the bus is a single pull-up resistor for each of the TWI bus lines. All devices connected to the
bus have individual addresses, and mechanisms for resolving bus contention are inherent in the TWI
protocol.
Figure 26-1. TWI Bus Interconnection
SDA
SCL
........ R1 R2
VCC
Device 1 Device 2 Device 3 Device n
26.2.1 TWI Terminology
The following definitions are frequently encountered in this section.
ATmega328/P
Two-Wire Serial Interface (TWI)
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Table 26-1. TWI Terminology
Term Description
Master The device that initiates and terminates a transmission. The master also generates the
SCL clock.
Slave The device addressed by a master.
Transmitter The device placing data on the bus.
Receiver The device reading data from the bus.
This device has one instance of TWI. For this reason, the instance index n is omitted.
The Power Reduction TWI bit in the Power Reduction Register (PRRn.PRTWI) must be written to '0' to
enable the two-wire Serial Interface.
TWI0 is in PRR.
Related Links
Power Management and Sleep Modes
26.2.2 Electrical Interconnection
As depicted in the TWI bus definition, both bus lines are connected to the positive supply voltage through
pull-up resistors. The bus drivers of all TWI-compliant devices are open-drain or open-collector. This
implements a wired-AND function, which is essential to the operation of the interface. A low level on a
TWI bus line is generated when one or more TWI devices output a zero. A high level is output when all
TWI devices tri-state their outputs, allowing the pull-up resistors to pull the line high. Note that all AVR
devices connected to the TWI bus must be powered in order to allow any bus operation.
The number of devices that can be connected to the bus is only limited by the bus capacitance limit of
400 pF and the 7-bit slave address space. Two different sets of specifications are presented there, one
relevant for bus speeds below 100 kHz, and one valid for bus speeds up to 400 kHz.
26.3 Data Transfer and Frame Format
26.3.1 Transferring Bits
Each data bit transferred on the TWI bus is accompanied by a pulse on the clock line. The level of the
data line must be stable when the clock line is high. The only exception to this rule is for generating start
and stop conditions.
Figure 26-2. Data Validity
SD
A
SCL
Data Stable Data Stable
Data Change
ATmega328/P
Two-Wire Serial Interface (TWI)
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26.3.2 START and STOP Conditions
The master initiates and terminates a data transmission. The transmission is initiated when the master
issues a START condition on the bus, and it is terminated when the master issues a STOP condition.
Between a START and a STOP condition, the bus is considered busy, and no other master should try to
seize control of the bus. A special case occurs when a new START condition is issued between a START
and STOP condition. This is referred to as a REPEATED START condition and is used when the master
wishes to initiate a new transfer without relinquishing control of the bus. After a REPEATED START, the
bus is considered busy until the next STOP. This is identical to the START behavior, and therefore START
is used to describe both START and REPEATED START for the remainder of this data sheet unless
otherwise noted. As depicted below, START and STOP conditions are signaled by changing the level of
the SDA line when the SCL line is high.
Figure 26-3. START, REPEATED START, and STOP Conditions
SDA
SCL
START STOPREPEATED STARTSTOP START
26.3.3 Address Packet Format
All address packets transmitted on the TWI bus are nine bits long, consisting of seven address bits, one
READ/WRITE control bit, and an acknowledge bit. If the READ/WRITE bit is set, a read operation is to be
performed, otherwise, a write operation should be performed. When a slave recognizes that it is being
addressed, it should acknowledge by pulling SDA low in the ninth SCL (ACK) cycle. If the addressed
slave is busy, or for some other reason cannot service the master’s request, the SDA line should be left
high in the ACK clock cycle. The master can then transmit a STOP condition, or a REPEATED START
condition to initiate a new transmission. An address packet consisting of a slave address and a READ or
a WRITE bit is called SLA+R or SLA+W, respectively.
The MSB of the address byte is transmitted first. Slave addresses can freely be allocated by the designer,
but the address '0000 000' is reserved for a general call.
When a general call is issued, all slaves should respond by pulling the SDA line low in the ACK cycle. A
general call is used when a master wishes to transmit the same message to several slaves in the system.
When the general call address followed by a Write bit is transmitted on the bus, all slaves set up to
acknowledge the general call will pull the SDA line low in the ACK cycle. The following data packets will
then be received by all the slaves that acknowledged the general call. Note that transmitting the general
call address followed by a Read bit is meaningless as this would cause contention if several slaves
started transmitting different data.
All addresses of the format '1111 xxx' should be reserved for future purposes.
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 264
Figure 26-4. Address Packet Format
SD A
SCL
ST AR T
1 2 7 8 9
Addr MSB Addr LSB R/W ACK
26.3.4 Data Packet Format
All data packets transmitted on the TWI bus are nine bits long, consisting of one data byte and an
acknowledge bit. During a data transfer, the ,master generates the clock and the START and STOP
conditions, while the receiver is responsible for acknowledging the reception. An Acknowledge (ACK) is
signaled by the receiver pulling the SDA line low during the ninth SCL cycle. If the receiver leaves the
SDA line high, a NACK is signaled. When the receiver has received the last byte, or for some reason
cannot receive any more bytes, it should inform the transmitter by sending a NACK after the final byte.
The MSB of the data byte is transmitted first.
Figure 26-5. Data Packet Format
1 2 7 8 9
Data MSB Data LSB ACK
Aggregate
SDA
SDA from
Transmitter
SDA from
Receiver
SCL from
Master
SLA+R/W Data Byte
STOP, REPEATED
START or Next
Data Byte
26.3.5 Combining Address and Data Packets Into a Transmission
A transmission basically consists of a START condition, a SLA+R/W, one or more data packets, and a
STOP condition. An empty message, consisting of a START followed by a STOP condition, is illegal. Note
that the "Wired-ANDing" of the SCL line can be used to implement handshaking between the master and
the slave. The slave can extend the SCL low period by pulling the SCL line low. This is useful if the clock
speed set up by the master is too fast for the slave, or the slave needs extra time for processing between
the data transmissions. The slave extending the SCL low period will not affect the SCL high period, which
is determined by the master. As a consequence, the slave can reduce the TWI data transfer speed by
prolonging the SCL duty cycle.
The following figure depicts a typical data transmission. Note that several data bytes can be transmitted
between the SLA+R/W and the STOP condition, depending on the software protocol implemented by the
application software.
ATmega328/P
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Figure 26-6. Typical Data Transmission
1 2 7 8 9
Data Byte
Data MSB Data LSB ACK
SDA
SCL
1 2 7 8 9
Addr MSB Addr LSB R/W ACK
SLA+R/W STOP
START
26.4 Multi-Master Bus Systems, Arbitration, and Synchronization
The TWI protocol allows bus systems with several masters. Special concerns have been taken in order to
ensure that transmissions will proceed as normal, even if two or more masters initiate a transmission at
the same time. Two problems arise in multi-master systems:
• An algorithm must be implemented allowing only one of the masters to complete the transmission.
All other masters should cease transmission when they discover that they have lost the selection
process. This selection process is called arbitration. When a contending master discovers that it
has lost the arbitration process, it should immediately switch to Slave mode to check whether it is
being addressed by the winning master. The fact that multiple masters have started transmission at
the same time should not be detectable to the slaves, i.e. the data being transferred on the bus
must not be corrupted.
• Different masters may use different SCL frequencies. A scheme must be devised to synchronize
the serial clocks from all masters, in order to let the transmission proceed in a lockstep fashion.
This will facilitate the arbitration process.
The wired-ANDing of the bus lines is used to solve both these problems. The serial clocks from all
masters will be wired-ANDed, yielding a combined clock with a high period equal to the one from the
master with the shortest high period. The low period of the combined clock is equal to the low period of
the master with the longest low period. Note that all masters listen to the SCL line, effectively starting to
count their SCL high and low time-out periods when the combined SCL line goes high or low,
respectively.
ATmega328/P
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Figure 26-7. SCL Synchronization Between Multiple Masters
TA low TA high
SCL from
Master A
SCL from
Master B
SCL Bus
Line
TBlow TBhigh
Masters Start
Counting Low Period
Masters Start
Counting High Period
Arbitration is carried out by all masters continuously monitoring the SDA line after outputting data. If the
value read from the SDA line does not match the value the master had output, it has lost the arbitration.
Note that a master can only lose arbitration when it outputs a high SDA value while another master
outputs a low value. The losing master should immediately go to Slave mode, checking if it is being
addressed by the winning master. The SDA line should be left high, but losing masters are allowed to
generate a clock signal until the end of the current data or address packet. Arbitration will continue until
only one master remains, and this may take many bits. If several masters are trying to address the same
slave, arbitration will continue into the data packet.
Figure 26-8. Arbitration Between Two Masters
SDA from
Master A
SDA from
Master B
SDA Line
Synchronized
SCL Line
START Master A Loses
Arbitration, SDAA
SDA
Note that arbitration is not allowed between:
• A REPEATED START condition and a data bit
• A STOP condition and a data bit
• A REPEATED START and a STOP condition
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It is the user software’s responsibility to ensure that these illegal arbitration conditions never occur. This
implies that in multi-master systems, all data transfers must use the same composition of SLA+R/W and
data packets. In other words; All transmissions must contain the same number of data packets,
otherwise, the result of the arbitration is undefined.
26.5 Overview of the TWI Module
The TWI module is comprised of several submodules, as shown in the following figure. The registers
drawn in a thick line are accessible through the AVR data bus.
Figure 26-9. Overview of the TWI Module
TWI Unit
Address Register
(TWAR)
Address Match Unit
Address Comparator
Control Unit
Control Register
(TWCR)
Status Register
(TWSR)
State Machine and
Status control
SCL
Slew-rate
Control
Spike
Filter
SDA
Control
Spike
Filter
Bit Rate Generator
Bit Rate Register
(TWBR)
Prescaler
Bus Interface Unit
START / STOP
Control
Arbitration detection Ack
Spike Suppression
Address/Data Shift
Register (TWDR)
Slew-rate
26.5.1 SCL and SDA Pins
These pins interface the AVR TWI with the rest of the MCU system. The output drivers contain a slew-
rate limiter in order to conform to the TWI specification. The input stages contain a spike suppression unit
removing spikes shorter than 50 ns. Note that the internal pull-ups in the AVR pads can be enabled by
setting the PORT bits corresponding to the SCL and SDA pins, as explained in the I/O Port section. The
internal pull-ups can in some systems eliminate the need for external ones.
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26.5.2 Bit Rate Generator Unit
This unit controls the period of SCL when operating in a Master mode. The SCL period is controlled by
settings in the TWI Bit Rate Register (TWBRn) and the Prescaler bits in the TWI Status Register
(TWSRn). Slave operation does not depend on bit rate or prescaler settings, but the CPU clock frequency
in the slave must be at least 16 times higher than the SCL frequency. Note that slaves may prolong the
SCL low period, thereby reducing the average TWI bus clock period.
The SCL frequency is generated according to the following equation:
SCLfrequency = CPUClockfrequency
16 + 2(TWBR) PrescalerValue
• TWBR = Value of the TWI Bit Rate Register TWBRn
• PrescalerValue = Value of the prescaler, see description of the TWI Prescaler bits in the TWSR
Status Register description (TWSRn.TWPS[1:0])
Note: Pull-up resistor values should be selected according to the SCL frequency and the capacitive bus
line load. See the Two-Wire Serial Interface Characteristics for a suitable value of the pull-up resistor.
Related Links
Two-Wire Serial Interface Characteristics
26.5.3 Bus Interface Unit
This unit contains the Data and Address Shift Register (TWDRn), a START/STOP controller, and
arbitration detection hardware. The TWDRn contains the address or data bytes to be transmitted, or the
address or data bytes received. In addition to the 8-bit TWDRn, the bus interface unit also contains a
register containing the (N)ACK bit to be transmitted or received. This (N)ACK register is not directly
accessible by the application software. However, when receiving, it can be set or cleared by manipulating
the TWI Control Register (TWCRn). When in Transmitter mode, the value of the received (N)ACK bit can
be determined by the value in the TWSRn.
The START/STOP controller is responsible for generation and detection of START, REPEATED START,
and STOP conditions. The START/STOP controller is able to detect the START and STOP conditions
even when the AVR MCU is in one of the sleep modes, enabling the MCU to wake up if addressed by a
master.
If the TWI has initiated a transmission as master, the arbitration detection hardware continuously monitors
the transmission trying to determine if arbitration is in process. If the TWI has lost an arbitration, the
control unit is informed. Correct action can then be taken and appropriate status codes generated.
26.5.4 Address Match Unit
The address match unit checks if received address bytes match the seven-bit address in the TWI
Address Register (TWARn). If the TWI General Call Recognition Enable bit (TWARn.TWGCE) is written
to '1', all incoming address bits will also be compared against the general call address. Upon an address
match, the control unit is informed, allowing the correct action to be taken. The TWI may or may not
acknowledge its address, depending on settings in the TWI Control Register (TWCRn). The address
match unit is able to compare addresses even when the AVR MCU is in Sleep mode, enabling the MCU
to wake up if addressed by a master.
26.5.5 Control Unit
The control unit monitors the TWI bus and generates responses corresponding to settings in the TWI
Control Register (TWCRn). When an event requiring the attention of the application occurs on the TWI
bus, the TWI Interrupt flag (TWINT) is asserted. In the next clock cycle, the TWI Status Register (TWSRn)
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is updated with a status code identifying the event. The TWSRn only contains relevant status information
when the TWI interrupt flag is asserted. At all other times, the TWSRn contains a special status code
indicating that no relevant status information is available. As long as the TWINT flag is set, the SCL line is
held low. This allows the application software to complete its tasks before allowing the TWI transmission
to continue.
The TWINT flag is set in the following situations:
• After the TWI has transmitted a START/REPEATED START condition
• After the TWI has transmitted SLA+R/W
• After the TWI has transmitted an address byte
• After the TWI has lost arbitration
• After the TWI has been addressed by own slave address or general call
• After the TWI has received a data byte
• After a STOP or REPEATED START has been received while still addressed as a slave
• When a bus error has occurred due to an illegal START or STOP condition
26.6 Using the TWI
The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus events, like
reception of a byte or transmission of a START condition. Because the TWI is interrupt-based, the
application software is free to carry on other operations during a TWI byte transfer. Note that the TWI
Interrupt Enable (TWIE) bit in TWCRn together with the Global Interrupt Enable bit in SREG allows the
application to decide whether or not an assertion of the TWINT flag should generate an interrupt request.
If the TWIE bit is cleared, the application must poll the TWINT flag in order to detect actions on the TWI
bus.
When the TWINT flag is asserted, the TWI has finished an operation and awaits application response. In
this case, the TWI Status Register (TWSRn) contains a value indicating the current state of the TWI bus.
The application software can then decide how the TWI should behave in the next TWI bus cycle by
manipulating the TWCRn and TWDRn registers.
The following figure illustrates a simple example of how the application can interface to the TWI
hardware. In this example, a master wishes to transmit a single data byte to a slave. A more detailed
explanation follows later in this section. Simple code examples are presented in the table below.
ATmega328/P
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Figure 26-10. Interfacing the Application to the TWI in a Typical Transmission
START SLA+W A Data A STOP
1. Application
writes to TWCR to
initiate
transmission of
START
2.TWINT set.
Status code indicates
START condition sent
4.TWINT set.
Status code indicates
SLA+W sent, ACK
received
6.TWINT set.
Status code indicates
data sent, ACK received
3. Check TWSR to see if START was
sent. Application loads SLA+W into
TWDR, and loads appropriate control
signals into TWCR, making sure that
TWINT is written to one,
and TWSTA is written to zero.
5. CheckTWSR to see if SLA+W was
sent and ACK received.
Application loads data into TWDR, and
loads appropriate control signals into
TWCR, making sure that TWINT is
written to one
7. CheckTWSR to see if data was sent
and ACK received.
Application loads appropriate control
signals to send STOP into TWCR,
making sure that TWINT is written to one
TWI bus
Indicates
TWINT set
Application
Action
TWI
Hardware
Action
1. The first step in a TWI transmission is to transmit a START condition. This is done by writing a
specific value into TWCRn, instructing the TWI n hardware to transmit a START condition. Which
value to write is described later on. However, it is important that the TWINT bit is set to the value
written. Writing a one to TWINT clears the flag. The TWI n will not start any operation as long as
the TWINT bit in TWCRn is set. Immediately after the application has cleared TWINT, the TWI n will
initiate transmission of the START condition.
2. When the START condition has been transmitted, the TWINT flag in TWCRn is set, and TWSRn is
updated with a status code indicating that the START condition has successfully been sent.
3. The application software should now examine the value of TWSRn to make sure that the START
condition was successfully transmitted. If TWSRn indicates otherwise, the application software
might take some special action, like calling an error routine. Assuming that the status code is as
expected, the application must load SLA+W into TWDR. Remember that TWDRn is used both for
address and data. After TWDRn has been loaded with the desired SLA+W, a specific value must
be written to TWCRn, instructing the TWIn hardware to transmit the SLA+W present in TWDRn.
Which value to write is described later on. However, it is important that the TWINT bit is set to the
value written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long
as the TWINT bit in TWCRn is set. Immediately after the application has cleared TWINT, the TWI
will initiate transmission of the address packet.
4. When the address packet has been transmitted, the TWINT flag in TWCRn is set, and TWSRn is
updated with a status code indicating that the address packet has successfully been sent. The
status code will also reflect whether a slave acknowledged the packet or not.
5. The application software should now examine the value of TWSRn, to make sure that the address
packet was successfully transmitted, and that the value of the ACK bit was as expected. If TWSRn
indicates otherwise, the application software might take some special action, like calling an error
routine. Assuming that the status code is as expected, the application must load a data packet into
TWDRn. Subsequently, a specific value must be written to TWCRn, instructing the TWI n hardware
to transmit the data packet present in TWDRn. Which value to write is described later on. However,
it is important that the TWINT bit is set to the value written. Writing a one to TWINT clears the flag.
The TWI n will not start any operation as long as the TWINT bit in TWCRn is set. Immediately after
the application has cleared TWINT, the TWI will initiate transmission of the data packet.
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6. When the data packet has been transmitted, the TWINT flag in TWCRn is set and TWSRn is
updated with a status code indicating that the data packet has successfully been sent. The status
code will also reflect whether a slave acknowledged the packet or not.
7. The application software should now examine the value of TWSRn, to make sure that the data
packet was successfully transmitted, and that the value of the ACK bit was as expected. If TWSR
indicates otherwise, the application software might take some special action, like calling an error
routine. Assuming that the status code is as expected, the application must write a specific value to
TWCRn, instructing the TWI n hardware to transmit a STOP condition. Which value to write is
described later on. However, it is important that the TWINT bit is set to the value written. Writing a
one to TWINT clears the flag. The TWI n will not start any operation as long as the TWINT bit in
TWCRn is set. Immediately after the application has cleared TWINT, the TWI will initiate
transmission of the STOP condition. Note that TWINT is not set after a STOP condition has been
sent.
Even though this example is simple, it shows the principles involved in all TWI transmissions. These can
be summarized as follows:
• When the TWI has finished an operation and expects application response, the TWINT flag is set.
The SCL line is pulled low until TWINT is cleared.
• When the TWINT flag is set, the user must update all TWI n registers with the value relevant for the
next TWI n bus cycle. As an example, TWDRn must be loaded with the value to be transmitted in
the next bus cycle.
• After all TWI n register updates and other pending application software tasks have been completed,
TWCRn is written. When writing TWCRn, the TWINT bit should be set. Writing a one to TWINT
clears the flag. The TWI n will then commence executing whatever operation was specified by the
TWCRn setting.
The following table lists assembly and C implementation examples for TWI0. Note that the code below
assumes that several definitions have been made, e.g. by using include-files.
Table 26-2. Assembly and C Code Example
Assembly Code Example C Example Comments
1ldi r16, (1<<TWINT)|
(1<<TWSTA)|(1<<TWEN)
out TWCR0, r16
TWCR0 = (1<<TWINT)|
(1<<TWSTA)|(1<<TWEN)
Send START condition
2wait1:
in r16,TWCR0
sbrs r16,TWINT
rjmp wait1
while (!(TWCR0 &
(1<<TWINT)));
Wait for TWINT Flag set. This indicates
that the START condition has been
transmitted.
3in r16,TWSR0
andi r16, 0xF8
cpi r16, START
brne ERROR
if ((TWSR0 & 0xF8) !=
START)
ERROR();
Check value of TWI Status Register. Mask
prescaler bits. If status different from
START go to ERROR.
ldi r16, SLA_W
out TWDR0, r16
ldi r16, (1<<TWINT) |
(1<<TWEN)
out TWCR0, r16
TWDR0 = SLA_W;
TWCR0 = (1<<TWINT) |
(1<<TWEN);
Load SLA_W into TWDR Register. Clear
TWINT bit in TWCR to start transmission of
address.
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Assembly Code Example C Example Comments
4wait2:
in r16,TWCR0
sbrs r16,TWINT
rjmp wait2
while (!(TWCR0 &
(1<<TWINT)));
Wait for TWINT Flag set. This indicates
that the SLA+W has been transmitted, and
ACK/NACK has been received.
5in r16,TWSR0
andi r16, 0xF8
cpi r16, MT_SLA_ACK
brne ERROR
if ((TWSR0 & 0xF8) !=
MT_SLA_ACK) ERROR();
Check value of TWI Status Register. Mask
prescaler bits. If status different from
MT_SLA_ACK go to ERROR.
ldi r16, DATA
out TWDR0, r16
ldi r16, (1<<TWINT) |
(1<<TWEN)
out TWCR, r16
TWDR0 = DATA;
TWCR0 = (1<<TWINT) |
(1<<TWEN);
Load DATA into TWDR Register. Clear
TWINT bit in TWCR to start transmission of
data.
6wait3:
in r16,TWCR0
sbrs r16,TWINT
rjmp wait3
while (!(TWCR0 &
(1<<TWINT)));
Wait for TWINT Flag set. This indicates
that the DATA has been transmitted, and
ACK/NACK has been received.
7in r16,TWSR0
andi r16, 0xF8
cpi r16, MT_DATA_ACK
brne ERROR
if ((TWSR0 & 0xF8) !=
MT_DATA_ACK) ERROR();
Check value of TWI Status Register. Mask
prescaler bits. If status different from
MT_DATA_ACK go to ERROR.
ldi r16, (1<<TWINT)|
(1<<TWEN)| (1<<TWSTO)
out TWCR0, r16
TWCR0 = (1<<TWINT)|
(1<<TWEN)|(1<<TWSTO);
Transmit STOP condition.
26.7 Transmission Modes
The TWI can operate in one of four major modes:
• Master Transmitter (MT)
• Master Receiver (MR)
• Slave Transmitter (ST)
• Slave Receiver (SR)
Several of these modes can be used in the same application. As an example, the TWI can use MT mode
to write data into a TWI EEPROM, MR mode to read the data back from the EEPROM. If other masters
are present in the system, some of these might transmit data to the TWI, and then SR mode would be
used. It is the application software that decides which modes are legal.
The following sections describe each of these modes. Possible status codes are described along with
figures detailing data transmission in each of the modes. These figures use the following abbreviations:
S START condition
Rs REPEATED START condition
R Read bit (high level at SDA)
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W Write bit (low level at SDA)
A Acknowledge bit (low level at SDA)
A Not acknowledge bit (high level at SDA)
Data 8-bit data byte
P STOP condition
SLA Slave Address
Circles are used to indicate that the TWINT flag is set. The numbers in the circles show the status code
held in TWSRn, with the prescaler bits masked to zero. At these points, actions must be taken by the
application to continue or complete the TWI transfer. The TWI transfer is suspended until the TWINT flag
is cleared by software.
When the TWINT flag is set, the status code in TWSRn is used to determine the appropriate software
action. For each status code, the required software action and details of the following serial transfer are
given below in the status code table for each mode. Note that the prescaler bits are masked to zero in
these tables.
26.7.1 Master Transmitter Mode
In the Master Transmitter (MT) mode, a number of data bytes are transmitted to a slave receiver, see the
figure below. In order to enter a Master mode, a START condition must be transmitted. The format of the
following address packet determines whether MT or Master Receiver (MR) mode is to be entered: If SLA
+W is transmitted the MT mode is entered, if SLA+R is transmitted the MR mode is entered. All the status
codes mentioned in this section assume that the prescaler bits are zero or masked to zero.
Figure 26-11. Data Transfer in Master Transmitter Mode
Device 1
MASTER
TRANSMITTER
Device 2
SLAVE
RECEIVER
Device 3 Device n
SDA
SCL
........ R1 R2
VCC
A START condition is sent by writing a value to the TWI Control Register n (TWCRn) of the type
TWCRn=1x10x10x:
• The TWI Enable bit (TWCRn.TWEN) must be written to '1' to enable the two-wire serial interface
• The TWI Start Condition bit (TWCRn.TWSTA) must be written to '1' to transmit a START condition
• The TWI Interrupt Flag (TWCRn.TWINT) must be written to '1' to clear the flag.
The TWI n will then test the two-wire serial bus and generate a START condition as soon as the bus
becomes free. After a START condition has been transmitted, the TWINT flag is set by hardware, and the
status code in TWSRn will be 0x08 (see Status Code table below). In order to enter MT mode, SLA+W
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must be transmitted. This is done by writing SLA+W to the TWI Data Register (TWDRn). Thereafter, the
TWCRn.TWINT flag should be cleared (by writing a '1' to it) to continue the transfer. This is accomplished
by writing a value to TWRC of the type TWCR=1x00x10x.
When SLA+W has been transmitted and an acknowledgment bit has been received, TWINT is set again
and a number of status codes in TWSR are possible. Possible status codes in Master mode are 0x18,
0x20, or 0x38. The appropriate action to be taken for each of these status codes is detailed in the status
code table below.
When SLA+W has been successfully transmitted, a data packet should be transmitted. This is done by
writing the data byte to TWDR. TWDR must only be written when TWINT is high. If not, the access will be
discarded, and the Write Collision bit (TWWC) will be set in the TWCRn register. After updating TWDRn,
the TWINT bit should be cleared (by writing '1' to it) to continue the transfer. This is accomplished by
writing again a value to TWCRn of the type TWCRn=1x00x10x.
This scheme is repeated until the last byte has been sent and the transfer is ended, either by generating
a STOP condition or a by a repeated START condition. A repeated START condition is accomplished by
writing a regular START value TWCRn=1x10x10x. A STOP condition is generated by writing a value of
the type TWCRn=1x01x10x.
After a repeated START condition (status code 0x10), the two-wire serial interface can access the same
slave again, or a new slave without transmitting a STOP condition. Repeated START enables the master
to switch between slaves, Master Transmitter mode, and Master Receiver mode without losing control of
the bus.
Table 26-3. Status Codes for Master Transmitter Mode
Status Code
(TWSR)
Prescaler
Bits are 0
Status of the Two-Wire
Serial Bus and Two-Wire
Serial Interface Hardware
Application Software Response Next Action Taken by TWI
Hardware
To/From
TWDR
To TWCRn
STA STO TWINT TWEA
0x08 A START condition has been
transmitted
Load SLA+W 0 0 1 X SLA+W will be transmitted;
ACK or NOT ACK will be
received
0x10 A repeated START condition
has been transmitted
Load SLA+W
or
0 0 1 X SLA+W will be transmitted;
ACK or NOT ACK will be
received
Load SLA+R 0 0 1 X SLA+R will be transmitted;
Logic will switch to Master
Receiver mode
0x18 SLA+W has been transmitted;
ACK has been received
Load data
byte or
0 0 1 X Data byte will be transmitted
and ACK or NOT ACK will be
received
No TWDR
action or
1 0 1 X Repeated START will be
transmitted
No TWDR
action or
0 1 1 X STOP condition will be
transmitted and TWSTO Flag
will be reset
No TWDR
action
1 1 1 X STOP condition followed by a
START condition will be
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Status Code
(TWSR)
Prescaler
Bits are 0
Status of the Two-Wire
Serial Bus and Two-Wire
Serial Interface Hardware
Application Software Response Next Action Taken by TWI
Hardware
To/From
TWDR
To TWCRn
STA STO TWINT TWEA
transmitted and TWSTO Flag
will be reset
0x20 SLA+W has been transmitted;
NOT ACK has been received
Load data
byte or
0 0 1 X Data byte will be transmitted
and ACK or NOT ACK will be
received
No TWDR
action or
1 0 1 X Repeated START will be
transmitted
No TWDR
action or
0 1 1 X STOP condition will be
transmitted and TWSTO Flag
will be reset
No TWDR
action
1 1 1 X STOP condition followed by a
START condition will be
transmitted and TWSTO Flag
will be reset
0x28 Data byte has been
transmitted;
ACK has been received
Load data
byte or
0 0 1 X Data byte will be transmitted
and ACK or NOT ACK will be
received
No TWDR
action or
1 0 1 X Repeated START will be
transmitted
No TWDR
action or
0 1 1 X STOP condition will be
transmitted and TWSTO Flag
will be reset
No TWDR
action
1 1 1 X STOP condition followed by a
START condition will be
transmitted and TWSTO Flag
will be reset
0x30 Data byte has been
transmitted;
NOT ACK has been received
Load data
byte or
0 0 1 X Data byte will be transmitted
and ACK or NOT ACK will be
received
No TWDR
action or
1 0 1 X Repeated START will be
transmitted
No TWDR
action or
0 1 1 X STOP condition will be
transmitted and TWSTO Flag
will be reset
No TWDR
action
1 1 1 X STOP condition followed by a
START condition will be
transmitted and TWSTO Flag
will be reset
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Status Code
(TWSR)
Prescaler
Bits are 0
Status of the Two-Wire
Serial Bus and Two-Wire
Serial Interface Hardware
Application Software Response Next Action Taken by TWI
Hardware
To/From
TWDR
To TWCRn
STA STO TWINT TWEA
0x38 Arbitration lost in SLA+W or
data bytes
No TWDR
action or
0 0 1 X two-wire Serial Bus will be
released and not addressed
Slave mode entered
No TWDR
action
1 0 1 X A START condition will be
transmitted when the bus
becomes free
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 277
Figure 26-12. Formats and States in the Master Transmitter Mode
S SLA W A DATA A P
0x08 0x18 0x28
R SLA W
0x10
A P
0x20
P
0x30
A or A
0x38
A
Other master
continues
A or A
0x38
Other master
continues
R
A
0x68
Other master
continues
0x78 0xB0
To corresponding
states in slave mode
MT
MR
Successfull
transmission
to a slave
receiver
Next transfer
started with a
repeated start
condition
Not acknowledge
received after the
slave address
Not acknowledge
received after a data
byte
Arbitration lost in slave
address or data byte
Arbitration lost and
addressed as slave
DATA A
n
From master to slave
From slave to master
Any number of data bytes
and their associated acknowledge bits
This number (contained in TWSR) corresponds
to a defined state of the Two-Wire Serial Bus. The
prescaler bits are zero or masked to zero.
S
26.7.2 Master Receiver Mode
In the Master Receiver (MR) mode, a number of data bytes are received from a slave transmitter (see
next figure). In order to enter a Master mode, a START condition must be transmitted. The format of the
following address packet determines whether Master Transmitter (MT) or MR mode is to be entered. If
SLA+W is transmitted the MT mode is entered, if SLA+R is transmitted the MR mode is entered. All the
status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero.
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 278
Figure 26-13. Data Transfer in Master Receiver Mode
Device 1
MASTER
RECEIVER
Device 2
SLAVE
TRANSMITTER
Device 3 Device n
SDA
SCL
........ R1 R2
VCC
A START condition is sent by writing to the TWI Control Register (TWCRn) a value of the type
TWCRn=1x10x10x:
• TWCRn.TWEN must be written to '1' to enable the two-wire serial interface
• TWCRn.TWSTA must be written to '1' to transmit a START condition
• TWCRn.TWINT must be cleared by writing a '1' to it
The TWI will then test the two-wire serial bus and generate a START condition as soon as the bus
becomes free. After a START condition has been transmitted, the TWINT flag is set by hardware and the
status code in TWSRn will be 0x08 (see the Status Code table below). In order to enter MR mode, SLA
+R must be transmitted. This is done by writing SLA+R to TWDR. Thereafter, the TWINT flag should be
cleared (by writing '1' to it) to continue the transfer. This is accomplished by writing a value to TWCRn of
the type TWCRn=1x00x10x.
When SLA+R has been transmitted and an acknowledgment bit has been received, TWINT is set again
and a number of status codes in TWSRn are possible. Possible status codes in Master mode are 0x38,
0x40, or 0x48. The appropriate action to be taken for each of these status codes is detailed in the table
below. Received data can be read from the TWDR register when the TWINT flag is set high by hardware.
This scheme is repeated until the last byte has been received. After the last byte has been received, the
MR should inform the ST by sending a NACK after the last received data byte. The transfer is ended by
generating a STOP condition or a repeated START condition. A repeated START condition is sent by
writing to the TWI Control Register (TWCRn) a value of the type TWCRn=1x10x10x again. A STOP
condition is generated by writing TWCRn=1x01x10x:
After a repeated START condition (status code 0x10) the two-wire Serial Interface can access the same
Slave again, or a new slave without transmitting a STOP condition. Repeated START enables the master
to switch between slaves, Master Transmitter mode and Master Receiver mode without losing control
over the bus.
Table 26-4. Status codes for Master Receiver Mode
Status Code
(TWSRn)
Prescaler Bits
are 0
Status of the Two-Wire Serial
Bus and Two-Wire Serial
Interface Hardware
Application Software Response Next Action Taken by TWI
Hardware
To/From
TWD
To TWCRn
STA STO TWINT TWEA
0x08 A START condition has been
transmitted
Load SLA+R 0 0 1 X SLA+R will be transmitted
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 279
Status Code
(TWSRn)
Prescaler Bits
are 0
Status of the Two-Wire Serial
Bus and Two-Wire Serial
Interface Hardware
Application Software Response Next Action Taken by TWI
Hardware
To/From
TWD
To TWCRn
STA STO TWINT TWEA
ACK or NOT ACK will be
received
0x10 A repeated START condition
has been transmitted
Load SLA+R 0 0 1 X SLA+R will be transmitted
ACK or NOT ACK will be
received
Load SLA+W 0 0 1 X SLA+W will be transmitted
Logic will switch to Master
Transmitter mode
0x38 Arbitration lost in SLA+R or
NOT ACK bit
No TWDR
action
0 0 1 X two-wire serial bus will be
released and not addressed
Slave mode will be entered
1 0 1 X A START condition will be
transmitted when the bus
becomes free
0x40 SLA+R has been transmitted;
ACK has been received
No TWDR
action
0 0 1 0 Data byte will be received and
NOT ACK will be returned
0 0 1 1 Data byte will be received and
ACK will be returned
0x48 SLA+R has been transmitted;
NOT ACK has been received
1 0 1 X Repeated START will be
transmitted
0 1 1 X STOP condition will be
transmitted and TWSTO flag
will be reset
1 1 1 X STOP condition followed by a
START condition will be
transmitted and TWSTO flag
will be reset
0x50 Data byte has been received;
ACK has been returned
Read data
byte
0 0 1 0 Data byte will be received and
NOT ACK will be returned
0 0 1 1 Data byte will be received and
ACK will be returned
0x58 Data byte has been received;
NOT ACK has been returned
Read data
byte
1 0 1 X Repeated START will be
transmitted
0 1 1 X STOP condition will be
transmitted and TWSTO flag
will be reset
1 1 1 X STOP condition followed by a
START condition will be
transmitted and TWSTO flag
will be reset
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 280
Figure 26-14. Formats and States in the Master Receiver Mode
S SLA R A A
0x08 0x40 0x50
SLA R
0x10
A P
0x48
A or A
0x38 0x38
W
A
0x68 0x78 0xB0
MR
MT
Next transfer
started with a
repeated start
condition
Successfull
reception
from a slave
receiver
Not acknowledge
received after the
slave address
Arbitration lost in slave
address or data byte
Arbitration lost and
addressed as slave
From master to slave
From slave to master
Any number of data bytes
and their associated acknowledge bits
This number (contained in TWSR) corresponds
to a defined state of the Two-Wire Serial Bus. The
prescaler bits are zero or masked to zero.
To corresponding
states in slave mode
Other master
continues
DATA A
n
PA
0x58
A
RS
Other master
continues
Other master
continues
DATA DATA
26.7.3 Slave Transmitter Mode
In the Slave Transmitter (ST) mode, a number of data bytes are transmitted to a master receiver, as in the
figure below. All the status codes mentioned in this section assume that the prescaler bits are zero or are
masked to zero.
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 281
Figure 26-15. Data Transfer in Slave Transmitter Mode
Device 3 Device n
SDA
SCL
........ R1 R2
VCC
Device 2
MASTER
RECEIVER
Device 1
SLA VE
TRANSMITTER
To initiate the SR mode, the TWI (Slave) Address Register (TWARn) and the TWI Control Register
(TWCRn) must be initialized as follows:
The upper seven bits of TWARn are the address to which the two-wire serial interface will respond when
addressed by a master (TWARn.TWA[6:0]). If the LSB of TWARn is written to TWARn.TWGCI=1, the TWI
will respond to the general call address (0x00), otherwise, it will ignore the general call address.
TWCRn must hold a value of the type TWCRn=0100010x - TWEN must be written to one to enable the
TWI. The TWEA bit must be written to one to enable the acknowledgment of the device’s own slave
address or the general call address. TWSTA and TWSTO must be written to zero.
When TWARn and TWCRn have been initialized, the TWI waits until it is addressed by its own slave
address (or the general call address if enabled) followed by the data direction bit. If the direction bit is “1”
(read), the TWI will operate in ST mode, otherwise, SR mode is entered. After its own slave address and
the write bit have been received, the TWINT flag is set and a valid status code can be read from TWSRb.
The status code is used to determine the appropriate sofTWARne action. The appropriate action to be
taken for each status code is detailed in the table below. The ST mode may also be entered if arbitration
is lost while the TWI is in the Master mode (see state 0xB0).
If the TWCRn.TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the
transfer. State 0xC0 or state 0xC8 will be entered, depending on whether the master receiver transmits a
NACK or ACK after the final byte. The TWI is switched to the not addressed Slave mode and will ignore
the master if it continues the transfer. Thus the master receiver receives all '1' as serial data. State 0xC8
is entered if the master demands additional data bytes (by transmitting ACK), even though the slave has
transmitted the last byte (TWEA zero and expecting NACK from the master).
While TWCRn.TWEA is zero, the TWI does not respond to its own slave address. However, the two-wire
serial bus is still monitored and address recognition may resume at any time by setting TWEA. This
implies that the TWEA bit may be used to temporarily isolate the TWI from the two-wire serial bus.
In all sleep modes other than the Idle mode, the clock system to the TWI is turned off. If the TWEA bit is
set, the interface can still acknowledge its own slave address or the general call address by using the
two-wire serial bus clock as a clock source. The part will then wake up from sleep and the TWI will hold
the SCL clock will low during the wake-up and until the TWINT Flag is cleared (by writing '1' to it). Further
data transmission will be carried out as normal, with the AVR clocks running as normal. Observe that if
the AVR is set up with a long start-up time, the SCL line may be held low for a long time, blocking other
data transmissions.
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 282
Note: The Two-wire serial interface Data Register (TWDRn) does not reflect the last byte present on the
bus when waking up from these Sleep modes.
Table 26-5. Status Codes for Slave Transmitter Mode
Status Code
(TWSRb)
Prescaler
Bits are 0
Status of the Two-Wire Serial
Bus and Two-Wire Serial
Interface Hardware
Application SofTWARne Response Next Action Taken by TWI
Hardware
To/From
TWDRn
To TWCRn
STA STO TWINT TWEA
0xA8 Own SLA+R has been
received;
ACK has been returned
Load data
byte
X 0 1 0 Last data byte will be transmitted
and NOT ACK should be received
X 0 1 1 Data byte will be transmitted and
ACK should be received
0xB0 Arbitration lost in SLA+R/W as
Master;
own SLA+R has been
received;
ACK has been returned
Load data
byte
X 0 1 0 Last data byte will be transmitted
and NOT ACK should be received
X 0 1 1 Data byte will be transmitted and
ACK should be received
0xB8 Data byte in TWDRn has been
transmitted;
ACK has been received
Load data
byte
X 0 1 0 Last data byte will be transmitted
and NOT ACK should be received
X 0 1 1 Data byte will be transmitted and
ACK should be received
0xC0 Data byte in TWDRn has been
transmitted;
NOT ACK has been received
No TWDRn
action
0 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA
0 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if
TWGCE = “1”
1 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA;
a START condition will be
transmitted when the bus
becomes free
1 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if
TWGCE = “1”; a START condition
will be transmitted when the bus
becomes free
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 283
Status Code
(TWSRb)
Prescaler
Bits are 0
Status of the Two-Wire Serial
Bus and Two-Wire Serial
Interface Hardware
Application SofTWARne Response Next Action Taken by TWI
Hardware
To/From
TWDRn
To TWCRn
STA STO TWINT TWEA
0xC8 Last data byte in TWDRn has
been transmitted (TWEA =
“0”);
ACK has been received
No TWDRn
action
0 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA
0 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if
TWGCE = “1”
1 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA;
a START condition will be
transmitted when the bus
becomes free
1 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if
TWGCE = “1”; a START condition
will be transmitted when the bus
becomes free
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 284
Figure 26-16. Formats and States in the Slave Transmitter Mode
S SLA R A A
0xA8 0xB8
A
0xB0
n
P or S
0xC0
DATA A
A
0xC8
P or SAll 1's
A
Reception of the own
slave address and one or
more data bytes
Arbitration lost as master
and addressed as slave
Last data byte transmitted.
Switched to not addressed
slave (TWEA = '0')
From master to slave
From slave to master
Any number of data bytes
and their associated acknowledge bits
This number (contained in TWSR) corresponds
to a defined state of the Two-Wire Ser ial Bus. The
prescaler bits are zero or masked to zero.
DATA DATA
26.7.4 Slave Receiver Mode
In the Slave Receiver (SR) mode, a number of data bytes are received from a master transmitter (see
figure below). All the status codes mentioned in this section assume that the prescaler bits are zero or are
masked to zero.
Figure 26-17. Data transfer in Slave Receiver mode
Device 3 Device n
SDA
SCL
........ R1 R2
VCC
Device 2
MASTER
TRANSMITTER
Device 1
SLAVE
RECEIVER
To initiate the SR mode, the TWI (Slave) Address Register n (TWARn) and the TWI Control Register n
(TWCRn) must be initialized as follows:
The upper seven bits of TWARn are the address to which the two-wire serial interface will respond when
addressed by a master (TWARn.TWA[6:0]). If the LSB of TWARn is written to TWARn.TWGCI=1, the TWI
n will respond to the general call address (0x00), otherwise, it will ignore the general call address.
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 285
TWCRn must hold a value of the type TWCRn=0100010x - TWCRn.TWEN must be written to '1' to
enable the TWI. TWCRn.TWEA bit must be written to '1' to enable the acknowledgment of the device’s
own slave address or the general call address. TWCRn.TWSTA and TWSTO must be written to zero.
When TWARn and TWCRn have been initialized, the TWI waits until it is addressed by its own slave
address (or the general call address, if enabled) followed by the data direction bit. If the direction bit is '0'
(write), the TWI will operate in SR mode, otherwise, ST mode is entered. After its own slave address and
the write bit have been received, the TWINT flag is set and a valid status code can be read from TWSR.
The status code is used to determine the appropriate software action, as detailed in the table below. The
SR mode may be entered if arbitration is lost while the TWI is in the Master mode (see states 0x68 and
0x78).
If the TWCRn.TWEA bit is reset during a transfer, the TWI will return a "Not Acknowledge" ('1') to SDA
after the next received data byte. This can be used to indicate that the slave is not able to receive any
more bytes. While TWEA is zero, the TWI does not acknowledge its own slave address. However, the
two-wire serial bus is still monitored and address recognition may resume at any time by setting TWEA.
This implies that the TWEA bit may be used to temporarily isolate the TWI from the two-wire serial bus.
In all sleep modes other than the Idle mode, the clock system to the TWI is turned off. If the TWEA bit is
set, the interface can still acknowledge its own slave address or the general call address by using the
two-wire serial bus clock as a clock source. The part will then wake up from sleep and the TWI will hold
the SCL clock low during the wake-up and until the TWINT flag is cleared (by writing '1' to it). Further data
reception will be carried out as normal, with the AVR clocks running as normal. Observe that if the AVR is
set up with a long start-up time, the SCL line may be held low for a long time, blocking other data
transmissions.
Note: The two-wire Serial Interface Data Register (TWDRn) does not reflect the last byte present on the
bus when waking up from these Sleep modes.
Table 26-6. Status Codes for Slave Receiver Mode
Status Code
(TWSR)
Prescaler
Bits are 0
Status of the Two-Wire Serial
Bus and Two-Wire Serial
Interface Hardware
Application SofTWARne Response Next Action Taken by TWI
Hardware
To/from
TWDRn
To TWCRn
STA STO TWINT TWEA
0x60 Own SLA+W has been
received;
ACK has been returned
No TWDRn
action
X 0 1 0 Data byte will be received and
NOT ACK will be returned
X 0 1 1 Data byte will be received and
ACK will be returned
0x68 Arbitration lost in SLA+R/W as
Master;
own SLA+W has been
received;
ACK has been returned
No TWDRn
action
X 0 1 0 Data byte will be received and
NOT ACK will be returned
X 0 1 1 Data byte will be received and
ACK will be returned
0x70 General call address has been
received;
ACK has been returned
No TWDRn
action
X 0 1 0 Data byte will be received and
NOT ACK will be returned
X 0 1 1 Data byte will be received and
ACK will be returned
0x78 Arbitration lost in SLA+R/W as
Master;
No TWDRn
action
X 0 1 0 Data byte will be received and
NOT ACK will be returned
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 286
Status Code
(TWSR)
Prescaler
Bits are 0
Status of the Two-Wire Serial
Bus and Two-Wire Serial
Interface Hardware
Application SofTWARne Response Next Action Taken by TWI
Hardware
To/from
TWDRn
To TWCRn
STA STO TWINT TWEA
General call address has been
received;
ACK has been returned
X 0 1 1 Data byte will be received and
ACK will be returned
0x80 Previously addressed with own
SLA+W;
data has been received;
ACK has been returned
Read data
byte
X 0 1 0 Data byte will be received and
NOT ACK will be returned
X 0 1 1 Data byte will be received and
ACK will be returned
0x88 Previously addressed with own
SLA+W;
data has been received;
NOT ACK has been returned
Read data
byte
0 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA
0 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if
TWGCE = “1”
1 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA;
a START condition will be
transmitted when the bus
becomes free
1 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if
TWGCE = “1”; a START condition
will be transmitted when the bus
becomes free
0x90 Previously addressed with
general call;
data has been received;
ACK has been returned
Read data
byte
X 0 1 0 Data byte will be received and
NOT ACK will be returned
X 0 1 1 Data byte will be received and
ACK will be returned
0x98 Previously addressed with
general call;
data has been received;
Read data
byte
0 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 287
Status Code
(TWSR)
Prescaler
Bits are 0
Status of the Two-Wire Serial
Bus and Two-Wire Serial
Interface Hardware
Application SofTWARne Response Next Action Taken by TWI
Hardware
To/from
TWDRn
To TWCRn
STA STO TWINT TWEA
NOT ACK has been returned 0 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if
TWGCE = “1”
1 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA;
a START condition will be
transmitted when the bus
becomes free
1 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if
TWGCE = “1”; a START condition
will be transmitted when the bus
becomes free
0xA0 A STOP condition or repeated
START condition has been
received while still addressed
as Slave
No action 0 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA
0 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
GCA will be recognized if
TWGCE = “1”
1 0 1 0 Switched to the not addressed
Slave mode;
no recognition of own SLA or
GCA;
a START condition will be
transmitted when the bus
becomes free
1 0 1 1 Switched to the not addressed
Slave mode;
own SLA will be recognized;
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 288
Status Code
(TWSR)
Prescaler
Bits are 0
Status of the Two-Wire Serial
Bus and Two-Wire Serial
Interface Hardware
Application SofTWARne Response Next Action Taken by TWI
Hardware
To/from
TWDRn
To TWCRn
STA STO TWINT TWEA
GCA will be recognized if
TWGCE = “1”; a START condition
will be transmitted when the bus
becomes free
Figure 26-18. Formats and States in the Slave Receiver Mode
S SLA W A DATA A
0x60 0x80
0x88
A
0x68
n
P or SDATA A
0x80 0xA0
P or SA
A DATA A
0x70 0x90
0x98
A
0x78
P or SDATA A
0x90 0xA0
P or SA
General Call
DATA A
Reception of the own
slave address and one or
more data bytes. All are
acknowledged
Last data byte received
is not acknowledged
Arbitration lost as master
and addressed as slave
Reception of the general call
address and one or more data
bytes
Last data byte received is
not acknowledged
Arbitration lost as master and
addressed as sla v e b y gene r al call
From master to slave
From slave to master
Any number of data bytes
and their associated acknowledge bits
This n umber (contained in TWSR) corresponds
to a defined state of the Two-Wire Serial Bus. The
prescaler bits are zero or masked to zero.
26.7.5 Miscellaneous States
There are two status codes that do not correspond to a defined TWI state, see the table in this section.
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 289
Status 0xF8 indicates that no relevant information is available because the TWINT flag is not set. This
occurs between other states, and when the TWI is not involved in a serial transfer.
Status 0x00 indicates that a bus error has occurred during a two-wire serial bus transfer. A bus error
occurs when a START or STOP condition occurs at an illegal position in the format frame. Examples of
such illegal positions are during the serial transfer of an address byte, a data byte, or an acknowledge bit.
When a bus error occurs, TWINT is set. To recover from a bus error, the TWSTO flag must set and
TWINT must be cleared by writing a logic one to it. This causes the TWI to enter the not addressed Slave
mode and to clear the TWSTO flag (no other bits in TWCRn are affected). The SDA and SCL lines are
released, and no STOP condition is transmitted.
Table 26-7. Miscellaneous States
Status Code
(TWSR)
Prescaler Bits are
0
Status of the Two-Wire Serial
Bus and Two-Wire Serial
Interface Hardware
Application Software Response Next Action Taken by
TWI Hardware
To/From
TWDRn
To TWCRn
STA STO TWINT TWEA
0xF8 No relevant state information
available; TWINT = “0”
No TWDRn
action
No TWCRn action Wait or proceed current
transfer
0x00 Bus error due to an illegal
START or STOP condition
No TWDRn
action
0 1 1 X Only the internal hardware
is affected, no STOP
condition is sent on the
bus. In all cases, the bus is
released and TWSTO is
cleared.
26.7.6 Combining Several TWI Modes
In some cases, several TWI modes must be combined in order to complete the desired action. Consider
for example reading data from a serial EEPROM. Typically, such a transfer involves the following steps:
1. The transfer must be initiated.
2. The EEPROM must be instructed what location should be read.
3. The reading must be performed.
4. The transfer must be finished.
Note that data is transmitted both from master to slave and vice versa. The master must instruct the slave
what location it wants to read, requiring the use of the MT mode. Subsequently, data must be read from
the slave, implying the use of the MR mode. Thus, the transfer direction must be changed. The master
must keep control of the bus during all these steps, and the steps should be carried out as an atomical
operation. If this principle is violated in a multi-master system, another master can alter the data pointer in
the EEPROM between steps 2 and 3, and the master will read the wrong data location. Such a change in
transfer direction is accomplished by transmitting a REPEATED START between the transmission of the
address byte and reception of the data. After a REPEATED START, the Master keeps ownership of the
bus. The flow in this transfer is depicted in the following figure:
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 290
Figure 26-19. Combining Several TWI Modes to Access a Serial EEPROM
Master Transmitter Master Receiver
S = START Rs = REPEATED START P = STOP
Transmitted from master to slave Transmitted from slave to master
S SLA+W A ADDRESS A Rs SLA+R A DATA A P
26.8 Multi-Master Systems and Arbitration
If multiple masters are connected to the same bus, transmissions may be initiated simultaneously by one
or more of them. The TWI standard ensures that such situations are handled in such a way that one of
the masters will be allowed to proceed with the transfer, and that no data will be lost in the process. An
example of an arbitration situation is depicted below, where two masters are trying to transmit data to a
slave receiver.
Figure 26-20. An Arbitration Example
Device 1
MASTER
TRANSMITTER
Device 2
MASTER
TRANSMITTER
Device 3
SLAVE
RECEIVER
Device n
SDA
SCL
........ R1 R2
VCC
Several different scenarios may arise during arbitration, as described below:
• Two or more masters are performing identical communication with the same slave. In this case,
neither the slave nor any of the masters will know about the bus contention.
• Two or more masters are accessing the same slave with different data or direction bit. In this case,
arbitration will occur, either in the READ/WRITE bit or in the data bits. The masters trying to output
a '1' on SDA while another master outputs a zero will lose the arbitration. Losing masters will switch
to not addressed Slave mode or wait until the bus is free and transmit a new START condition,
depending on application software action.
• Two or more masters are accessing different slaves. In this case, arbitration will occur in the SLA
bits. Masters trying to output a '1' on SDA while another master outputs a zero will lose the
arbitration. Masters losing arbitration in SLA will switch to Slave mode to check if they are being
addressed by the winning master. If addressed, they will switch to SR or ST mode, depending on
the value of the READ/WRITE bit. If they are not being addressed, they will switch to not addressed
Slave mode or wait until the bus is free and transmit a new START condition, depending on
application software action.
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 291
This is summarized in the next figure. Possible status values are given in circles.
Figure 26-21. Possible Status Codes Caused by Arbitration
Own
Address / General Call
received
Arbitration lost in SLA
TWI bus will be released and not addressed slave mode will be entered
A START condition will be transmitted when the bus becomes free
No
Arbitration lost in Data
Direction
Yes
Data byte will be received and NOT ACK will be returned
Data byte will be received and ACK will be returned
Last data byte will be transmitted and NOT ACK should be received
Data byte will be transmitted and ACK should be received
SLASTART Data STOP
Write
Read
B0
68/78
38
26.9 Register Description
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 292
26.9.1 TWI Bit Rate Register
Name: TWBR
Offset: 0xB8
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
TWBR [7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TWBR [7:0] TWI Bit Rate Register
TWBR selects the division factor for the bit rate generator. The bit rate generator is a frequency divider
which generates the SCL clock frequency in the Master modes.
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 293
26.9.2 TWI Status Register
Name: TWSR
Offset: 0xB9
Reset: 0xF8
Property: -
Bit 7 6 5 4 3 2 1 0
TWS7 TWS6 TWS5 TWS4 TWS3 TWPS[1:0]
Access R R R R R R R/W R/W
Reset 1 1 1 1 1 0 0 0
Bits 3, 4, 5, 6, 7 – TWS TWI Status Bit
The TWS[7:3] reflect the status of the TWI logic and the 2-wire serial bus. The different status codes are
described later in this section. Note that the value read from TWSR contains both the 5-bit status value
and the 2-bit prescaler value. The application designer should mask the prescaler bits to zero when
checking the Status bits. This makes status checking independent of prescaler setting. This approach is
used in this datasheet, unless otherwise noted.
Bits 1:0 – TWPS[1:0] TWI Prescaler
These bits can be read and written, and control the bit rate prescaler.
Table 26-8. TWI Bit Rate Prescaler
TWS[1:0] Prescaler Value
00 1
01 4
10 16
11 64
To calculate bit rates, refer to Bit Rate Generator Unit. The value of TWPS1...0 is used in the equation.
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 294
26.9.3 TWI (Slave) Address Register
Name: TWAR
Offset: 0xBA
Reset: 0xFE
Property: -
The TWAR should be loaded with the 7-bit slave address (in the seven most significant bits of TWAR) to
which the TWI will respond when programmed as a slave transmitter or receiver, and not needed in the
Master modes. In multi master systems, TWAR must be set in masters which can be addressed as slaves
by other masters.
The LSB of TWAR is used to enable recognition of the general call address (0x00). There is an
associated address comparator that looks for the slave address (or general call address if enabled) in the
received serial address. If a match is found, an interrupt request is generated.
Bit 7 6 5 4 3 2 1 0
TWA[6:0] TWGCE
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 1 1 1 1 1 1 1 0
Bits 7:1 – TWA[6:0] TWI (Slave) Address
These seven bits constitute the slave address of the TWI unit.
Bit 0 – TWGCE TWI General Call Recognition Enable Bit
If set, this bit enables the recognition of a general call given over the 2-wire serial bus.
ATmega328/P
Two-Wire Serial Interface (TWI)
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26.9.4 TWI Data Register
Name: TWDR
Offset: 0xBB
Reset: 0xFF
Property: -
In Transmit mode, TWDR contains the next byte to be transmitted. In Receive mode, the TWDR contains
the last byte received. It is writable while the TWI is not in the process of shifting a byte. This occurs when
the TWI Interrupt flag (TWINT) is set by hardware. Note that the data register cannot be initialized by the
user before the first interrupt occurs. The data in TWDR remains stable as long as TWINT is set. While
data is shifted out, data on the bus is simultaneously shifted in. TWDR always contains the last byte
present on the bus, except after a wake up from a sleep mode by the TWI interrupt. In this case, the
contents of TWDR is undefined. In the case of a lost bus arbitration, no data is lost in the transition from
master to slave. Handling of the ACK bit is controlled automatically by the TWI logic, the CPU cannot
access the ACK bit directly.
Bit 7 6 5 4 3 2 1 0
TWD[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 1 1 1 1 1 1 1 1
Bits 7:0 – TWD[7:0] TWI Data
These eight bits constitute the next data byte to be transmitted, or the latest data byte received on the 2-
wire Serial Bus.
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 296
26.9.5 TWI Control Register
Name: TWCR
Offset: 0xBC
Reset: 0x00
Property: -
The TWCR is used to control the operation of the TWI. It is used to enable the TWI, to initiate a master
access by applying a START condition to the bus, to generate a receiver acknowledge, to generate a stop
condition, and to control halting of the bus while the data to be written to the bus are written to the TWDR.
It also indicates a write collision if data is attempted written to TWDR while the register is inaccessible.
Bit 7 6 5 4 3 2 1 0
TWINT TWEA TWSTA TWSTO TWWC TWEN TWIE
Access R/W R/W R/W R/W R/W R/W R R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – TWINT TWI Interrupt Flag
This bit is set by hardware when the TWI has finished its current job and expects application software
response. If the I-bit in SREG and TWIE in TWCR are set, the MCU will jump to the TWI interrupt vector.
While the TWINT flag is set, the SCL low period is stretched. The TWINT flag must be cleared by
software by writing a logic one to it.
Note that this flag is not automatically cleared by hardware when executing the interrupt routine. Also
note that clearing this flag starts the operation of the TWI, so all accesses to the TWI Address Register
(TWAR), TWI Status Register (TWSR), and TWI Data Register (TWDR) must be complete before clearing
this flag.
Bit 6 – TWEA TWI Enable Acknowledge
The TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is written to one, the
ACK pulse is generated on the TWI bus if the following conditions are met:
1. The device’s own slave address has been received.
2. A general call has been received, while the TWGCE bit in the TWAR is set.
3. A data byte has been received in Master Receiver or Slave Receiver mode.
By writing the TWEA bit to zero, the device can be virtually disconnected from the 2-wire serial bus
temporarily. Address recognition can then be resumed by writing the TWEA bit to one again.
Bit 5 – TWSTA TWI START Condition
The application writes the TWSTA bit to one when it desires to become a master on the 2-wire serial bus.
The TWI hardware checks if the bus is available, and generates a START condition on the bus if it is free.
However, if the bus is not free, the TWI waits until a STOP condition is detected, and then generates a
new START condition to claim the bus master status. TWSTA must be cleared by software when the
START condition has been transmitted.
Bit 4 – TWSTO TWI STOP Condition
Writing the TWSTO bit to one in Master mode will generate a STOP condition on the 2-wire serial bus.
When the STOP condition is executed on the bus, the TWSTO bit is cleared automatically. In Slave
mode, setting the TWSTO bit can be used to recover from an error condition. This will not generate a
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 297
STOP condition, but the TWI returns to a well-defined unaddressed Slave mode and releases the SCL
and SDA lines to a high impedance state.
Bit 3 – TWWC TWI Write Collision Flag
The TWWC bit is set when attempting to write to the TWDR when TWINT is low. This flag is cleared by
writing the TWDR when TWINT is high.
Bit 2 – TWEN TWI Enable
The TWEN bit enables TWI operation and activates the TWI interface. When TWEN is written to one, the
TWI takes control over the I/O pins connected to the SCL and SDA pins, enabling the slew-rate limiters
and spike filters. If this bit is written to zero, the TWI is switched off and all TWI transmissions are
terminated, regardless of any ongoing operation.
Bit 0 – TWIE TWI Interrupt Enable
When this bit is written to one, and the I-bit in SREG is set, the TWI interrupt request will be activated for
as long as the TWINT flag is high.
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 298
26.9.6 TWI (Slave) Address Mask Register
Name: TWAMR
Offset: 0xBD
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
TWAM6 TWAM5 TWAM4 TWAM3 TWAM2 TWAM1 TWAM0
Access R/W R/W R/W R/W R/W R/W R/W R
Reset 0 0 0 0 0 0 0 0
Bits 1, 2, 3, 4, 5, 6, 7 – TWAM TWI (Slave) Address
The TWAMR can be loaded with a 7-bit slave address mask. Each of the bits in TWAMR can mask
(disable) the corresponding address bits in the TWI Address Register (TWAR). If the mask bit is set to
one then the address match logic ignores the compare between the incoming address bit and the
corresponding bit in TWAR.
Figure 26-22. TWI Address Match Logic
TWAR0
TWAMR0
Address
Bit 0
Address
Match
Address Bit Comparator 6:1
Address Bit Comparator 0
ATmega328/P
Two-Wire Serial Interface (TWI)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 299
27. Analog Comparator (AC)
27.1 Overview
The analog comparator evaluates the input values on the positive pin AIN0 and negative pin AIN1. When
the voltage on the positive pin AIN0 is higher than the voltage on the negative pin AIN1, the Analog
Comparator Output (ACO) is set. The comparator’s output can be set to trigger the timer/counter1 input
capture function. In addition, the comparator can trigger a separate interrupt, exclusive to the analog
comparator. The user can select Interrupt triggering on comparator output rise, fall or toggle. A block
diagram of the comparator and its surrounding logic is shown below.
The power reduction ADC bit in the Power Reduction Register (PRR.PRADC) must be written to '0' in
order to be able to use the ADC input MUX.
Figure 27-1. Analog Comparator Block Diagram
ACBG
BANDGAP
REFERENCE
ADC MULTIPLEXER
OUTPUT
ACME
ADEN
(1)
AIN0
AIN1
INTERRUPT
SELECT
ACIS1 ACIS0 ACIC
ACIE
ACO
TO T/C1 CAPTURE
TRIGGER MUX
ACI
ANALOG
COMPARATOR
IRQ
ACD
VCC
Note: Refer to the Pin Configuration and the I/O Ports description for Analog Comparator pin placement
Related Links
I/O-Ports
PRR
Power Management and Sleep Modes
Minimizing Power Consumption
27.2 Analog Comparator Multiplexed Input
It is possible to select any of the ADC[7:0] pins to replace the negative input to the analog comparator.
The ADC multiplexer is used to select this input, and consequently, the ADC must be switched off to
utilize this feature. If the Analog Comparator Multiplexer Enable bit in the ADC Control and Status
Register B (ADCSRB.ACME) is '1' and the ADC is switched off (ADCSRA.ADEN=0), the three least
significant analog channel selection bits in the ADC Multiplexer Selection register (ADMUX.MUX[2:0])
select the input pin to replace the negative input to the analog comparator, as shown in the table below.
When ADCSRB.ACME=0 or ADCSRA.ADEN=1, AIN1 is applied to the negative input of the analog
comparator.
ATmega328/P
Analog Comparator (AC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 300
Table 27-1. Analog Comparator Multiplexed Input
ACME ADEN MUX[2:0] Analog Comparator Negative Input
0 x xxx AIN1
1 1 xxx AIN1
1 0 000 ADC0
1 0 001 ADC1
1 0 010 ADC2
1 0 011 ADC3
1 0 100 ADC4
1 0 101 ADC5
1 0 110 ADC6
1 0 111 ADC7
27.3 Register Description
ATmega328/P
Analog Comparator (AC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 301
27.3.1 Analog Comparator Control and Status Register
Name: ACSR
Offset: 0x50
Reset: N/A
Property: When addressing as I/O Register: address offset is 0x30
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
ACD ACBG ACO ACI ACIE ACIC ACIS[1:0]
Access R/W R/W R R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – ACD Analog Comparator Disable
When this bit is written logic one, the power to the analog comparator is switched off. This bit can be set
at any time to turn off the analog comparator. This will reduce power consumption in Active and Idle
mode. When changing the ACD bit, the analog comparator interrupt must be disabled by clearing the
ACIE bit in ACSR. Otherwise, an interrupt can occur when the bit is changed.
Bit 6 – ACBG Analog Comparator Bandgap Select
When this bit is set, a fixed bandgap reference voltage replaces the positive input to the analog
comparator. When this bit is cleared, AIN0 is applied to the positive input of the analog comparator. When
the bandgap reference is used as input to the analog comparator, it will take a certain time for the voltage
to stabilize. If not stabilized, the first conversion may give a wrong value.
Bit 5 – ACO Analog Comparator Output
The output of the analog comparator is synchronized and then directly connected to ACO. The
synchronization introduces a delay of 1 - 2 clock cycles.
Bit 4 – ACI Analog Comparator Interrupt Flag
This bit is set by hardware when a comparator output event triggers the interrupt mode defined by ACIS1
and ACIS0. The analog comparator interrupt routine is executed if the ACIE bit is set and the I-bit in
SREG is set. ACI is cleared by hardware when executing the corresponding interrupt handling vector.
Alternatively, ACI is cleared by writing a logic one to the flag.
Bit 3 – ACIE Analog Comparator Interrupt Enable
When the ACIE bit is written logic one and the I-bit in the status register is set, the analog comparator
interrupt is activated. When written logic zero, the interrupt is disabled.
Bit 2 – ACIC Analog Comparator Input Capture Enable
When written logic one, this bit enables the input capture function in Timer/Counter1 to be triggered by
the analog comparator. The comparator output is in this case directly connected to the input capture front-
end logic, making the comparator utilize the noise canceler and edge select features of the Timer/
Counter1 input capture interrupt. When written logic zero, no connection between the analog comparator
and the input capture function exists. To make the comparator trigger the Timer/Counter1 input capture
interrupt, the ICIE1 bit in the Timer Interrupt Mask Register (TIMSK1) must be set.
ATmega328/P
Analog Comparator (AC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 302
Bits 1:0 – ACIS[1:0] Analog Comparator Interrupt Mode Select
These bits determine which comparator events that trigger the analog comparator interrupt.
Table 27-2. ACIS[1:0] Settings
ACIS1 ACIS0 Interrupt Mode
0 0 Comparator interrupt on output toggle.
0 1 Reserved
1 0 Comparator interrupt on falling output edge.
1 1 Comparator interrupt on rising output edge.
When changing the ACIS1/ACIS0 bits, the analog comparator Interrupt must be disabled by clearing its
interrupt enable bit in the ACSR register. Otherwise, an interrupt can occur when the bits are changed.
ATmega328/P
Analog Comparator (AC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 303
27.3.2 Digital Input Disable Register 1
Name: DIDR1
Offset: 0x7F
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
AIN1D AIN0D
Access R R R R R R R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 0, 1 – AIND AIN Digital Input Disable
When this bit is written logic one, the digital input buffer on the AIN1/0 pin is disabled. The corresponding
PIN Register bit will always read as zero when this bit is set. When an analog signal is applied to the
AIN1/0 pin and the digital input from this pin is not needed, this bit should be written logic one to reduce
power consumption in the digital input buffer.
ATmega328/P
Analog Comparator (AC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 304
28. Analog-to-Digital Converter (ADC)
28.1 Features
• 10-bit Resolution
• 0.5 LSB Integral Non-Linearity
• ±2 LSB Absolute Accuracy
• 13 - 260 μs Conversion Time
• Up to 76.9 kSPS (Up to 15 kSPS at Maximum Resolution)
• Six Multiplexed Single Ended Input Channels
• Two Additional Multiplexed Single Ended Input Channels (TQFP and QFN Package only)
• Temperature Sensor Input Channel
• Optional Left Adjustment for ADC Result Readout
• 0 - VCC ADC Input Voltage Range
• Selectable 1.1V ADC Reference Voltage
• Free Running or Single Conversion Mode
• Interrupt on ADC Conversion Complete
• Sleep Mode Noise Canceler
28.2 Overview
The device features a 10-bit successive approximation ADC. The ADC is connected to an 8-channel
analog multiplexer which allows eight single-ended voltage inputs constructed from the pins of Port A.
The single-ended voltage inputs refer to 0V (GND).
The ADC contains a sample and hold circuit, which ensures that the input voltage to the ADC is held at a
constant level during conversion. A block diagram of the ADC is shown below.
The ADC has a separate analog supply voltage pin, AVCC. AVCC must not differ more than ±0.3V from
VCC. See section ADC Noise Canceler on how to connect this pin.
The Power Reduction ADC bit in the Power Reduction Register (PRR.PRADC) must be written to '0' in
order to enable the ADC.
The ADC converts an analog input voltage to a 10-bit digital value through successive approximation.
The minimum value represents GND and the maximum value represents the voltage on the AREF pin
minus 1 LSB. Optionally, AVCC or an internal 1.1V reference voltage may be connected to the AREF pin
by writing to the REFSn bits in the ADMUX Register. The internal voltage reference must be decoupled
by an external capacitor at the AREF pin to improve noise immunity.
ATmega328/P
Analog-to-Digital Converter (ADC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 305
Figure 28-1. Analog-to-Digital Converter Block Schematic Operation
ADC CONVERSION
COMPLETE IRQ
8-BIT DATA BUS
15 0
ADC MULTIPLEXER
SELECT (ADMUX) ADC CTRL. & STATUS
REGISTER (ADCSRA) ADC DATA REGISTER
(ADCH/ADCL)
MUX2
ADIE
ADFR
ADSC
ADEN
ADIF ADIF
MUX1
MUX0
ADPS0
ADPS1
ADPS2
MUX3
CONVERSION LOGIC
10-BIT DAC
+
-
SAMPLE & HOLD
COMPARATOR
INTERNAL 1.1V
REFERENCE
MUX DECODER
AVCC
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADC1
ADC0
REFS0
REFS1
ADLAR
CHANNEL SELECTION
ADC[9:0]
ADC MULTIPLEXER
OUTPUT
AREF
BANDGAP
REFERENCE
PRESCALER
GND
INPUT
MUX
TEMPERATURE
SENSOR
The analog input channel is selected by writing to the MUX bits in the ADC Multiplexer Selection register
ADMUX.MUX[3:0]. Any of the ADC input pins, as well as GND and a fixed bandgap voltage reference,
can be selected as single ended inputs to the ADC. The ADC is enabled by writing a '1' to the ADC
Enable bit in the ADC Control and Status Register A (ADCSRA.ADEN). Voltage reference and input
channel selections will not take effect until ADEN is set. The ADC does not consume power when ADEN
is cleared, so it is recommended to switch the ADC OFF before entering the power-saving sleep modes.
The ADC generates a 10-bit result which is presented in the ADC Data registers, ADCH and ADCL. By
default, the result is presented right adjusted, but can optionally be presented left adjusted by setting the
ADC Left Adjust Result bit ADMUX.ADLAR.
If the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read ADCH.
Otherwise, ADCL must be read first, then ADCH, to ensure that the content of the data registers belongs
ATmega328/P
Analog-to-Digital Converter (ADC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 306
to the same conversion: Once ADCL is read, the ADC access to the data registers is blocked. This
means that if ADCL has been read, and a second conversion completes before ADCH is read, neither
register is updated and the result from the second conversion is lost. When ADCH is read, ADC access to
the ADCH and ADCL registers is re-enabled.
The ADC has its own interrupt which can be triggered when a conversion completes. When ADC access
to the data registers is prohibited between the reading of ADCH and ADCL, the interrupt will trigger even
if the result is lost.
Related Links
Power Management and Sleep Modes
Power Reduction Register
28.3 Starting a Conversion
A single conversion is started by writing a '0' to the Power Reduction ADC bit in the Power Reduction
Register (PRR.PRADC), and writing a '1' to the ADC Start Conversion bit in the ADC Control and Status
Register A (ADCSRA.ADSC). ADCS will stay high as long as the conversion is in progress, and will be
cleared by hardware when the conversion is completed. If a different data channel is selected while a
conversion is in progress, the ADC will finish the current conversion before performing the channel
change.
Alternatively, a conversion can be triggered automatically by various sources. Auto triggering is enabled
by setting the ADC Auto Trigger Enable bit (ADCSRA.ADATE). The trigger source is selected by setting
the ADC Trigger Select bits in the ADC Control and Status Register B (ADCSRB.ADTS). See the
description of the ADCSRB.ADTS for a list of available trigger sources.
When a positive edge occurs on the selected trigger signal, the ADC prescaler is reset and a conversion
is started. This provides a method of starting conversions at fixed intervals. If the trigger signal still is set
when the conversion completes, a new conversion will not be started. If another positive edge occurs on
the trigger signal during conversion, the edge will be ignored. Note that an interrupt flag will be set even if
the specific interrupt is disabled or the Global Interrupt Enable bit in the AVR Status Register (SREG.I) is
cleared. A conversion can thus be triggered without causing an interrupt. However, the interrupt flag must
be cleared in order to trigger a new conversion at the next interrupt event.
Figure 28-2. ADC Auto Trigger Logic
ADSC
ADIF
SOURCE 1
SOURCE n
ADTS[2:0]
CONVERSION
LOGIC
PRESCALER
START CLKADC
.
.
.
.EDGE
DETECTOR
ADATE
Using the ADC interrupt flag as a trigger source makes the ADC start a new conversion as soon as the
ongoing conversion has finished. The ADC then operates in Free Running mode, constantly sampling
ATmega328/P
Analog-to-Digital Converter (ADC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 307
and updating the ADC data register. The first conversion must be started by writing a '1' to
ADCSRA.ADSC. In this mode, the ADC will perform successive conversions independently of whether
the ADC Interrupt Flag (ADIF) is cleared or not.
If Auto triggering is enabled, single conversions can be started by writing ADCSRA.ADSC to '1'. ADSC
can also be used to determine if a conversion is in progress. The ADSC bit will be read as '1' during a
conversion, independently of how the conversion was started.
28.4 Prescaling and Conversion Timing
Figure 28-3. ADC Prescaler
7-BIT ADC PRESCALER
ADC CLOCK SOURCE
CK
ADPS0
ADPS1
ADPS2
CK/128
CK/2
CK/4
CK/8
CK/16
CK/32
CK/64
Reset
ADEN
START
By default, the successive approximation circuitry requires an input clock frequency between 50 kHz and
200 kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the input clock frequency
to the ADC can be higher than 200 kHz to get a higher sample rate.
The ADC module contains a prescaler, which generates an acceptable ADC clock frequency from any
CPU frequency above 100 kHz. The prescaling is selected by the ADC Prescaler Select bits in the ADC
Control and Status Register A (ADCSRA.ADPS). The prescaler starts counting from the moment the ADC
is switched on by writing the ADC Enable bit ADCSRA.ADEN to '1'. The prescaler keeps running for as
long as ADEN=1 and is continuously reset when ADEN=0.
When initiating a single ended conversion by writing a '1' to the ADC Start Conversion bit
(ADCSRA.ADSC), the conversion starts at the following rising edge of the ADC clock cycle.
A normal conversion takes 13 ADC clock cycles. The first conversion after the ADC is switched on (i.e.,
ADCSRA.ADEN is written to '1') takes 25 ADC clock cycles in order to initialize the analog circuitry.
When the bandgap reference voltage is used as input to the ADC, it will take a certain time for the voltage
to stabilize. If not stabilized, the first value read after the first conversion may be wrong.
The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal conversion and
13.5 ADC clock cycles after the start of a first conversion. When a conversion is complete, the result is
written to the ADC Data Registers (ADCL and ADCH), and the ADC Interrupt Flag (ADCSRA.ADIF) is set.
In Single Conversion mode, ADCSRA.ADSC is cleared simultaneously. The software may then set
ADCSRA.ADSC again, and a new conversion will be initiated on the first rising ADC clock edge.
ATmega328/P
Analog-to-Digital Converter (ADC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 308
When auto triggering is used, the prescaler is reset when the trigger event occurs. This assures a fixed
delay from the trigger event to the start of conversion. In this mode, the sample-and-hold takes place two
ADC clock cycles after the rising edge on the trigger source signal. Three additional CPU clock cycles are
used for synchronization logic.
In Free Running mode, a new conversion will be started immediately after the conversion completes,
while ADCRSA.ADSC remains high. See also the ADC conversion time table below.
Figure 28-4. ADC Timing Diagram, First Conversion (Single Conversion Mode)
Sign and MSB of Result
LSB of Result
ADC Clock
ADSC
Sample and Hold
ADIF
ADCH
ADCL
Cycle Number
ADEN
1 2 12 13 14 15 16 17 18 19 20 21 22 23 24 25 1 2
First Conversion Next
Conversion
3
MUX and REFS
Update
MUX and REFS
Update
Conversion
Complete
Figure 28-5. ADC Timing Diagram, Single Conversion
12 3 4 5 6 7 8 9 10 11 12 13
Sign and MSB of Result
LSB of Result
ADC Clock
ADSC
ADIF
ADCH
ADCL
Cycle Number 1 2
One Conversion Next Conversion
3
Sample and Hold
MUX and REFS
Update
Conversion
Complete MUX and REFS
Update
ATmega328/P
Analog-to-Digital Converter (ADC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 309
Figure 28-6. ADC Timing Diagram, Auto Triggered Conversion
1 2 3 4 5 6 7 8 910 11 12 13
Sign and MSB of Result
LSB of Result
ADC Clock
Trigger
Source
ADIF
ADCH
ADCL
Cycle Number 1 2
One Conversion Next Conversion
Conversion
Complete
Prescaler
Reset
ADATE
Prescaler
Reset
Sample &
Hold
MUX and REFS
Update
Figure 28-7. ADC Timing Diagram, Free Running Conversion
11 12 13
Sign and MSB of Result
LSB of Result
ADC Clock
ADSC
ADIF
ADCH
ADCL
Cycle Number 12
One Conversion Next Conversion
3 4
Conversion
Complete
Sample and Hold
MUX and REFS
Update
Table 28-1. ADC Conversion Time
Condition Sample & Hold
[Cycles from Start of Conversion]
Conversion Time
[Cycles]
First conversion 13.5 25
Normal conversions, single ended 1.5 13
Auto Triggered conversions 2 13.5
28.5 Changing Channel or Reference Selection
The analog channel selection bits (MUX) and the Reference Selection bits (REFS) bits in the ADC
Multiplexer Selection Register (ADMUX.MUX and ADMUX.REFS) are single buffered through a
temporary register to which the CPU has random access. This ensures that the channels and reference
ATmega328/P
Analog-to-Digital Converter (ADC)
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selection only takes place at a safe point during the conversion. The channel and reference selection is
continuously updated until a conversion is started. Once the conversion starts, the channel and reference
selection is locked to ensure a sufficient sampling time for the ADC. Continuous updating resumes in the
last ADC clock cycle before the conversion completes (indicated by ADCSRA.ADIF set). Note that the
conversion starts on the following rising ADC clock edge after ADSC is written. The user is thus advised
not to write new channel or reference selection values to ADMUX until one ADC clock cycle after the
ADC Start Conversion bit (ADCRSA.ADSC) was written.
If auto triggering is used, the exact time of the triggering event can be indeterministic. Special care must
be taken when updating the ADMUX register, in order to control which conversion will be affected by the
new settings.
If both the ADC Auto Trigger Enable and ADC Enable bits (ADCRSA.ADATE, ADCRSA.ADEN) are
written to '1', an interrupt event can occur at any time. If the ADMUX Register is changed in this period,
the user cannot tell if the next conversion is based on the old or the new settings. ADMUX can be safely
updated in the following ways:
1. When ADATE or ADEN is cleared.
1.1. During conversion, minimum one ADC clock cycle after the trigger event.
1.2. After a conversion, before the Interrupt Flag used as trigger source is cleared.
When updating ADMUX in one of these conditions, the new settings will affect the next ADC conversion.
28.5.1 ADC Input Channels
When changing channel selections, the user should observe the following guidelines to ensure that the
correct channel is selected:
• In Single Conversion mode, always select the channel before starting the conversion. The channel
selection may be changed one ADC clock cycle after writing one to ADSC. However, the simplest
method is to wait for the conversion to complete before changing the channel selection.
• In Free Running mode, always select the channel before starting the first conversion. The channel
selection may be changed one ADC clock cycle after writing one to ADSC. However, the simplest
method is to wait for the first conversion to complete and then change the channel selection. Since
the next conversion has already started automatically, the next result will reflect the previous
channel selection. Subsequent conversions will reflect the new channel selection.
The user is advised not to write new channel or reference selection values during the Free Running
mode.
28.5.2 ADC Voltage Reference
The reference voltage for the ADC (VREF) indicates the conversion range for the ADC. Single-ended
channels that exceed VREF will result in codes close to 0x3FF. VREF can be selected as either AVCC,
internal 1.1V reference, or external AREF pin.
AVCC is connected to the ADC through a passive switch. The internal 1.1V reference is generated from
the internal bandgap reference (VBG) through an internal amplifier. In either case, the external AREF pin
is directly connected to the ADC, and the reference voltage can be made more immune to noise by
connecting a capacitor between the AREF pin and ground. VREF can also be measured at the AREF pin
with a high impedance voltmeter. Note that VREF is a high-impedance source, and only a capacitive load
should be connected to a system.
If the user has a fixed voltage source connected to the AREF pin, the user may not use the other
reference voltage options in the application, as they will be shorted to the external voltage. If no external
voltage is applied to the AREF pin, the user may switch between AVCC and 1.1V as reference selection.
ATmega328/P
Analog-to-Digital Converter (ADC)
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The first ADC conversion result after switching reference voltage source may be inaccurate, and the user
is advised to discard this result.
28.6 ADC Noise Canceler
The ADC features a noise canceler that enables conversion during Sleep mode to reduce noise induced
from the CPU core and other I/O peripherals. The noise canceler can be used with ADC Noise Reduction
and Idle mode. To make use of this feature, the following procedure should be used:
1. Make sure that the ADC is enabled and is not busy converting. Single Conversion mode must be
selected and the ADC conversion complete interrupt must be enabled.
2. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion once the CPU
has been halted.
3. If no other interrupts occur before the ADC conversion completes, the ADC interrupt will wake up
the CPU and execute the ADC conversion complete interrupt routine. If another interrupt wakes up
the CPU before the ADC conversion is complete, that interrupt will be executed, and an ADC
conversion complete interrupt request will be generated when the ADC conversion completes. The
CPU will remain in Active mode until a new sleep command is executed.
Note: The ADC will not be automatically turned off when entering other Sleep modes than Idle mode
and ADC Noise Reduction mode. The user is advised to write zero to ADCRSA.ADEN before entering
such Sleep modes to avoid excessive power consumption.
28.6.1 Analog Input Circuitry
The analog input circuitry for single ended channels is illustrated below. An analog source applied to
ADCn is subjected to the pin capacitance and input leakage of that pin, regardless of whether that
channel is selected as input for the ADC. When the channel is selected, the source must drive the S/H
capacitor through the series resistance (combined resistance in the input path).
The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or less. If such
a source is used, the sampling time will be negligible. If a source with higher impedance is used, the
sampling time will depend on how long of a time the source needs to charge the S/H capacitor, which can
vary widely. It is recommended to use only low impedance sources with slowly varying signals since this
minimizes the required charge transfer to the S/H capacitor.
Signal components higher than the Nyquist frequency (fADC/2) should not be present for either kind of
channels to avoid distortion from unpredictable signal convolution. The user is advised to remove high
frequency components with a low-pass filter before applying the signals as inputs to the ADC.
Figure 28-8. Analog Input Circuitry
ADCn
IIH
1..100 kΩ
C
S/H
= 14pF
IIL
VCC/2
ATmega328/P
Analog-to-Digital Converter (ADC)
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28.6.2 Analog Noise Canceling Techniques
Digital circuitry inside and outside the device generates EMI which might affect the accuracy of analog
measurements. If conversion accuracy is critical, the noise level can be reduced by applying the following
techniques:
1. Keep analog signal paths as short as possible. Make sure analog tracks run over the analog
ground plane, and keep them well away from high-speed switching digital tracks.
1.1. The AVCC pin on the device should be connected to the digital VCC supply voltage via an LC
network as shown in the figure below.
1.2. Use the ADC noise canceler function to reduce induced noise from the CPU.
1.3. If any ADC [3:0] port pins are used as digital outputs, it is essential that these do not switch
while a conversion is in progress. However, using the two-wire Interface (ADC4 and ADC5) will only
affect the conversion on ADC4 and ADC5 and not the other ADC channels.
Figure 28-9. ADC Power Connections
GND
VCC
PC5 (ADC5/SCL)
PC4 (ADC4/SDA)
PC3 (ADC3)
PC2 (ADC2)
PC1 (ADC1)
PC0 (ADC0)
ADC7
GND
AREF
AVCC
ADC6
PB5
10 µH
100 nF Analog Ground Plane
ATmega328/P
Analog-to-Digital Converter (ADC)
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Note: If the resistivity in the inductor is too high, the AVCC may exceed its range, VCC - 0.3V < AVCC <
VCC + 0.3V.
28.6.3 ADC Accuracy Definitions
An n-bit single-ended ADC converts a voltage linearly between GND and VREF in 2n steps (LSBs). The
lowest code is read as 0, and the highest code is read as 2n-1.
Several parameters describe the deviation from the ideal behavior:
• Offset: The deviation of the first transition (0x000 to 0x001) compared to the ideal transition (at 0.5
LSB). Ideal value: 0 LSB.
Figure 28-10. Offset Error
Output Code
VREF Input Voltage
Ideal ADC
Actual ADC
Offset
Error
• Gain error: After adjusting for offset, the gain error is found as the deviation of the last transition
(0x3FE to 0x3FF) compared to the ideal transition (at 1.5 LSB below maximum). Ideal value: 0
LSB.
Figure 28-11. Gain Error
Output Code
VREF Input Voltage
Ideal ADC
Actual ADC
Gain
Error
• Integral Non-linearity (INL): After adjusting for offset and gain error, the INL is the maximum
deviation of an actual transition compared to an ideal transition for any code. Ideal value: 0 LSB.
ATmega328/P
Analog-to-Digital Converter (ADC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 314
Figure 28-12. Integral Non-Linearity (INL)
Output Code
VREF Input Voltage
Ideal ADC
Actual ADC
INL
• Differential Non-linearity (DNL): The maximum deviation of the actual code width (the interval
between two adjacent transitions) from the ideal code width (1 LSB). Ideal value: 0 LSB.
Figure 28-13. Differential Non-Linearity (DNL)
Output Code
0x3FF
0x000
0VREF Input Voltage
DNL
1 LSB
• Quantization Error: Due to the quantization of the input voltage into a finite number of codes, a
range of input voltages (1 LSB wide) will code to the same value. Always ±0.5 LSB.
• Absolute accuracy: The maximum deviation of an actual (unadjusted) transition compared to an
ideal transition for any code. This is the compound effect of offset, gain error, differential error, non-
linearity, and quantization error. Ideal value: ±0.5 LSB.
28.7 ADC Conversion Result
After the conversion is complete (ADCSRA.ADIF is set), the conversion result can be found in the ADC
Result registers (ADCL, ADCH).
For single-ended conversion, the result is
ADC = IN 1024
REF
ATmega328/P
Analog-to-Digital Converter (ADC)
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where VIN is the voltage on the selected input pin, and VREF the selected voltage reference (see also
descriptions of ADMUX.REFSn and ADMUX.MUX). 0x000 represents analog ground, and 0x3FF
represents the selected reference voltage minus one LSB.
28.8 Temperature Measurement
The temperature measurement is based on an on-chip temperature sensor that is coupled to a single
ended temperature sensor channel. Selecting the temperature sensor channel by writing
ADMUX.MUX[3:0] to '1000' enables the temperature sensor. The internal 1.1V voltage reference must
also be selected for the ADC voltage reference source in the temperature sensor measurement. When
the temperature sensor is enabled, the ADC converter can be used in Single Conversion mode to
measure the voltage over the temperature sensor.
The measured voltage has a linear relationship to the temperature as described in the following table.
The voltage sensitivity is approximately 1 LSB/°C, the accuracy of the temperature measurement is
±10°C assuming calibration at room temperature. Better accuracies are achieved by using two
temperature points for calibration.
Table 28-2. Temperature vs. Sensor Output Voltage (Typical Case)
Temperature -40°C 25°C +105°C
ADC 205 LSB 270 LSB 350 LSB
The values described in the table above are typical values. However, due to process variation the
temperature sensor output voltage varies from one chip to another. To be capable of achieving more
accurate results the temperature measurement can be calibrated in the application software. The
software calibration can be done using the formula:
T = { [(ADCH << 8) | ADCL] - TOS} / k
where ADCH and ADCL are the ADC data registers, k is a fixed coefficient and TOS is the temperature
sensor offset. Typically, k is very close to 1.0 and in single-point calibration the coefficient may be omitted.
Gain and offset varies from device to device, so calibration has to be done for each device. Refer to
AVR122: Calibration of the AVR's Internal Temperature Reference for the detail.
28.9 Register Description
ATmega328/P
Analog-to-Digital Converter (ADC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 316
28.9.1 ADC Multiplexer Selection Register
Name: ADMUX
Offset: 0x7C
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
REFS[1:0] ADLAR MUX[3:0]
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bits 7:6 – REFS[1:0] Reference Selection
These bits select the voltage reference for the ADC. If these bits are changed during a conversion, the
change will not go into effect until this conversion is complete (ADIF in ADCSRA is set). The internal
voltage reference options may not be used if an external reference voltage is being applied to the AREF
pin.
Table 28-3. ADC Voltage Reference Selection
REFS[1:0] Voltage Reference Selection
00 AREF, Internal Vref turned OFF
01 AVCC with external capacitor at AREF pin
10 Reserved
11 Internal 1.1V voltage reference with external capacitor at AREF pin
Bit 5 – ADLAR ADC Left Adjust Result
The ADLAR bit affects the presentation of the ADC conversion result in the ADC data register. Write one
to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the ADLAR bit will
affect the ADC data register immediately, regardless of any ongoing conversions. For a complete
description of this bit, refer to ADCL and ADCH.
Bits 3:0 – MUX[3:0] Analog Channel Selection
The value of these bits selects which analog inputs are connected to the ADC. If these bits are changed
during a conversion, the change will not go in effect until this conversion is complete (ADIF in ADCSRA is
set).
Table 28-4. Input Channel Selection
MUX[3:0] Single Ended Input
0000 ADC0
0001 ADC1
0010 ADC2
0011 ADC3
0100 ADC4
ATmega328/P
Analog-to-Digital Converter (ADC)
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MUX[3:0] Single Ended Input
0101 ADC5
0110 ADC6
0111 ADC7
1000 Temperature sensor
1001 Reserved
1010 Reserved
1011 Reserved
1100 Reserved
1101 Reserved
1110 1.1V (VBG)
1111 0V (GND)
Related Links
ADCL and ADCH
ATmega328/P
Analog-to-Digital Converter (ADC)
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28.9.2 ADC Control and Status Register A
Name: ADCSRA
Offset: 0x7A
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
ADEN ADSC ADATE ADIF ADIE ADPS [2:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – ADEN ADC Enable
Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off. Turning the ADC off
while a conversion is in progress, will terminate this conversion.
Bit 6 – ADSC ADC Start Conversion
In Single Conversion mode, write this bit to one to start each conversion. In Free Running mode, write
this bit to one to start the first conversion. The first conversion after ADSC has been written after the ADC
has been enabled, or if ADSC is written at the same time as the ADC is enabled, will take 25 ADC clock
cycles instead of the normal 13. This first conversion performs initialization of the ADC.
ADSC will read as one as long as a conversion is in progress. When the conversion is complete, it
returns to zero. Writing zero to this bit has no effect.
Bit 5 – ADATE ADC Auto Trigger Enable
When this bit is written to one, auto triggering of the ADC is enabled. The ADC will start a conversion on
a positive edge of the selected trigger signal. The trigger source is selected by setting the ADC trigger
select bits, ADTS in ADCSRB.
Bit 4 – ADIF ADC Interrupt Flag
This bit is set when an ADC conversion completes and the data registers are updated. The ADC
conversion complete interrupt is executed if the ADIE bit and the I-bit in SREG are set. ADIF is cleared by
hardware when executing the corresponding interrupt handling vector. Alternatively, ADIF is cleared by
writing a logical one to the flag. Beware that if doing a Read-Modify-Write on ADCSRA, a pending
interrupt can be disabled. This also applies if the SBI and CBI instructions are used.
Bit 3 – ADIE ADC Interrupt Enable
When this bit is written to one and the I-bit in SREG is set, the ADC conversion complete interrupt is
activated.
Bits 2:0 – ADPS [2:0] ADC Prescaler Select
These bits determine the division factor between the system clock frequency and the input clock to the
ADC.
ATmega328/P
Analog-to-Digital Converter (ADC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 319
Table 28-5. Input Channel Selection
ADPS[2:0] Division Factor
000 2
001 2
010 4
011 8
100 16
101 32
110 64
111 128
ATmega328/P
Analog-to-Digital Converter (ADC)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 320
28.9.3 ADC Data Register Low and High Byte (ADLAR=0)
Name: ADCL and ADCH
Offset: 0x78
Reset: 0x00
Property: ADLAR = 0
The ADCL and ADCH register pair represents the 16-bit value, ADC data register. The low byte [7:0]
(suffix L) is accessible at the original offset. The high byte [15:8] (suffix H) can be accessed at offset
+ 0x01. For more details on reading and writing 16-bit registers, refer to Accessing 16-bit Timer/Counter
Registers.
When an ADC conversion is complete, the result is found in these two registers.
When ADCL is read, the ADC data register is not updated until ADCH is read. Consequently, if the result
is left adjusted and no more than 8-bit precision is required, it is sufficient to read ADCH. Otherwise,
ADCL must be read first, then ADCH.
The ADLAR bit and the MUXn bits in ADMUX affect the way the result is read from the registers. If
ADLAR is set (ADLAR=1), the result is left adjusted. If ADLAR is cleared (ADLAR=0, which is the default
value), the result is right adjusted.
Bit 15 14 13 12 11 10 9 8
ADC[9:8]
Access R R
Reset 0 0
Bit 7 6 5 4 3 2 1 0
ADC[7:0]
Access R R R R R R R R
Reset 0 0 0 0 0 0 0 0
Bits 9:0 – ADC[9:0] ADC Conversion Result
These bits represent the result from the conversion. Refer to ADC Conversion Result for details.
Related Links
Accessing 16-bit Timer/Counter Registers
ATmega328/P
Analog-to-Digital Converter (ADC)
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28.9.4 ADC Data Register Low and High Byte (ADLAR=1)
Name: ADCL and ADCH (ADLAR = 1)
Offset: 0x78
Reset: 0x00
The ADCL and ADCH register pair represents the 16-bit value, ADC data register. The low byte [7:0]
(suffix L) is accessible at the original offset. The high byte [15:8] (suffix H) can be accessed at offset
+ 0x01. For more details on reading and writing 16-bit registers, refer to Accessing 16-bit Timer/Counter
Registers.
When an ADC conversion is complete, the result is found in these two registers.
When ADCL is read, the ADC data register is not updated until ADCH is read. Consequently, if the result
is left adjusted and no more than 8-bit precision is required, it is sufficient to read ADCH. Otherwise,
ADCL must be read first, then ADCH.
The ADLAR bit and the MUXn bits in ADMUX affect the way the result is read from the registers. If
ADLAR is set (ADLAR=1), the result is left adjusted. If ADLAR is cleared (ADLAR=0, which is the default
value), the result is right adjusted.
Bit 15 14 13 12 11 10 9 8
ADC[9:2]
Access R R R R R R R R
Reset 0 0 0 0 0 0 0 0
Bit 7 6 5 4 3 2 1 0
ADC[1:0]
Access R R
Reset 0 0
Bits 15:6 – ADC[9:0] ADC Conversion Result
These bits represent the result from the conversion. Refer to ADC Conversion Result for details.
Related Links
Accessing 16-bit Timer/Counter Registers
ATmega328/P
Analog-to-Digital Converter (ADC)
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28.9.5 ADC Control and Status Register B
Name: ADCSRB
Offset: 0x7B
Reset: 0x00
Property: -
Bit 7 6 5 4 3 2 1 0
ACME ADTS[2:0]
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 6 – ACME Analog Comparator Multiplexer Enable
When this bit is written logic one and the ADC is switched off (ADEN in ADCSRA is zero), the ADC
multiplexer selects the negative input to the analog comparator. When this bit is written logic zero, AIN1 is
applied to the negative input of the analog comparator. For a detailed description of this bit, see Analog
Comparator Multiplexed Input.
Bits 2:0 – ADTS[2:0] ADC Auto Trigger Source
If ADATE in ADCSRA is written to one, the value of these bits selects which source will trigger an ADC
conversion. If ADATE is cleared, the ADTS[2:0] settings will have no effect. A conversion will be triggered
by the rising edge of the selected interrupt flag. Note that switching from a trigger source that is cleared to
a trigger source that is set, will generate a positive edge on the trigger signal. If ADEN in ADCSRA is set,
this will start a conversion. Switching to Free Running mode (ADTS[2:0]=0) will not cause a trigger event,
even if the ADC interrupt flag is set.
Table 28-6. ADC Auto Trigger Source Selection
ADTS[2:0] Trigger Source
000 Free Running mode
001 Analog Comparator
010 External Interrupt Request 0
011 Timer/Counter0 Compare Match A
100 Timer/Counter0 Overflow
101 Timer/Counter1 Compare Match B
110 Timer/Counter1 Overflow
111 Timer/Counter1 Capture Event
Related Links
Analog Comparator Multiplexed Input
ATmega328/P
Analog-to-Digital Converter (ADC)
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28.9.6 Digital Input Disable Register 0
Name: DIDR0
Offset: 0x7E
Reset: 0x00
Property: -
When the respective bits are written to logic one, the digital input buffer on the corresponding ADC pin is
disabled. The corresponding PIN Register bit will always read as zero when this bit is set. When an
analog signal is applied to the ADC7...0 pin and the digital input from this pin is not needed, this bit
should be written logic one to reduce power consumption in the digital input buffer.
Bit 7 6 5 4 3 2 1 0
ADC5D ADC4D ADC3D ADC2D ADC1D ADC0D
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bits 0, 1, 2, 3, 4, 5 – ADCD ADC Digital Input Disable
ATmega328/P
Analog-to-Digital Converter (ADC)
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29. debugWIRE On-chip Debug System
29.1 Features
• Complete Program Flow Control
• Emulates All On-chip Functions, Both Digital and Analog, except RESET Pin
• Real-time Operation
• Symbolic Debugging Support (Both at C and Assembler Source Level, or for Other HLLs)
• Unlimited Number of Program Break Points (Using Software Break Points)
• Non-intrusive Operation
• Electrical Characteristics Identical to Real Device
• Automatic Configuration System
• High-speed Operation
• Programming of Nonvolatile Memories
29.2 Overview
The debugWIRE on-chip debug system uses a wire with bi-directional interface to control the program
flow and execute AVR instructions in the CPU and to program the different nonvolatile memories.
29.3 Physical Interface
When the debugWIRE Enable (DWEN) bit is programmed to '0' and Lock bits are unprogrammed ('1'), the
debugWIRE system within the target device is activated. The RESET port pin is configured as a wire-
AND (open-drain) bi-directional I/O pin with pull-up enabled and becomes the communication gateway
between target and emulator.
Figure 29-1. The debugWIRE Setup
dW
GND
dW(RESET)
VCC
1.8 - 5.5V
The debugWIRE Setup shows the schematic of a target MCU, with debugWIRE enabled, and the
emulator connector. The system clock is not affected by debugWIRE and will always be the clock source
selected by the CKSEL Fuses.
ATmega328/P
debugWIRE On-chip Debug System
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 325
When designing a system where debugWIRE will be used, the following observations must be made for
correct operation:
• Pull-up resistors on the dW/(RESET) line must not be smaller than 10 kΩ. The pull-up resistor is
not required for debugWIRE functionality.
• Connecting the RESET pin directly to VCC will not work.
• Capacitors connected to the RESET pin must be disconnected when using debugWire.
• All external reset sources must be disconnected.
29.4 Software Breakpoints
debugWIRE supports the breakpoint functions in program memory by the AVR BREAK instruction.
Setting a breakpoint in Atmel Studio will insert a BREAK instruction in the program memory. The
instruction replaced by the BREAK instruction will be stored. When program execution is continued, the
stored instruction will be executed before continuing from the program memory. A break can be inserted
manually by putting the BREAK instruction in the program.
The Flash must be re-programmed each time when a breakpoint is changed. This is automatically
handled by Atmel Studio through the debugWIRE interface. The use of breakpoints will, therefore, reduce
the Flash data retention. Devices used for debugging purposes should not be shipped to end customers.
29.5 Limitations of debugWIRE
The debugWIRE communication pin (dW) is physically located on the same pin as external Reset
(RESET). An external Reset source is therefore not supported when the debugWIRE is enabled.
A programmed DWEN fuse enables some parts of the clock system to be running in all sleep modes. This
will increase the power consumption while in sleep. Thus, the DWEN fuse should be disabled when
debugWire is not used.
29.6 Register Description
The following section describes the registers used with the debugWire.
ATmega328/P
debugWIRE On-chip Debug System
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 326
29.6.1 debugWire Data Register
Name: DWDR
Offset: 0x51 [ID-000004d0]
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x31
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
DWDR[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – DWDR[7:0] debugWire Data
The DWDR register provides a communication channel from the running program in the MCU to the
debugger. This register is only accessible by the debugWIRE and can therefore not be used as a general
purpose register in the normal operations.
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30. Boot Loader Support – Read-While-Write Self-programming (BTLDR)
30.1 Features
• Read-While-Write Self-Programming
• Flexible Boot Memory Size
• High Security (Separate Boot Lock Bits for a Flexible Protection)
• Separate Fuse to Select Reset Vector
• Optimized Page(1) Size
• Code Efficient Algorithm
• Efficient Read-Modify-Write Support
Note: 1. A page is a section in the Flash consisting of several bytes (see Table. Number of words in a
page and number of pages in the Flash in Page Size) used during programming. The page organization
does not affect normal operation.
30.2 Overview
In this device, the boot loader support provides a real read-while-write self-programming mechanism for
downloading and uploading program code by the MCU itself. This feature allows flexible application
software updates controlled by the MCU using a Flash-resident boot loader program. The boot loader
program can use any available data interface and associated protocol to read code and write (program)
that code into the Flash memory, or read the code from the program memory. The program code within
the boot loader section has the capability to write into the entire Flash, including the boot loader memory.
The boot loader can thus even modify itself, and it can also erase itself from the code if the feature is not
needed anymore. The size of the boot loader memory is configurable with fuses and the boot loader has
two separate sets of boot lock bits, which can be set independently. This gives the user a unique flexibility
to select different levels of protection.
30.3 Application and Boot Loader Flash Sections
The Flash memory is organized into two main sections; the application section and the boot loader
section. The size of the different sections is configured by the BOOTSZ fuses. These two sections can
have different level of protection since they have different sets of Lock bits.
30.3.1 Application Section
The application section is the section of the Flash that is used for storing the application code. The
protection level for the application section can be selected by the application boot lock bits (Boot Lock bits
0). The application section can never store any boot loader code since the SPM instruction is disabled
when executed from the application section.
30.3.2 Boot Loader Section (BLS)
While the application section is used for storing the application code, the boot loader software must be
located in the Boot Loader Section (BLS) since the SPM instruction can initiate a programming when
executing from the BLS only. The SPM instruction can access the entire Flash, including the BLS itself.
The protection level for the BLS can be selected by the Boot Loader Lock bits (Boot Lock bits 1).
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30.4 Read-While-Write and No Read-While-Write Flash Sections
Whether the CPU supports Read-While-Write (RWW) or if the CPU is halted during a boot loader
software update is dependent on which address that is being programmed. In addition to the two sections
that are configurable by the BOOTSZ fuses as described above, the Flash is also divided into two fixed
sections; the RWW section and the No Read-While-Write (NRWW) section. The limit between the RWW
and NRWW sections is given in the Boot Loader Parameters section and Figure 30-2. The main
differences between the two sections are:
• When erasing or writing a page located inside the RWW section, the NRWW section can be read
during the operation
• When erasing or writing a page located inside the NRWW section, the CPU is halted during the
entire operation
The user software can never read any code that is located inside the RWW section during a boot loader
software operation. The syntax “Read-While-Write section” refers to which section that is being
programmed (erased or written), not which section that actually is being read during a boot loader
software update.
30.4.1 Read-While-Write (RWW) Section
If a boot loader software update is programming a page inside the RWW section, it is possible to read
code from the Flash, but only code that is located in the NRWW section. During an ongoing programming,
the software must ensure that the RWW section is never being read. If the user software is trying to read
code that is located inside the RWW section (i.e., by a call/jmp/lpm or an interrupt) during programming,
the software might end up in an unknown state. To avoid this, the interrupts should either be disabled or
moved to the boot loader section. The boot loader section is always located in the NRWW section. The
RWW Section Busy bit (RWWSB) in the Store Program Memory Control and Status Register (SPMCSR)
will be read as logical one as long as the RWW section is blocked for reading. After a programming is
completed, the RWWSB must be cleared by software before reading code located in the RWW section.
Refer to SPMCSR – Store Program Memory Control and Status Register in this chapter for details on
how to clear RWWSB.
30.4.2 No Read-While-Write (NRWW) Section
The code located in the NRWW section can be read when the boot loader software is updating a page in
the RWW section. When the boot loader code updates the NRWW section, the CPU is halted during the
entire page erase or page write operation.
Table 30-1. Read-While-Write Features
Which Section does the Z-
pointer Address During the
Programming?
Which Section can be Read
During Programming?
CPU Halted? Read-While-Write
Supported?
RWW Section NRWW Section No Yes
NRWW Section None Yes No
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Figure 30-1. Read-While-Write vs. No Read-While-Write
Read-While-Write
(RWW) Section
No Read-While-Write
(NRWW) Section
Z-pointer
Addresses RWW
Section
Z-pointer
Addresses NRWW
Section
CPU is Halted
During the Operation
Code Located in
NRWW Section
Can be Read During
the Operation
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Figure 30-2. Memory Sections
30.5 Boot Loader Lock Bits
If no boot loader capability is needed, the entire Flash is available for application code. The boot loader
has two separate sets of boot lock bits which can be set independently. This gives the user a unique
flexibility to select different levels of protection.
The user can select:
• To protect the entire Flash from a software update by the MCU
• To protect only the boot loader Flash section from a software update by the MCU
• To protect only the application Flash section from a software update by the MCU
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• Allow software update in the entire Flash
The boot lock bits can be set in software and in Serial or Parallel Programming mode, but they can be
cleared by a chip erase command only. The general Write Lock (Lock Bit mode 2) does not control the
programming of the Flash memory by SPM instruction. Similarly, the general Read/Write Lock (Lock Bit
mode 1) does not control reading nor writing by LPM/SPM, if it is attempted.
Table 30-2. Boot Lock Bit0 Protection Modes (Application Section)
BLB0
Mode
BLB02 BLB01 Protection
1 1 1 No restrictions for SPM or LPM accessing the application section.
2 1 0 SPM is not allowed to write to the application section.
3 0 0 SPM is not allowed to write to the application section, and LPM executing
from the boot loader section is not allowed to read from the application
section. If interrupt vectors are placed in the boot loader section, interrupts
are disabled while executing from the application section.
4 0 1 LPM executing from the boot loader section is not allowed to read from the
application section. If interrupt vectors are placed in the boot loader
section, interrupts are disabled while executing from the application
section.
Note: “1” means unprogrammed, “0” means programmed.
Table 30-3. Boot Lock Bit1 Protection Modes (Boot Loader Section)
BLB1
Mode
BLB12 BLB11 Protection
1 1 1 No restrictions for SPM or LPM accessing the boot loader section.
2 1 0 SPM is not allowed to write to the boot loader section.
3 0 0 SPM is not allowed to write to the boot loader section, and LPM executing
from the application section is not allowed to read from the boot loader
section. If interrupt vectors are placed in the application section, interrupts
are disabled while executing from the boot loader section.
4 0 1 LPM executing from the application section is not allowed to read from the
boot loader section. If interrupt vectors are placed in the application
section, interrupts are disabled while executing from the boot loader
section.
Note: “1” means unprogrammed, “0” means programmed.
30.6 Entering the Boot Loader Program
Entering the boot loader takes place by a jump or call from the application program. This may be initiated
by a trigger such as a command received via USART or SPI interface. Alternatively, the boot Reset fuse
can be programmed so that the Reset vector is pointing to the boot Flash start address after a reset. In
this case, the boot loader is started after a Reset. After the application code is loaded, the program can
start executing the application code. The fuses cannot be changed by the MCU itself. This means that
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once the boot Reset fuse is programmed, the Reset vector will always point to the boot loader Reset and
the fuse can only be changed through the serial or parallel programming interface.
Table 30-4. Boot Reset Fuse
BOOTRST Reset Address
1 Reset vector = application Reset (address 0x0000)
0 Reset vector = boot loader Reset, as described by the boot loader parameters
Note: '1' means unprogrammed, '0' means programmed.
30.7 Addressing the Flash During Self-Programming
The Z-pointer is used to address the SPM commands. The Z-pointer consists of the Z-registers ZL and
ZH in the register file. The number of bits actually used is implementation dependent.
Bit 15 14 13 12 11 10 9 8
ZH (R31) Z15 Z14 Z13 Z12 Z11 Z10 Z9 Z8
ZL (R30) Z7 Z6 Z5 Z4 Z3 Z2 Z1 Z0
7 6 5 4 3 2 1 0
Since the Flash is organized in pages, the program counter can be treated as having two different
sections. One section, consisting of the least significant bits, is addressing the words within a page, while
the most significant bits are addressing the pages. This is shown in the following figure. The page erase
and page write operations are addressed independently. Therefore, it is of major importance that the Boot
Loader software addresses the same page in both the page erase and page write operation. Once a
programming operation is initiated, the address is latched and the Z-pointer can be used for other
operations.
The only SPM operation that does not use the Z-pointer is setting the boot loader lock bits. The content of
the Z-pointer is ignored and will have no effect on the operation. The LPM instruction does also use the Z-
pointer to store the address. Since this instruction addresses the Flash byte-by-byte, also the LSB (bit Z0)
of the Z-pointer is used.
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Figure 30-3. Addressing the Flash During SPM
PROGRAM MEMORY
0115
Z - REGISTER
BIT
0
ZPAGEMSB
WORD ADDRESS
WITHIN A PAGE
PAGE ADDRESS
WITHIN THE FLASH
ZPCMSB
INSTRUCTION WORD
PAGE PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
PAGE
PCWORDPCPAGE
PCMSB PAGEMSB
PROGRAM
COUNTER
Note: The different variables used in this figure are listed in the Related Links.
Related Links
Page Size
30.8 Self-Programming the Flash
The program memory is updated in a page-by-page fashion. Before programming a page with the data
stored in the temporary page buffer, the page must be erased. The temporary page buffer is filled one
word at a time using SPM and the buffer can be filled either before the page erase command or between
a page erase and a page write operation:
Alternative 1. Fill the Buffer Before a Page Erase
• Fill temporary page buffer
• Perform a page erase
• Perform a page write
Alternative 2. Fill the Buffer After Page Erase
• Perform a page erase
• Fill temporary page buffer
• Perform a page write
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If only a part of the page needs to be changed, the rest of the page must be stored (for example in the
temporary page buffer) before the erase, and then be rewritten. When using Alternative 1, the boot loader
provides an effective Read-Modify-Write feature which allows the user software to first read the page, do
the necessary changes, and then write back the modified data. If Alternative 2 is used, it is not possible to
read the old data while loading since the page is already erased. The temporary page buffer can be
accessed in a random sequence. It is essential that the page address used in both the page erase and
page write operations are addressing the same page. Refer to Simple Assembly Code Example for a
Boot Loader.
30.8.1 Performing Page Erase by SPM
To execute page erase, set up the address in the Z-pointer, write “0x0000011” to Store Program Memory
Control and Status Register (SPMCSR), and execute SPM within four clock cycles after writing SPMCSR.
The data in R1 and R0 is ignored. The page address must be written to PCPAGE in the Z-register. Other
bits in the Z-pointer will be ignored during this operation.
• Page erase to the RWW section: The NRWW section can be read during the page erase.
• Page erase to the NRWW section: The CPU is halted during the operation.
30.8.2 Filling the Temporary Buffer (Page Loading)
To write an instruction word, set up the address in the Z-pointer and data in [R1:R0], write “0x00000001”
to SPMCSR, and execute SPM within four clock cycles after writing SPMCSR. The content of PCWORD
([Z5:Z1]) in the Z-register is used to address the data in the temporary buffer. The temporary buffer will
auto-erase after a page write operation or by writing the RWWSRE bit in SPMCSR
(SPMCSR.RWWSRE). It is also erased after a system reset. It is not possible to write more than one time
to each address without erasing the temporary buffer.
If the EEPROM is written in the middle of an SPM page load operation, all data loaded will be lost.
30.8.3 Performing a Page Write
To execute page write, setup the address in the Z-pointer, write “0x0000101” to SPMCSR, and execute
SPM within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored. The page address
must be written to PCPAGE ([Z5:Z1]). Other bits in the Z-pointer must be written to zero during this
operation.
• Page write to the RWW section: The NRWW section can be read during the page write
• Page write to the NRWW section: The CPU is halted during the operation
30.8.4 Using the SPM Interrupt
If the SPM interrupt is enabled, the SPM interrupt will generate a constant interrupt when the SPMEN bit
in SPMCSR is cleared (SPMCSR.SPMEN). This means that the interrupt can be used instead of polling
the SPMCSR register in software. When using the SPM interrupt, the interrupt vectors should be moved
to the Boot Loader Section (BLS) section to avoid that an interrupt is accessing the RWW section when it
is blocked for reading. How to move the interrupts is described in Interrupts chapter.
Related Links
Interrupts
30.8.5 Consideration While Updating Boot Loader Section (BLS)
Special care must be taken if the user allows the Boot Loader Section (BLS) to be updated by leaving
Boot Lock bit11 unprogrammed. An accidental write to the boot loader itself can corrupt the entire boot
loader, and further software updates might be impossible. If it is not necessary to change the boot loader
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software itself, it is recommended to program the Boot Lock bit11 to protect the boot loader software from
any internal software changes.
30.8.6 Prevent Reading the RWW Section During Self-Programming
During self-programming (either page erase or page write), the RWW section is always blocked for
reading. The user software itself must prevent that this section is addressed during the self-programming
operation. The RWWSB in the SPMCSR (SPMCSR.RWWSB) will be set as long as the RWW section is
busy. During self-programming the interrupt vector table should be moved to the BLS as described in
Watchdog Timer chapter or the interrupts must be disabled. Before addressing the RWW section after the
programming is completed, the user software must clear the SPMCSR.RWWSB by writing the
SPMCSR.RWWSRE. Refer to Simple Assembly Code Example for a Boot Loader for an example.
Related Links
Watchdog System Reset
30.8.7 Setting the Boot Loader Lock Bits by SPM
To set the Boot Loader Lock bits and general Lock bits, write the desired data to R0, write “0x0001001” to
SPMCSR and execute SPM within four clock cycles after writing SPMCSR.
Bit 7 6 5 4 3 2 1 0
R0 1 1 BLB12 BLB11 BLB02 BLB01 LB2 LB1
The tables in Boot Loader Lock Bits show how the different settings of the Boot Loader bits affect the
Flash access.
If bits 5...0 in R0 are cleared (zero), the corresponding Lock bit will be programmed if an SPM instruction
is executed within four cycles after BLBSET and SPMEN are set in SPMCSR (SPMCSR.BLBSET and
SPMCSR.SPMEN). For future compatibility, it is recommended to load the Z-pointer with 0x0001 (same
as used for reading the l/Ock bits). It is also recommended to set bits 7 and 6 in R0 to “1” when writing the
Lock bits. When programming the Lock bits the entire Flash can be read during the operation.
30.8.8 EEPROM Write Prevents Writing to SPMCSR
An EEPROM write operation will block all software programming to Flash. Reading the fuses and Lock
bits from the software will be prevented during the EEPROM write operation. It is recommended to check
the status bit (EEPE) in the EECR Register (EECR.EEPE) and verify that the bit is cleared before writing
to the SPMCSR register.
30.8.9 Reading the Fuse and Lock Bits from Software
It is possible to read both the Fuse and Lock bits (LB) from software. To read the Lock bits, load the Z-
pointer with 0x0001 and set the BLBSET and SPMEN bits in SPMCSR (SPMCSR.BLBSET and
SPMCSR.SPMEN). When an LPM instruction is executed within three CPU cycles after the BLBSET and
SPMEN bits are set in SPMCSR (SPMCSR.BLBSET and SPMCSR.SPMEN), the value of the Lock bits
will be loaded in the destination register. The SPMCSR.BLBSET and SPMCSR.SPMEN will auto-clear
upon completion of reading the Lock bits or if no LPM instruction is executed within three CPU cycles or
no SPM instruction is executed within four CPU cycles. When SPMCSR.BLBSET and SPMCSR.SPMEN
are cleared, LPM will work as described in the Instruction set Manual.
Bit 7 6 5 4 3 2 1 0
Rd - - BLB12 BLB11 BLB02 BLB01 LB2 LB1
The algorithm for reading the Fuse Low byte (FLB) is similar to the one described above for reading the
Lock bits. To read the Fuse Low byte, load the Z-pointer with 0x0000 and set the BLBSET and SPMEN
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bits in SPMCSR (SPMCSR.BLBSET and SPMCSR.SPMEN). When an LPM instruction is executed within
three cycles after the SPMCSR.BLBSET and SPMCSR.SPMEN are set, the value of the Fuse Low byte
(FLB) will be loaded into the destination register as shown below.
Bit76543210
Rd FLB7 FLB6 FLB5 FLB4 FLB3 FLB2 FLB1 FLB0
Similarly, when reading the Fuse High byte (FHB), load 0x0003 in the Z-pointer. When an LPM instruction
is executed within three cycles after the SPMCSR.BLBSET and SPMCSR.SPMEN are set, the value of
the Fuse High byte (FHB) will be loaded into the destination register as shown below.
Bit 7 6 5 4 3 2 1 0
Rd FHB7 FHB6 FHB5 FHB4 FHB3 FHB2 FHB1 FHB0
When reading the Extended Fuse byte (EFB), load 0x0002 in the Z-pointer. When an LPM instruction is
executed within three cycles after the SPMCSR.BLBSET and SPMCSR.SPMEN are set, the value of the
Extended Fuse byte (EFB) will be loaded into the destination register as shown below.
Bit 7 6 5 4 3 2 1 0
Rd - - - - EFB3 EFB2 EFB1 EFB0
Fuse and Lock bits that are programmed read as '0'. Fuse and Lock bits that are unprogrammed, will read
as '1'.
30.8.10 Reading the Signature Row from Software
To read the signature row from software, load the Z-pointer with the signature byte address given in the
following table and set the SIGRD and SPMEN bits in SPMCSR (SPMCSR.SIGRD and
SPMCSR.SPMEN). When an LPM instruction is executed within three CPU cycles after the
SPMCSR.SIGRD and SPMCSR.SPMEN are set, the signature byte value will be loaded in the
destination register. The SPMCSR.SIGRD and SPMCSR.SPMEN will auto-clear upon completion of
reading the Signature Row Lock bits or if no LPM instruction is executed within three CPU cycles. When
SPMCSR.SIGRD and SPMCSR.SPMEN are cleared, LPM will work as described in the instruction set
manual.
30.8.11 Preventing Flash Corruption
During periods of low VCC, the Flash program can be corrupted because the supply voltage is too low for
the CPU and the Flash to operate properly. These issues are the same as for board level systems using
the Flash, and the same design solutions should be applied.
A Flash program corruption can be caused by two situations when the voltage is too low. First, a regular
write sequence to the Flash requires a minimum voltage to operate correctly. Secondly, the CPU itself can
execute instructions incorrectly, if the supply voltage for executing instructions is too low.
Flash corruption can easily be avoided by following these design recommendations (one is sufficient):
1. If it is no need for a boot loader update in the system, program the Boot Loader Lock bits to prevent
any boot loader software updates.
2. Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be
done by enabling the internal Brown-out Detector (BOD) if the operating voltage matches the
detection level. If not, an external low VCC Reset protection circuit can be used. If a Reset occurs
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while a write operation is in progress, the write operation will be completed provided that the power
supply voltage is sufficient.
3. Keep the AVR core in Power-Down Sleep mode during periods of low VCC. This will prevent the
CPU from attempting to decode and execute instructions, effectively protecting the SPMCSR
register and thus the Flash from unintentional writes.
30.8.12 Programming Time for Flash when Using SPM
The calibrated RC Oscillator is used to time Flash accesses. The following table shows the typical
programming time for Flash accesses from the CPU.
Table 30-5. SPM Programming Time
Symbol Min. Programming
Time
Max. Programming
Time
Flash write (Page Erase, Page Write, and write Lock
bits by SPM)
3.2 ms 3.4 ms
Note: Minimum and maximum programming time is per individual operation.
30.8.13 Simple Assembly Code Example for a Boot Loader
;-the routine writes one page of data from RAM to Flash
; the first data location in RAM is pointed to by the Y pointer
; the first data location in Flash is pointed to by the Z-pointer
;-error handling is not included
;-the routine must be placed inside the Boot space
; (at least the Do_spm sub routine). Only code inside NRWW section can
; be read during Self-Programming (Page Erase and Page Write).
;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo (r24),
; loophi (r25), spmcrval (r20)
; storing and restoring of registers is not included in the routine
; register usage can be optimized at the expense of code size
;-It is assumed that either the interrupt table is moved to the Boot
; loader section or that the interrupts are disabled.
.equ PAGESIZEB = PAGESIZE*2 ;PAGESIZEB is page size in BYTES, not words
.org SMALLBOOTSTART
Write_page:
; Page Erase
ldi spmcrval, (1<<PGERS) | (1<<SPMEN)
call Do_spm
; re-enable the RWW section
.
; must be avoided if the page buffer is pre-filled. Will flush the page
buffer.
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ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
; transfer data from RAM to Flash page buffer
ldi looplo, low(PAGESIZEB) ;init loop variable
ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256
Wrloop:
ld r0, Y+
ld r1, Y+
ldi spmcrval, (1<<SPMEN)
call Do_spm
adiw ZH:ZL, 2
sbiw loophi:looplo, 2 ;use subi for PAGESIZEB<=256
brne Wrloop
; execute Page Write
subi ZL, low(PAGESIZEB) ;restore pointer
sbci ZH, high(PAGESIZEB) ;not required for PAGESIZEB<=256
ldi spmcrval, (1<<PGWRT) | (1<<SPMEN)
call Do_spm
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
; read back and check, optional
ldi looplo, low(PAGESIZEB) ;init loop variable
ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256
subi YL, low(PAGESIZEB) ;restore pointer
sbci YH, high(PAGESIZEB)
Rdloop:
lpm r0, Z+
ld r1, Y+
cpse r0, r1
jmp Error
sbiw loophi:looplo, 1 ;use subi for PAGESIZEB<=256
brne Rdloop
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; return to RWW section
; verify that RWW section is safe to read
Return:
in temp1, SPMCSR
sbrs temp1, RWWSB ; If RWWSB is set, the RWW section is not ready yet
ret
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
rjmp Return
Do_spm:
; check for previous SPM complete
Wait_spm:
in temp1, SPMCSR
sbrc temp1, SPMEN
rjmp Wait_spm
; input: spmcrval determines SPM action
; disable interrupts if enabled, store status
in temp2, SREG
cli
; check that no EEPROM write access is present
Wait_ee:
sbic EECR, EEPE
rjmp Wait_ee
; SPM timed sequence
out SPMCSR, spmcrval
spm
; restore SREG (to enable interrupts if originally enabled)
out SREG, temp2
ret
30.8.14 ATmega328/P Boot Loader Parameters
In the following tables, the parameters used in the description of the self programming are given.
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Table 30-6. Boot Size Configuration, ATmega328/P
BOOTSZ1 BOOTSZ0 Boot
Size
Pages Application
Flash Section
Boot
Loader
Flash
Section
End
Application
Section
Boot Reset
Address
(Start Boot
Loader
Section)
1 1 256
words
4 0x0000 -
0x3EFF
0x3F00 -
0x3FFF
0x3EFF 0x3F00
1 0 512
words
8 0x0000 -
0x3DFF
0x3E00 -
0x3FFF
0x3DFF 0x3E00
0 1 1024
words
16 0x0000 -
0x3BFF
0x3C00 -
0x3FFF
0x3BFF 0x3C00
0 0 2048
words
32 0x0000 -
0x37FF
0x3800 -
0x3FFF
0x37FF 0x3800
Note: The different BOOTSZ Fuse configurations are shown in Figure 30-2
Table 30-7. Read-While-Write Limit, ATmega328/P
Section Pages Address
Read-While-Write section (RWW) 224 0x0000 - 0x37FF
No Read-While-Write section (NRWW) 32 0x3800 - 0x3FFF
Note: For details about these two section, see No Read-While-Write (NRWW) Section and Read-While-
Write (RWW) Section.
Table 30-8. Explanation of Different Variables used in Figure and the Mapping to the Z-pointer,
ATmega328/P
Variable Corresponding
Variable (1)
Description
PCMSB 11 Most significant bit in the Program Counter. (The Program
Counter is 12 bits PC[11:0])
PAGEMSB 4 Most significant bit which is used to address the words
within one page (32 words in a page requires 5 bits PC
[4:0]).
ZPCMSB Z12 Bit in Z-register that is mapped to PCMSB. Because Z0 is
not used, the ZPCMSB equals PCMSB + 1.
ZPAGEMSB Z5 Bit in Z-register that is mapped to PAGEMSB. Because Z0
is not used, the ZPAGEMSB equals PAGEMSB + 1.
PCPAGE PC[11:5] Z12:Z6 Program counter page address: Page select, for page
erase and page write
PCWORD PC[4:0] Z5:Z1 Program counter word address: Word select, for filling
temporary buffer (must be zero during page write
operation)
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Note:
1. Z15:Z13: always ignored
2. Z0: should be zero for all SPM commands, byte select for the LPM instruction.
See Addressing the Flash During Self-Programming for details about the use of Z-pointer during
Self- Programming.
30.9 Register Description
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30.9.1 Store Program Memory Control and Status Register (SPMCSR)
Name: SPMCSR
Offset: 0x57 [ID-000004d0]
Reset: 0x00
Property: When addressing as I/O register: address offset is 0x37
The Store Program Memory Control and Status Register (SPMCSR) contains the control bits needed to
control the boot loader operations.
When addressing I/O registers as data space using LD and ST instructions, the provided offset must be
used. When using the I/O specific commands IN and OUT, the offset is reduced by 0x20, resulting in an
I/O address offset within 0x00 - 0x3F.
Bit 7 6 5 4 3 2 1 0
SPMIE RWWSB SIGRD RWWSRE BLBSET PGWRT PGERS SPMEN
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – SPMIE SPM Interrupt Enable
When the SPMIE bit is written to one, and the I-bit in the Status register is set (one), the SPM ready
interrupt will be enabled. The SPM ready Interrupt will be executed as long as the SPMEN bit in the
SPMCSR register is cleared.
Bit 6 – RWWSB Read-While-Write Section Busy
When a self-programming (page erase or page write) operation to the RWW section is initiated, the
RWWSB will be set (one) by hardware. When the RWWSB bit is set, the RWW section cannot be
accessed. The RWWSB bit will be cleared if the RWWSRE bit is written to one after a self-programming
operation is completed. Alternatively, the RWWSB bit will automatically be cleared if a page load
operation is initiated.
Bit 5 – SIGRD Signature Row Read
If this bit is written to one at the same time as SPMEN, the next LPM instruction within three clock cycles
will read a byte from the signature row into the destination register. Refer to Reading the Fuse and Lock
Bits from Software in this chapter. An SPM instruction within four cycles after SIGRD and SPMEN are set
will have no effect. This operation is reserved for future use and should not be used.
Bit 4 – RWWSRE Read-While-Write Section Read Enable
When programming (page erase or page write) to the RWW section, the RWW section is blocked for
reading (the RWWSB will be set by hardware). To re-enable the RWW section, the user software must
wait until the programming is completed (SPMEN will be cleared). Then, if the RWWSRE bit is written to
one at the same time as SPMEN, the next SPM instruction within four clock cycles re-enables the RWW
section. The RWW section cannot be re-enabled while the Flash is busy with a page erase or a page
write (SPMEN is set). If the RWWSRE bit is written while the Flash is being loaded, the Flash load
operation will abort and the data loaded will be lost.
Bit 3 – BLBSET Boot Lock Bit Set
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles
sets Boot Lock bits and Memory Lock bits, according to the data in R0. The data in R1 and the address in
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the Z-pointer are ignored. The BLBSET bit will automatically be cleared upon completion of the Lock bit
set, or if no SPM instruction is executed within four clock cycles.
An LPM instruction within three cycles after BLBSET and SPMEN are set in the SPMCSR register
(SPMCSR.BLBSET and SPMCSR.SPMEN), will read either the Lock bits or the Fuse bits (depending on
Z0 in the Z-pointer) into the destination register. Refer to Reading the Fuse and Lock Bits from Software
in this chapter.
Bit 2 – PGWRT Page Write
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles
executes page write, with the data stored in the temporary buffer. The page address is taken from the
high part of the Z-pointer. The data in R1 and R0 are ignored. The PGWRT bit will auto-clear upon
completion of a page write, or if no SPM instruction is executed within four clock cycles. The CPU is
halted during the entire page write operation if the NRWW section is addressed.
Bit 1 – PGERS Page Erase
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles
executes page erase. The page address is taken from the high part of the Z-pointer. The data in R1 and
R0 are ignored. The PGERS bit will auto-clear upon completion of a page erase, or if no SPM instruction
is executed within four clock cycles. The CPU is halted during the entire page write operation if the
NRWW section is addressed.
Bit 0 – SPMEN Store Program Memory
This bit enables the SPM instruction for the next four clock cycles. If written to one together with either
RWWSRE, BLBSET, PGWRT, or PGERS, the following SPM instruction will have a special meaning (see
the description above). If only SPMEN is written, the following SPM instruction will store the value in
R1:R0 in the temporary page buffer addressed by the Z-pointer. The LSB of the Z-pointer is ignored. The
SPMEN bit will auto-clear upon completion of an SPM instruction, or if no SPM instruction is executed
within four clock cycles. During page erase and page write, the SPMEN bit remains high until the
operation is completed.
Writing any other combination than “0x10001”, “0x01001”, “0x00101”, “0x00011” or “0x00001” in the lower
five bits will have no effect.
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31. Memory Programming (MEMPROG)
31.1 Program And Data Memory Lock Bits
The device provides six Lock bits. These can be left unprogrammed ('1') or can be programmed ('0') to
obtain the additional features listed in the table Lock Bit Protection Modes below. The Lock bits can only
be erased to '1' with the Chip Erase command.
Table 31-1. Lock Bit Byte(1)
Lock Bit Byte Bit No. Description Default Value
7 – 1 (unprogrammed)
6 – 1 (unprogrammed)
BLB12 5 Boot Lock bit 1 (unprogrammed)
BLB11 4 Boot Lock bit 1 (unprogrammed)
BLB02 3 Boot Lock bit 1 (unprogrammed)
BLB01 2 Boot Lock bit 1 (unprogrammed)
LB2 1 Lock bit 1 (unprogrammed)
LB1 0 Lock bit 1 (unprogrammed)
Note:
1. '1' means unprogrammed, '0' means programmed.
Table 31-2. Lock Bit Protection Modes(1)(2)
Memory Lock Bits Protection Type
LB Mode LB2 LB1
1 1 1 No memory lock features enabled.
2 1 0 Further programming of the Flash and EEPROM is disabled in Parallel and Serial
Programming modes. The Fuse bits are locked in both Serial and Parallel
Programming modes.(1)
3 0 0 Further programming and verification of the Flash and EEPROM is disabled in
Parallel and Serial Programming modes. The Boot Lock bits and Fuse bits are
locked in both Serial and Parallel Programming modes.(1)
Note:
1. Program the Fuse bits and Boot Lock bits before programming the LB1 and LB2.
2. '1' means unprogrammed, '0' means programmed.
ATmega328/P
Memory Programming (MEMPROG)
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Table 31-3. Lock Bit Protection - BLB0 Mode(1)(2)
BLB0
Mode
BLB02 BLB01
1 1 1 No restrictions for SPM or Load Program Memory (LPM) instruction
accessing the application section.
2 1 0 SPM is not allowed to write to the application section.
3 0 0 SPM is not allowed to write to the application section, and LPM executing
from the boot loader section is not allowed to read from the application
section. If interrupt vectors are placed in the boot loader section, interrupts
are disabled while executing from the application section.
4 0 1 LPM executing from the boot loader section is not allowed to read from the
application section. If interrupt vectors are placed in the boot loader
section, interrupts are disabled while executing from the application
section.
Table 31-4. Lock Bit Protection - BLB1 Mode(1)(2)
BLB1
Mode
BLB12 BLB11
1 1 1 No restrictions for SPM or LPM accessing the boot loader section.
2 1 0 SPM is not allowed to write to the boot loader section.
3 0 0 SPM is not allowed to write to the boot loader section, and LPM executing
from the application section is not allowed to read from the boot loader
section. If interrupt vectors are placed in the application section, interrupts
are disabled while executing from the boot loader section.
4 0 1 LPM executing from the application section is not allowed to read from the
boot loader section. If interrupt vectors are placed in the application
section, interrupts are disabled while executing from the boot loader
section.
Note:
1. Program the Fuse bits and Boot Lock bits before programming the LB1 and LB2.
2. '1' means unprogrammed; '0' means programmed.
31.2 Fuse Bits
The device has three Fuse bytes. The following tables describe briefly the functionality of all the fuses
and how they are mapped into the Fuse bytes. Note that the fuses are read as logical zero, “0”, if they are
programmed.
ATmega328/P
Memory Programming (MEMPROG)
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Table 31-5. Extended Fuse Byte for ATmega328/P
Extended Fuse Byte Bit No. Description Default Value
– 7 – 1
– 6 – 1
– 5 – 1
– 4 – 1
– 3 – 1
BODLEVEL2(1) 2 Brown-out Detector trigger level 1 (unprogrammed)
BODLEVEL1(1) 1 Brown-out Detector trigger level 1 (unprogrammed)
BODLEVEL0(1) 0 Brown-out Detector trigger level 1 (unprogrammed)
Note:
1. Refer to Table BODLEVEL Fuse Coding in System and Reset Characteristics for BODLEVEL Fuse
decoding.
Table 31-6. Fuse High Byte
High Fuse Byte Bit No. Description Default Value
RSTDISBL(1) 7 External Reset Disable 1 (unprogrammed)
DWEN 6 debugWIRE Enable 1 (unprogrammed)
SPIEN(2) 5 Enable Serial Program and Data
Downloading
0 (programmed, SPI programming
enabled)
WDTON(3) 4 Watchdog Timer Always On 1 (unprogrammed)
EESAVE 3 EEPROM memory is preserved through
the Chip Erase
1 (unprogrammed), EEPROM not
preserved
BOOTSZ1 2 Select Boot Size
(see Boot Loader Parameters)
0 (programmed)(4)
BOOTSZ0 1 Select Boot Size
(see Boot Loader Parameters)
0 (programmed)(4)
BOOTRST 0 Select Reset Vector 1 (unprogrammed)
Note:
1. Refer to Alternate Functions of Port C in I/O-Ports chapter for description of RSTDISBL Fuse.
2. The SPIEN Fuse is not accessible in serial programming mode.
3. Refer to WDTCSR – Watchdog Timer Control Register for details.
4. The default value of BOOTSZ[1:0] results in maximum Boot Size. See table Boot Size
Configuration in subsection Boot Loader Parameters in the previous chapter for details.
ATmega328/P
Memory Programming (MEMPROG)
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Table 31-7. Fuse Low Byte
Low Fuse Byte Bit No. Description Default Value
CKDIV8(4) 7 Divide clock by 8 0 (programmed)
CKOUT(3) 6 Clock output 1 (unprogrammed)
SUT1 5 Select start-up time 1 (unprogrammed)(1)
SUT0 4 Select start-up time 0 (programmed)(1)
CKSEL3 3 Select Clock source 0 (programmed)(2)
CKSEL2 2 Select Clock source 0 (programmed)(2)
CKSEL1 1 Select Clock source 1 (unprogrammed)(2)
CKSEL0 0 Select Clock source 0 (programmed)(2)
Note:
1. The default value of SUT[1:0] results in maximum start-up time for the default clock source. See
table Start-Up Times for the Internal Calibrated RC Oscillator Clock Selection - SUT in Calibrated
Internal RC Oscillator of System Clock and Clock Options chapter for details.
2. The default setting of CKSEL[3:0] results in internal RC Oscillator @ 8 MHz. See table Internal
Calibrated RC Oscillator Operating Modes in Calibrated Internal RC Oscillator of the System Clock
and Clock Options chapter for details.
3. The CKOUT Fuse allows the system clock to be output on PORTB0. Refer to Clock Output Buffer
section in the System Clock and Clock Options chapter for details.
4. Refer to System Clock Prescaler section in the System Clock and Clock Options chapter for details.
The status of the Fuse bits is not affected by Chip Erase. Note that the Fuse bits are locked if Lock bit1
(LB1) is programmed. Program the Fuse bits before programming the Lock bits.
Related Links
Alternate Functions of Port C
WDTCSR
Calibrated Internal RC Oscillator
Clock Output Buffer
System Clock Prescaler
31.2.1 Latching of Fuses
The fuse values are latched when the device enters programming mode and changes of the fuse values
will have no effect until the part leaves Programming mode. This does not apply to the EESAVE fuse,
which will take effect once it is programmed. The fuses are also latched on power-up in Normal mode.
31.3 Signature Bytes
The device have a three-byte signature code. This code can be read in both serial and parallel mode,
also when the device is locked. The three bytes reside in a separate address space. For the device, the
signature bytes are given in the following table.
ATmega328/P
Memory Programming (MEMPROG)
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Table 31-8. Device ID
Part Signature Bytes Address
0x000 0x001 0x002
ATmega328 0x1E 0x95 0x14
ATmega328P 0x1E 0x95 0x0F
31.4 Calibration Byte
The device has a byte calibration value for the Internal RC oscillator. This byte resides in the high byte of
address 0x000 in the signature address space. During Reset, this byte is automatically written into the
OSCCAL register to ensure correct frequency of the calibrated RC oscillator.
Related Links
Calibrated Internal RC Oscillator
31.5 Serial Number
The product has a serial number which offers a unique ID to identify a specified part while it is in the field.
It consists of several bytes, which can be accessed from the signature address space.
The Signature row includes factory-programmed data:
• ID for each device type
• Serial number for each device
• Calibration bytes for factory calibrated peripherals
31.6 Page Size
Table 31-9. No. of Words in a Page and No. of Pages in the Flash
Device Flash Size Page Size PCWORD No. of
Pages
PCPAGE PCMSB
ATmega328/P 16K words
(32 KB)
64 words PC[5:0] 256 PC[13:6] 13
Table 31-10. No. of Words in a Page and No. of Pages in the EEPROM
Device EEPROM
Size
Page
Size
PCWORD No. of
Pages
PCPAGE EEAMSB
ATmega328/P 1 KB 4 bytes EEA[1:0] 256 EEA[9:2] 9
31.7 Parallel Programming Parameters, Pin Mapping, and Commands
This section describes how to parallel program and verify Flash program memory, EEPROM data
memory, Memory Lock bits, and Fuse bits in the device. Pulses are assumed to be at least 250 ns unless
otherwise noted.
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Memory Programming (MEMPROG)
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31.7.1 Signal Names
In this section, some pins of this device are referenced by signal names describing their functionality
during parallel programming. Refer to figure Parallel Programming and table Pin Name Mapping below.
Pins not described in the following table are referenced by pin names.
The XA1/XA0 pins determine the action executed when the XTAL1 pin is given a positive pulse. The bit
coding is shown in table XA1 and XA0 Coding below.
When pulsing WR or OE, the command loaded determines the action executed. The different commands
are shown in table Command Byte Bit Coding below.
Figure 31-1. Parallel Programming
VCC
GND
XTAL1
PD1
PD2
PD3
PD4
PD5
PD6
PC[1:0]:PB[5:0]
DATA
RESET
PD7
+12V
BS1
XA0
XA1
OE
RDY/BSY
PAGEL
PC2
WR
BS2
AVCC
+4.5 - 5.5V
+4.5 - 5.5V
Note: VCC - 0.3V < AVCC < VCC + 0.3V; however, AVCC should always be within 4.5 - 5.5V
Table 31-11. Pin Name Mapping
Signal Name in
Programming Mode
Pin Name I/O Function
RDY/BSY PD1 O 0: Device is busy programming, 1: Device is ready for
new command
OE PD2 I Output Enable (Active low)
WR PD3 I Write Pulse (Active low)
BS1 PD4 I Byte Select 1 (“0” selects Low byte, “1” selects High
byte)
XA0 PD5 I XTAL Action Bit 0
XA1 PD6 I XTAL Action Bit 1
PAGEL PD7 I Program memory and EEPROM Data Page Load
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Memory Programming (MEMPROG)
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Signal Name in
Programming Mode
Pin Name I/O Function
BS2 PC2 I Byte Select 2 (“0” selects Low byte, “1” selects 2’nd
High byte)
DATA {PC[1:0]: PB[5:0]} I/O Bi-directional Data bus (Output when OE is low)
Table 31-12. Pin Values Used to Enter Programming Mode
Pin Symbol Value
PAGEL Prog_enable[3] 0
XA1 Prog_enable[2] 0
XA0 Prog_enable[1] 0
BS1 Prog_enable[0] 0
Table 31-13. XA1 and XA0 Coding
XA1 XA0 Action When XTAL1 is Pulsed
0 0 Load Flash or EEPROM Address (High or low address byte determined by BS1)
0 1 Load Data (High or Low data byte for Flash determined by BS1)
1 0 Load Command
1 1 No Action, Idle
Table 31-14. Command Byte Bit Coding
Command Byte Command Executed
1000 0000 Chip Erase
0100 0000 Write Fuse bits
0010 0000 Write Lock bits
0001 0000 Write Flash
0001 0001 Write EEPROM
0000 1000 Read Signature Bytes and Calibration byte
0000 0100 Read Fuse and Lock bits
0000 0010 Read Flash
0000 0011 Read EEPROM
31.8 Parallel Programming
31.8.1 Entering Programming Mode
Follow the steps below to put the device in Parallel (High-voltage) Programming mode:
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Memory Programming (MEMPROG)
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1. Set the Prog_enable pins listed in the table Pin Values Used to Enter Programming Mode above to
“0x0000”, RESET pin to 0V and VCC to 0V.
2. Apply 4.5–5.5V between VCC and GND.
Ensure that VCC reaches at least 1.8V within the next 20 μs.
3. Wait for 20–60 μs, and apply 11.5–12.5V to RESET.
4. Keep the Prog_enable pins unchanged for at least 10 μs after the high voltage has been applied to
ensure the Prog_enable signature has been latched.
5. Wait at least 300 μs before giving any parallel programming commands.
6. Exit Programming mode by powering down the device or by bringing RESET pin to 0V.
If the rise time of VCC is unable to fulfill the requirements listed above, the following alternative method
can be used to put the device in Parallel (High-voltage) Programming mode:
1. Set the Prog_enable pins listed in the table Pin Values Used to Enter Programming Mode above to
“0000”, RESET pin to 0V and VCC to 0V.
2. Apply 4.5–5.5V between VCC and GND.
3. Monitor VCC, and as soon as VCC reaches 0.9–1.1V, apply 11.5–12.5V to RESET.
4. Keep the Prog_enable pins unchanged for at least 10 μs after the high voltage has been applied to
ensure the Prog_enable signature has been latched.
5. Wait until VCC reaches 4.5–5.5V before giving any parallel programming commands.
6. Exit Programming mode by powering down the device or by bringing RESET pin to 0V.
31.8.2 Considerations for Efficient Programming
The loaded command and address are retained in the device during programming. For efficient
programming, the following should be considered.
• The command needs only be loaded once when writing or reading multiple memory locations.
• Skip writing the data value 0xFF, that is the contents of the entire EEPROM (unless the EESAVE
fuse is programmed) and Flash after a chip erase.
• Address high byte needs only be loaded before programming or reading a new 256-word window in
Flash or 256 byte EEPROM. This consideration also applies to Signature bytes reading.
31.8.3 Chip Erase
The chip erase will erase the Flash, the SRAM and the EEPROM memories plus Lock bits. The Lock bits
are not Reset until the program memory has been completely erased. The Fuse bits are not changed. A
chip erase must be performed before the Flash and/or EEPROM are reprogrammed.
Note: The EEPROM memory is preserved during chip erase if the EESAVE fuse is programmed.
Load Command “Chip Erase”:
1. Set XA1, XA0 to “10”. This enables command loading.
2. Set BS1 to “0”.
3. Set DATA to “1000 0000”. This is the command for chip erase.
4. Give XTAL1 a positive pulse. This loads the command.
5. Give WR a negative pulse. This starts the chip erase. RDY/BSY goes low.
6. Wait until RDY/BSY goes high before loading a new command.
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Memory Programming (MEMPROG)
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31.8.4 Programming the Flash
The Flash is organized in pages as a number of words in a page and number of pages in the Flash.
When programming the Flash, the program data is latched into a page buffer. This allows one page of
program data to be programmed simultaneously. The following procedure describes how to program the
entire Flash memory:
Step A. Load Command “Write Flash”
1. Set XA1, XA0 to “10”. This enables command loading.
2. Set BS1 to “0”.
3. Set DATA to “0001 0000”. This is the command for write Flash.
4. Give XTAL1 a positive pulse. This loads the command.
Step B. Load Address Low Byte
1. Set XA1, XA0 to “00”. This enables address loading.
2. Set BS1 to “0”. This selects low address.
3. Set DATA = Address low byte (0x00 - 0xFF).
4. Give XTAL1 a positive pulse. This loads the address low byte.
Step C. Load Data Low Byte
1. Set XA1, XA0 to “01”. This enables data loading.
2. Set DATA = Data low byte (0x00 - 0xFF).
3. Give XTAL1 a positive pulse. This loads the data byte.
Step D. Load Data High Byte
1. Set BS1 to “1”. This selects high data byte.
2. Set XA1, XA0 to “01”. This enables data loading.
3. Set DATA = Data high byte (0x00 - 0xFF).
4. Give XTAL1 a positive pulse. This loads the data byte.
Step E. Latch Data
1. Set BS1 to “1”. This selects high data byte.
2. Give PAGEL a positive pulse. This latches the data bytes. (Refer to figure Programming the Flash
Waveforms, in this section, for signal waveforms.)
Step F. Repeat B Through E Until the Entire Buffer is Filled or Until All Data Within the Page is
Loaded
While the lower bits in the address are mapped to words within the page, the higher bits address the
pages within the FLASH. This is illustrated in the following figure, Addressing the Flash Which is
Organized in Pages, in this section. Note that if less than eight bits are required to address words in the
page (page size < 256), the most significant bit(s) in the address low byte are used to address the page
when performing a page write.
Step G. Load Address High Byte
1. Set XA1, XA0 to “00”. This enables address loading.
2. Set BS1 to “1”. This selects high address.
ATmega328/P
Memory Programming (MEMPROG)
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3. Set DATA = Address high byte (0x00 - 0xFF).
4. Give XTAL1 a positive pulse. This loads the address high byte.
Step H. Program Page
1. Give WR a negative pulse. This starts programming of the entire page of data. RDY/BSY goes low.
2. Wait until RDY/BSY goes high. (Refer to the figure, Programming the Flash Waveforms, in this
section for signal waveforms.)
Step I. Repeat B Through H Until the Entire Flash is Programmed or Until All Data Has Been
Programmed
Step J. End Page Programming
1. 1. Set XA1, XA0 to “10”. This enables command loading.
2. Set DATA to “0000 0000”. This is the command for no operation.
3. Give XTAL1 a positive pulse. This loads the command, and the internal write signals are Reset.
Figure 31-2. Addressing the Flash Which is Organized in Pages
PROGRAM MEMORY
WORD ADDRESS
WITHIN A PAGE
PAGE ADDRESS
WITHIN THE FLASH
INSTRUCTION WORD
PAGE PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
PAGE
PCWORDPCPAGE
PCMSB PAGEMSB
PROGRAM
COUNTER
Note: PCPAGE and PCWORD are listed in table No. of Words in a Page and No. of Pages in the Flash
in Page Size section.
ATmega328/P
Memory Programming (MEMPROG)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 354
Programming the Flash Waveforms
RDY/BSY
WR
OE
RESET+12V
PAGEL
BS2
0x10 ADDR. LOW ADDR. HIGH
DATA DATA LOW DATA HIGH ADDR. LOW DATA LOW DATA HIGH
XA1
XA0
BS1
XTAL1
XX XX XX
A B C D E B C D E G H
F
Note: “XX” is don’t care. The letters refer to the programming description above.
31.8.5 Programming the EEPROM
The EEPROM is organized in pages, refer to the table, number of words in a page and number of pages
in the EEPROM, in the page size section. When programming the EEPROM, the program data is latched
into a page buffer. This allows one page of data to be programmed simultaneously. The programming
algorithm for the EEPROM data memory is as follows (for details on Command, Address, and Data
loading, refer to Programming the Flash):
1. Step A: Load Command “0001 0001”.
2. Step G: Load Address High Byte (0x00 - 0xFF).
3. Step B: Load Address Low Byte (0x00 - 0xFF).
4. Step C: Load Data (0x00 - 0xFF).
5. Step E: Latch data (give PAGEL a positive pulse).
6. Step K: Repeat 3 through 5 until the entire buffer is filled.
7. Step L: Program EEPROM page.
7.1. Set BS1 to “0”.
7.2. Give WR a negative pulse. This starts programming of the EEPROM page. RDY/BSY goes
low.
7.3. Wait until to RDY/BSY goes high before programming the next page (refer to the following
figure for signal waveforms).
ATmega328/P
Memory Programming (MEMPROG)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 355
Figure 31-3. Programming the EEPROM Waveforms
RDY/BSY
WR
OE
RESET+12V
PAGEL
BS2
0x11 ADDR. HIGH
DATA ADDR. LOW DATA ADDR. LOW DATA XX
XA1
XA0
BS1
XTAL1
XX
A G B C E B C E L
K
31.8.6 Reading the Flash
The algorithm for reading the Flash memory is as follows (refer to Programming the Flash in this chapter
for details on Command and Address loading):
1. Step A: Load Command “0000 0010”.
2. Step G: Load Address High Byte (0x00 - 0xFF).
3. Step B: Load Address Low Byte (0x00 - 0xFF).
4. Set OE to “0”, and BS1 to “0”. The Flash word low byte can now be read at DATA.
5. Set BS1 to “1”. The Flash word high byte can now be read at DATA.
6. Set OE to “1”.
31.8.7 Reading the EEPROM
The algorithm for reading the EEPROM memory is as follows (refer to Programming the Flash for details
on Command and Address loading):
1. Step A: Load Command “0000 0011”.
2. Step G: Load Address High Byte (0x00 - 0xFF).
3. Step B: Load Address Low Byte (0x00 - 0xFF).
4. Set OE to “0”, and BS1 to “0”. The EEPROM Data byte can now be read at DATA.
5. Set OE to “1”.
31.8.8 Programming the Fuse Low Bits
The algorithm for programming the Fuse Low bits is as follows (refer to Programming the Flash for details
on Command and Data loading):
1. Step A: Load Command “0100 0000”.
2. Step C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.
3. Give WR a negative pulse and wait for RDY/BSY to go high.
31.8.9 Programming the Fuse High Bits
The algorithm for programming the Fuse High bits is as follows (refer to Programming the Flash for
details on Command and Data loading):
ATmega328/P
Memory Programming (MEMPROG)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 356
1. Step A: Load Command “0100 0000”.
2. Step C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.
3. Set BS1 to “1” and BS2 to “0”. This selects high data byte.
4. Give WR a negative pulse and wait for RDY/BSY to go high.
5. Set BS1 to “0”. This selects low data byte.
31.8.10 Programming the Extended Fuse Bits
The algorithm for programming the Extended Fuse bits is as follows (refer to Programming the Flash for
details on Command and Data loading):
1. Step A: Load Command “0100 0000”.
2. Step C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.
3. Set BS1 to “0” and BS2 to “1”. This selects extended data byte.
4. Give WR a negative pulse and wait for RDY/BSY to go high.
5. Set BS2 to “0”. This selects low data byte.
Figure 31-4. Programming the FUSES Waveforms
RDY/BSY
WR
OE
RESET +12V
PAGEL
0x40
DATA DATA XX
XA1
XA0
BS1
XTAL1
A C
0x40 DATA XX
A C
Write Fuse Low byte Write Fuse high byte
0x40 DATA XX
A C
Write Extended Fuse byte
BS2
31.8.11 Programming the Lock Bits
The algorithm for programming the Lock bits is as follows (refer to Programming the Flash for details on
command and data loading):
1. Step A: Load Command “0010 0000”.
2. Step C: Load Data Low Byte. Bit n = “0” programs the Lock bit. If LB mode 3 is programmed (LB1
and LB2 is programmed), it is not possible to program the Boot Lock bits by any External
Programming mode.
3. Give WR a negative pulse and wait for RDY/BSY to go high.
The Lock bits can only be cleared by executing chip erase.
ATmega328/P
Memory Programming (MEMPROG)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 357
31.8.12 Reading the Fuse and Lock Bits
The algorithm for reading the Fuse and Lock bits is as follows (refer to Programming the Flash for details
on Command loading):
1. Step A: Load Command “0000 0100”.
2. Set OE to “0”, BS2 to “0” and BS1 to “0”. The status of the Fuse Low bits can now be read at DATA
(“0” means programmed).
3. Set OE to “0”, BS2 to “1” and BS1 to “1”. The status of the Fuse High bits can now be read at DATA
(“0” means programmed).
4. Set OE to “0”, BS2 to “1”, and BS1 to “0”. The status of the Extended Fuse bits can now be read at
DATA (“0” means programmed).
5. Set OE to “0”, BS2 to “0” and BS1 to “1”. The status of the Lock bits can now be read at DATA (“0”
means programmed).
6. Set OE to “1”.
Figure 31-5. Mapping Between BS1, BS2 and the Fuse and Lock Bits During Read
Lock Bits 0
1
BS2
Fuse High Byte
0
1
BS1
DATA
Fuse Low Byte 0
1
BS2
Extended Fuse Byte
31.8.13 Reading the Signature Bytes
The algorithm for reading the Signature bytes is as follows (refer to Programming the Flash for details on
command and address loading):
1. Step A: Load Command “0000 1000”.
2. Step B: Load Address Low Byte (0x00 - 0x02).
3. Set OE to “0”, and BS1 to “0”. The selected Signature byte can now be read at DATA.
4. Set OE to “1”.
31.8.14 Reading the Calibration Byte
The algorithm for reading the Calibration byte is as follows (refer to Programming the Flash for details on
command and address loading):
1. Step A: Load Command “0000 1000”.
2. Step B: Load Address Low Byte, 0x00.
3. Set OE to “0”, and BS1 to “1”. The Calibration byte can now be read at DATA.
4. Set OE to “1”.
ATmega328/P
Memory Programming (MEMPROG)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 358
31.8.15 Parallel Programming Characteristics
For characteristics of the Parallel Programming, refer to Parallel Programming Characteristics.
Related Links
Parallel Programming Characteristics
31.9 Serial Downloading
Both the Flash and EEPROM memory arrays can be programmed using the serial SPI bus while RESET
is pulled to GND. The serial interface consists of pins SCK, MOSI (input) and MISO (output). After
RESET is set low, the programming enable instruction needs to be executed first before the program/
erase operations can be executed.
Figure 31-6. Serial Programming and Verify, VCC = 1.8 - 5.5V
VCC
GND
RESET
MOSI
MISO
SCK
XTAL1
PB5
PB6
PB7
+1.8 - 5.5V
AVCC
+1.8 - 5.5V (2)
Note:
1. If the device is clocked by the internal Oscillator, it is no need to connect a clock source to the
XTAL1 pin.
2. VCC - 0.3V < AVCC < VCC + 0.3V, however, AVCC should always be within the specified voltage
range (VCC) for the device.
When programming the EEPROM, an auto-erase cycle is built into the self-timed programming operation
(in the Serial mode ONLY) and there is no need to first execute the chip erase instruction. The chip erase
operation turns the content of every memory location in both the program and EEPROM arrays into 0xFF.
Depending on CKSEL fuses, a valid clock must be present. The minimum low and high periods for the
serial clock (SCK) input are defined as follows:
• Low: > 2 CPU clock cycles for fck < 12 MHz, 3 CPU clock cycles for fck ≥ 12 MHz
• High: > 2 CPU clock cycles for fck < 12 MHz, 3 CPU clock cycles for fck ≥ 12 MHz
ATmega328/P
Memory Programming (MEMPROG)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 359
31.9.1 Serial Programming Pin Mapping
Table 31-15. Pin Mapping Serial Programming
Symbol Pins I/O Description
MOSI PB3 I Serial Data in
MISO PB4 O Serial Data out
SCK PB5 I Serial Clock
Note: The pin mapping for SPI programming is listed. Not all parts use the SPI pins dedicated for the
internal SPI interface.
31.9.2 Serial Programming Algorithm
When writing serial data to the device, data is clocked on the rising edge of SCK.
When reading data from the device, data is clocked on the falling edge of SCK. Please refer to the figure,
serial programming waveforms in SPI serial programming characteristics section for timing details.
To program and verify the device in the Serial Programming mode, the following sequence is
recommended (See serial programming instruction set in Table 31-17:
1. Power-up sequence:
Apply power between VCC and GND while RESET and SCK are set to “0”. In some systems, the
programmer can not assure that SCK is held low during power-up. In this case, RESET must be
given a positive pulse of at least two CPU clock cycles duration after SCK has been set to “0”.
2. Wait for at least 20ms and enable serial programming by sending the programming enable serial
instruction to pin MOSI.
3. The serial programming instructions will not work if the communication is out of synchronization.
When in sync the second byte (0x53) will echo back when issuing the third byte of the programming
enable instruction. Whether the echo is correct or not, all four bytes of the instruction must be
transmitted. If the 0x53 did not echo back, give RESET a positive pulse and issue a new
programming enable command.
4. The Flash is programmed one page at a time. The memory page is loaded one byte at a time by
supplying the 6 LSB of the address and data together with the load program memory page
instruction. To ensure correct loading of the page, the data low byte must be loaded before data
high byte is applied for a given address. The program memory page is stored by loading the write
program memory page instruction with the 7 MSB of the address. If polling (RDY/BSY) is not used,
the user must wait at least tWD_FLASH before issuing the next page . Accessing the serial
programming interface before the Flash write operation completes can result in incorrect
programming.
5. A: The EEPROM array is programmed one byte at a time by supplying the address and data
together with the appropriate write instruction. An EEPROM memory location is first automatically
erased before new data is written. If polling (RDY/BSY) is not used, the user must wait at least
tWD_EEPROM before issuing the next byte. In a chip erased device, no 0xFFs in the data file(s) need
to be programmed.
B: The EEPROM array is programmed one page at a time. The memory page is loaded one byte at
a time by supplying the 6 LSB of the address and data together with the Load EEPROM memory
page instruction. The EEPROM memory page is stored by loading the Write EEPROM memory
page instruction with the 7 MSB of the address. When using EEPROM page access only byte
locations loaded with the load EEPROM memory page instruction is altered. The remaining
ATmega328/P
Memory Programming (MEMPROG)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 360
locations remain unchanged. If polling (RDY/BSY) is not used, the used must wait at least
tWD_EEPROM before issuing the next byte. In a chip erased device, no 0xFF in the data file(s) need
to be programmed.
6. Any memory location can be verified by using the read instruction which returns the content at the
selected address at serial output MISO.
7. At the end of the programming session, RESET can be set high to commence normal operation.
8. Power-off sequence (if needed):
Set RESET to “1”.
Turn VCC power off.
Table 31-16. Typical Wait Delay Before Writing the Next Flash or EEPROM Location
Symbol Minimum Wait Delay
tWD_FLASH 4.5 ms
tWD_EEPROM 3.6 ms
tWD_ERASE 10.5 ms
tWD_FUSE 4.5 ms
31.9.3 Serial Programming Instruction Set
This section describes the instruction set.
Table 31-17. Serial Programming Instruction Set (Hexadecimal Values)
Instruction/Operation Instruction Format
Byte 1 Byte 2 Byte 3 Byte 4
Programming Enable 0xAC 0x53 0x00 0x00
Chip Erase (Program Memory/EEPROM) 0xAC 0x80 0x00 0x00
Poll RDY/BSY 0xF0 0x00 0x00 data byte out
Load Instructions
Load Extended Address byte(1) 0x4D 0x00 Extended adr 0x00
Load Program Memory Page, High byte 0x48 0x00 adr LSB high data byte in
Load Program Memory Page, Low byte 0x40 0x00 adr LSB low data byte in
Load EEPROM Memory Page (page access) 0xC1 0x00 0000 000aa(2) data byte in
Read Instructions(5)
Read Program Memory, High byte 0x28 adr MSB adr LSB high data byte out
Read Program Memory, Low byte 0x20 adr MSB adr LSB low data byte out
Read EEPROM Memory 0xA0 0000 00aa(2) aaaa aaaa(2) data byte out
Read Lock bits(3) 0x58 0x00 0x00 data byte out
Read Signature Byte 0x30 0x00 0000 000aa(2) data byte out
Read Fuse bits(3) 0x50 0x00 0x00 data byte out
Read Fuse High bits(3) 0x58 0x08 0x00 data byte out
Read Extended Fuse Bits(3) 0x50 0x08 0x00 data byte out
Read Calibration Byte 0x38 0x00 0x00 data byte out
Write Instructions(5)
ATmega328/P
Memory Programming (MEMPROG)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 361
Instruction/Operation Instruction Format
Byte 1 Byte 2 Byte 3 Byte 4
Write Program Memory Page(6) 0x4C adr MSB(8) adr LSB(8) 0x00
Write EEPROM Memory 0xC0 0000 00aa(2) aaaa aaaa(2) data byte in
Write EEPROM Memory Page (page access) 0xC2 0000 00aa(2) aaaa aa00(2) 0x00
Write Lock bits(3)(4) 0xAC 0xE0 0x00 data byte in
Write Fuse bits(3)(4) 0xAC 0xA0 0x00 data byte in
Write Fuse High bits(3)(4) 0xAC 0xA8 0x00 data byte in
Write Extended Fuse Bits(3)(4) 0xAC 0xA4 0x00 data byte in
Note:
1. Not all instructions are applicable for all parts.
2. a = address.
3. Bits are programmed ‘0’, unprogrammed ‘1’.
4. To ensure future compatibility, unused Fuses and Lock bits should be unprogrammed (‘1’).
5. Refer to the corresponding section for Fuse and Lock bits, Calibration and Signature bytes and
page size.
6. Instructions accessing program memory use a word address. This address may be random within
the page range.
Note: See http://www.microchip.com/design-centers/8-bit/microchip-avr-mcus for application notes
regarding programming and programmers.
If the LSB in RDY/BSY data byte out is ‘1’, a programming operation is still pending. Wait until this bit
returns ‘0’ before the next instruction is carried out.
Within the same page, the low data byte must be loaded prior to the high data byte.
After data is loaded into the page buffer, program the EEPROM page. Refer to the following figure.
Within the same moisture group, the user should not configure all the sensors to the single multi-touch
group.
ATmega328/P
Memory Programming (MEMPROG)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 362
Figure 31-7. Serial Programming Instruction Example
Byte 1 Byte 2 Byte 3 Byte 4
Adr MSB Adr LSB
Bit 15 B 0
Serial Programming Instruction
Program Memory/
EEPROM Memory
Page 0
Page 1
Page 2
Page N-1
Page Buffer
Write Program Memory Page/
Write EEPROM Memory Page
Load Program Memory Page (High/Low Byte)/
Load EEPROM Memory Page (page access)
Byte 1 Byte 2 Byte 3 Byte 4
Bit 15 B 0
Adr MSB Adr LSB
Page Offset
Page Number
31.9.4 SPI Serial Programming Characteristics
Figure 31-8. Serial Programming Waveforms
MSB
MSB
LSB
LSB
SERIAL CLOCK INPUT
(SCK)
SERIAL DATA INPUT
(MOSI)
(MISO)
SAMPLE
SERIAL DATA OUTPUT
ATmega328/P
Memory Programming (MEMPROG)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 363
32. Electrical Characteristics
32.1 Absolute Maximum Ratings
Table 32-1. Absolute Maximum Ratings
Operating Temperature -55°C to +125°C
Storage Temperature -65°C to +150°C
Voltage on any Pin except RESET
with respect to Ground
-0.5V to VCC+0.5V
Voltage on RESET
with respect to Ground
-0.5V to +13.0V
Maximum Operating Voltage 6.0V
DC Current per I/O Pin 40.0mA
DC Current VCC and GND Pins 200.0mA
Note: Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage
to the device. This is a stress rating only and functional operation of the device at these or other
conditions beyond those indicated in the operational sections of this specification is not implied. Exposure
to absolute maximum rating conditions for extended periods may affect device reliability.
32.2 Common DC Characteristics
Table 32-2. Common DC characteristics TA = -40°C to 105°C, VCC = 1.8V to 5.5V (unless otherwise noted)
Symbol Parameter Condition Min. Typ. Max. Units
VIL Input Low Voltage, except XTAL1 and
RESET pin
VCC = 1.8V - 2.4V -0.5 0.2VCC(1) V
VCC = 2.4V - 5.5V -0.5 0.3VCC(1)
VIH Input High Voltage, except XTAL1 and
RESET pins
VCC = 1.8V - 2.4V 0.7VCC(2) VCC + 0.5 V
VCC = 2.4V - 5.5V 0.6VCC(2) VCC + 0.5
VIL1 Input Low Voltage,
XTAL1 pin
VCC = 1.8V - 5.5V -0.5 0.1VCC(1) V
VIH1 Input High Voltage,
XTAL1 pin
VCC = 1.8V - 2.4V 0.8VCC(2) VCC + 0.5 V
VCC = 2.4V - 5.5V 0.7VCC(2) VCC + 0.5
VIL2 Input Low Voltage, RESET pin VCC = 1.8V - 5.5V -0.5 0.1VCC(1) V
VIH2 Input High Voltage, RESET pin VCC = 1.8V - 5.5V 0.9VCC(2) VCC + 0.5 V
VIL3 Input Low Voltage,
RESET pin as I/O
VCC = 1.8V - 2.4V -0.5 0.2VCC(1) V
VCC = 2.4V - 5.5V -0.5 0.3VCC(1)
ATmega328/P
Electrical Characteristics
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 364
Symbol Parameter Condition Min. Typ. Max. Units
VIH3 Input High Voltage,
RESET pin as I/O
VCC = 1.8V - 2.4V 0.7VCC(2) VCC + 0.5 V
VCC = 2.4V - 5.5V 0.6VCC(2) VCC + 0.5
VOL Output Low Voltage(4)
except RESET pin IOL = 20mA,
VCC = 5V
TA=85°C 0.9 V
TA=105°C(5) 1.0 V
IOL = 10mA,
VCC = 3V
TA=85°C 0.6 V
TA=105°C(5) 0.7 V
VOH Output High Voltage(3)
except Reset pin IOH = -20mA,
VCC = 5V
TA=85°C 4.2 V
TA=105°C(5) 4.1 V
IOH = -10mA,
VCC = 3V
TA=85°C 2.3 V
TA=105°C(5) 2.1 V
IIL Input Leakage
Current I/O Pin
VCC = 5.5V, pin low
(absolute value)
1 μA
IIH Input Leakage
Current I/O Pin
VCC = 5.5V, pin high
(absolute value)
1 μA
RRST Reset Pull-up Resistor 30 60 kΩ
RPU I/O Pin Pull-up Resistor 20 50 kΩ
VACIO Analog Comparator
Input Offset Voltage
VCC = 5V
Vin = VCC/2
<10 40 mV
IACLK Analog Comparator
Input Leakage Current
VCC = 5V
Vin = VCC/2
-50 50 nA
tACPD Analog Comparator
Propagation Delay
VCC = 2.7V
VCC = 4.0V
750
500
ns
Note:
1. “Max.” means the highest value where the pin is guaranteed to be read as low.
2. “Min.” means the lowest value where the pin is guaranteed to be read as high.
3. Although each I/O port can source more than the test conditions (20mA at VCC = 5V, 10mA at VCC
= 3V) under steady state conditions (non-transient), the following must be observed:
3.1. The sum of all IOH, for ports C0 - C5, D0- D4, ADC7, RESET should not exceed 100mA.
3.2. The sum of all IOH, for ports B0 - B5, D5 - D7, ADC6, XTAL1, XTAL2 should not exceed
100mA.
ATmega328/P
Electrical Characteristics
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 365
If IIOH exceeds the test condition, VOH may exceed the related specification. Pins are not
guaranteed to source current greater than the listed test condition.
4. Although each I/O port can sink more than the test conditions (20mA at VCC = 5V, 10mA at VCC =
3V) under steady state conditions (non-transient), the following must be observed:
4.1. The sum of all IOL, for ports C0 - C5, ADC7, ADC6 should not exceed 100mA.
4.2. The sum of all IOL, for ports B0 - B5, D5 - D7, XTAL1, XTAL2 should not exceed 100mA.
4.3. The sum of all IOL, for ports D0 - D4, RESET should not exceed 100mA.
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed
to sink current greater than the listed test condition.
5. Only for ATmega328P
Related Links
Minimizing Power Consumption
32.2.1 ATmega328 DC Characteristics – Current Consumption
Table 32-3. DC characteristics - TA = -40°C to 85°C, VCC = 1.8V to 5.5V (unless otherwise noted)
Symbol Parameter Condition Min. Typ.(2) Max. Units
ICC Power Supply Current(1) Active 1MHz, VCC = 2V T = 85°C 0.3 0.5 mA
Active 4MHz, VCC = 3V T = 85°C 1.7 3.5
Active 8MHz, VCC = 5V T = 85°C 5.2 12
Idle 1MHz, VCC = 2V T = 85°C 0.04 0.5
Idle 4MHz, VCC = 3V T = 85°C 0.3 1.5
Idle 8MHz,VCC = 5V T = 85°C 1.2 5.5
Power-save mode(3) 32kHz TOSC enabled,
VCC = 1.8V
T = 85°C 0.8 μA
32kHz TOSC enabled,
VCC = 3V
T = 85°C 0.9
Power-down mode(3) WDT enabled, VCC = 3V T = 85°C 4.2 15
WDT disabled, VCC = 3V T = 85°C 0.1 2
Note:
1. Values with Minimizing Power Consumption enabled (0xFF).
2. Typical values at 25°C. Maximum values are test limits in production.
3. The current consumption values include input leakage current.
32.2.2 ATmega328P DC Characteristics – Current Consumption
Table 32-4. ATmega328P DC characteristics - TA = -40°C to 85/105°C, VCC = 1.8V to 5.5V (unless otherwise
noted)
Symbol Parameter Condition Min. Typ.(2) Max. Units
ICC Power Supply Current(1) Active 1MHz, VCC = 2V T = 85°C 0.3 0.5 mA
T = 105°C 0.3 0.5
ATmega328/P
Electrical Characteristics
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 366
Symbol Parameter Condition Min. Typ.(2) Max. Units
Active 4MHz, VCC = 3V T = 85°C 1.7 2.5
T = 105°C 1.7 2.5
Active 8MHz, VCC = 5V T = 85°C 5.2 9.0
T = 105°C 5.2 9.0
Idle 1MHz, VCC = 2V T = 85°C 0.04 0.15
T = 105°C 0.04 0.15
Idle 4MHz, VCC = 3V T = 85°C 0.3 0.7
T = 105°C 0.3 0.7
Idle 8MHz,VCC = 5V T = 85°C 1.2 2.7
T = 105°C 1.2 2.7
Power-save mode(3) 32kHz TOSC enabled,
VCC = 1.8V
T = 85°C 0.8 μA
T = 105°C 0.8
32kHz TOSC enabled,
VCC = 3V
T = 85°C 0.9
T = 105°C 0.9
Power-down mode(3)(4) WDT enabled, VCC = 3V T = 85°C 4.2 8
T = 105°C 4.2 10
WDT disabled, VCC = 3V T = 85°C 0.1 2
T = 105°C 0.1 5
Note:
1. Values with Minimizing Power Consumption enabled (0xFF).
2. Typical values at 25°C. Maximum values are test limits in production.
3. The current consumption values include input leakage current.
4. No clock is applied to the pad during power-down mode.
32.3 Speed Grades
Maximum frequency is dependent on VCC. As shown in the figure Maximum Frequency vs. VCC, where
the curve is linear between 1.8V < VCC < 2.7V and between 2.7V < VCC < 4.5V.
ATmega328/P
Electrical Characteristics
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 367
Figure 32-1. Maximum Frequency vs. VCC
4MHz
1.8V 2.7V 4.5V
10MHz
20MHz
5.5V
Safe Operating Area
32.4 Clock Characteristics
Related Links
Calibrated Internal RC Oscillator
OSCCAL
32.4.1 Calibrated Internal RC Oscillator Accuracy
Table 32-5. Calibration Accuracy of Internal RC Oscillator
Frequency VCC Temperature Calibration Accuracy
Factory
Calibration
8.0 MHz 3.0V 25°C ±10%
User
Calibration Fixed frequency within:
7.3 - 8.1 MHz
Fixed voltage within:
1.8V - 5.5V
Fixed temperature within:
-40°C to - 85°C
±1%
Related Links
OSCCAL
32.4.2 External Clock Drive Waveforms
Figure 32-2. External Clock Drive Waveforms
VIL1
VIH1
Related Links
OSCCAL
ATmega328/P
Electrical Characteristics
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 368
32.4.3 External Clock Drive
Table 32-6. External Clock Drive
Symbol Parameter VCC= 1.8 - 5.5V VCC= 2.7 - 5.5V VCC= 4.5 - 5.5V Units
Min. Max. Min. Max. Min. Max.
1/tCLCL Oscillator Frequency 0 4 0 10 0 20 MHz
tCLCL Clock Period 250 - 100 - 50 - ns
tCHCX High Time 100 - 40 - 20 - ns
tCLCX Low Time 100 - 40 - 20 - ns
tCLCH Rise Time - 2.0 - 1.6 - 0.5 μs
tCHCL Fall Time - 2.0 - 1.6 - 0.5 μs
ΔtCLCL Change in period from one clock
cycle to the next
- 2 - 2 - 2 %
Related Links
OSCCAL
32.5 System and Reset Characteristics
Table 32-7. Reset, Brown-out and Internal Voltage Characteristics(1)
Symbol Parameter Condition Min. Typ Max Units
VPOT Power-on Reset Threshold Voltage (rising) 1.1 1.5 1.7 V
Power-on Reset Threshold Voltage (falling)(2) 0.6 1.0 1.7 V
SRON Power-on Slope Rate 0.01 - 10 V/ms
VRST RESET Pin Threshold Voltage 0.2 VCC - 0.9 VCC V
tRST Minimum pulse width on RESET Pin - - 2.5 μs
VHYST Brown-out Detector Hysteresis - 50 - mV
tBOD Min. Pulse Width on Brown-out Reset - 2 - μs
VBG Bandgap reference voltage VCC=2.7
TA=25°C
1.0 1.1 1.2 V
tBG Bandgap reference start-up time VCC=2.7
TA=25°C
- 40 70 μs
IBG Bandgap reference current consumption VCC=2.7
TA=25°C
- 10 - μA
Note:
1. Values are guidelines only.
2. The Power-on Reset will not work unless the supply voltage has been below VPOT (falling)
ATmega328/P
Electrical Characteristics
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 369
Table 32-8. BODLEVEL Fuse Coding(1)(2)
BODLEVEL [2:0] Fuses Min. VBOT Typ. VBOT Max VBOT Units
111 BOD Disabled
110 1.7 1.8 2.0 V
101 2.5 2.7 2.9
100 4.1 4.3 4.5
011 - 000 Reserved
Note: VBOT may be below nominal minimum operating voltage for some devices. For devices where this
is the case, the device is tested down to VCC = VBOT during the production test. This assures that a
Brown-Out Reset will occur before VCC drops to a voltage where correct operation of the microcontroller
is no longer certain. The test is performed using BODLEVEL = 110, 101 and 100.
Note: VBOT tested at 25°C and 85°C in production
32.6 SPI Timing Characteristics
Table 32-9. SPI Timing Parameters
Description Mode Min. Typ Max Units
1 SCK period Master - See Table. Relationship Between SCK and the Oscillator
Frequency in "SPCR – SPI Control Register"
- ns
2 SCK high/low Master - 50% duty cycle -
3 Rise/Fall time Master - 3.6 -
4 Setup Master - 10 -
5 Hold Master - 10 -
6 Out to SCK Master - 0.5 • tsck -
7 SCK to out Master - 10 -
8 SCK to out high Master - 10 -
9 SS low to out Slave - 15 -
10 SCK period Slave 4 • tck - -
11 SCK high/low(1) Slave 2 • tck - -
12 Rise/Fall time Slave - - 1600
13 Setup Slave 10 - -
14 Hold Slave tck - -
15 SCK to out Slave - 15 -
16 SCK to SS high Slave 20 - -
ATmega328/P
Electrical Characteristics
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 370
Description Mode Min. Typ Max Units
17 SS high to tri-state Slave 10 -
18 SS low to SCK Slave 2 • tck - -
Note: In SPI Programming mode the minimum SCK high/low period is:
• 2 • tCLCLCL for fCK < 12MHz
• 3 • tCLCL for fCK > 12MHz
Figure 32-3. SPI Interface Timing Requirements (Master Mode)
MOSI
(Data Output)
SCK
(CPOL = 1)
MISO
(Data Input)
SCK
(CPOL = 0)
SS
MSB LSB
LSBMSB
...
...
6 1
2 2
34 5
8
7
Figure 32-4. SPI Interface Timing Requirements (Slave Mode)
MISO
(Data Output)
SCK
(CPOL = 1)
MOSI
(Data Input)
SCK
(CPOL = 0)
SS
MSB LSB
LSBMSB
...
...
10
11 11
1213 14
17
15
9
X
16
32.7 Two-Wire Serial Interface Characteristics
Table in this section describes the requirements for devices connected to the two-wire serial bus. The
two-wire serial interface meets or exceeds these requirements under the noted conditions.
Timing symbols refer to Figure 32-5.
ATmega328/P
Electrical Characteristics
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 371
Table 32-10. Two-Wire Serial Bus Requirements
Symbol Parameter Condition Min. Max Units
VIL Input Low-voltage -0.5 0.3 VCC V
VIH Input High-voltage 0.7 VCC VCC + 0.5 V
Vhys(1) Hysteresis of Schmitt Trigger
Inputs
0.05 VCC(2) – V
VOL(1) Output Low-voltage 3mA sink current 0 0.4 V
tr(1) Rise Time for both SDA and
SCL
20 + 0.1Cb(3)(2) 300 ns
tof(1) Output Fall Time from VIHmin to
VILmax
10 pF < Cb < 400 pF(3) 20 + 0.1Cb(3)(2) 250 ns
tSP(1) Spikes Suppressed by Input
Filter
0 50(2) ns
IiInput Current each I/O Pin 0.1VCC < Vi < 0.9VCC -10 10 μA
Ci(1) Capacitance for each I/O Pin – 10 pF
fSCL SCL Clock Frequency fCK(4) > max(16fSCL, 250
kHz)(5)
0 400 kHz
Rp Value of Pull-up resistor fSCL ≤ 100 kHz CC 0.4V
3mA
1000ns
fSCL > 100 kHz CC 0.4V
3mA
300ns
tHD;STA Hold Time (repeated) START
Condition
fSCL ≤ 100 kHz 4.0 – μs
fSCL > 100 kHz 0.6 – μs
tLOW Low Period of the SCL Clock fSCL ≤ 100 kHz 4.7 – μs
fSCL > 100 kHz 1.3 – μs
tHIGH High period of the SCL clock fSCL ≤ 100 kHz 4.0 – μs
fSCL > 100 kHz 0.6 – μs
tSU;STA Setup time for a repeated
START condition
fSCL ≤ 100 kHz 4.7 – μs
fSCL > 100 kHz 0.6 – μs
tHD;DAT Data hold time fSCL ≤ 100 kHz 0 3.45 μs
fSCL > 100 kHz 0 0.9 μs
tSU;DAT Data setup time fSCL ≤ 100 kHz 250 – ns
fSCL > 100 kHz 100 – ns
tSU;STO Setup time for STOP condition fSCL ≤ 100 kHz 4.0 – μs
fSCL > 100 kHz 0.6 – μs
ATmega328/P
Electrical Characteristics
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 372
Symbol Parameter Condition Min. Max Units
tBUF Bus free time between a STOP
and START condition
fSCL ≤ 100 kHz 4.7 – μs
fSCL > 100 kHz 1.3 – μs
Note:
1. This parameter is characterized and not 100% tested.
2. Required only for fSCL > 100 kHz.
3. Cb = capacitance of one bus line in pF.
4. fCK = CPU clock frequency.
5. This requirement applies to all two-wire serial interface operation. Other devices connected to the
two-wire serial bus need only obey the general fSCL requirement.
Figure 32-5. Two-Wire Serial Bus Timing
tSU;STA
tLOW
tHIGH
tLOW
tof
tHD;STA tHD;DAT tSU;DAT tSU;STO
tBUF
SCL
SDA
tr
32.8 ADC Characteristics
Table 32-11. ADC Characteristics
Symbol Parameter Condition Min. Typ Max Units
Resolution - 10 - Bits
Absolute accuracy (Including
INL, DNL, quantization error,
gain and offset error)
VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
- 2 - LSB
VREF = 4V, VCC = 4V,
ADC clock = 1 MHz
- 4 - LSB
VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
Noise Reduction Mode
- 2 - LSB
VREF = 4V, VCC = 4V,
ADC clock = 1 MHz
Noise Reduction Mode
- 4 - LSB
Integral Non-Linearity (INL) VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
- 0.5 - LSB
Differential Non-Linearity (DNL) VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
- 0.25 - LSB
ATmega328/P
Electrical Characteristics
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 373
Symbol Parameter Condition Min. Typ Max Units
Gain Error VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
- 2 - LSB
Offset Error VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
- 2 - LSB
Conversion Time Free Running Conversion 13 - 260 μs
Clock Frequency 50 - 1000 kHz
AVCC(1) Analog Supply Voltage VCC - 0.3 - VCC + 0.3 V
VREF Reference Voltage 1.0 - AVCC V
VIN Input Voltage GND - VREF V
Input Bandwidth - 38.5 kHz
VINT Internal Voltage Reference 1.0 1.1 1.2 V
RREF Reference Input Resistance - 50 - kΩ
RAIN Analog Input Resistance - 100 - MΩ
Note:
1. AVCC absolute min./max: 1.8V/5.5V
32.9 Parallel Programming Characteristics
Table 32-12. Parallel Programming Characteristics, VCC = 5V ± 10%
Symbol Parameter Min. Max Units
VPP Programming Enable Voltage 11.5 12.5 V
IPP Programming Enable Current - 250 μA
tDVXH Data and Control Valid before XTAL1 High 67 - ns
tXLXH XTAL1 Low to XTAL1 High 200 - ns
tXHXL XTAL1 Pulse Width High 150 - ns
tXLDX Data and Control Hold after XTAL1 Low 67 - ns
tXLWL XTAL1 Low to WR Low 0 - ns
tXLPH XTAL1 Low to PAGEL high 0 - ns
tPLXH PAGEL low to XTAL1 high 150 - ns
tBVPH BS1 Valid before PAGEL High 67 - ns
tPHPL PAGEL Pulse Width High 150 - ns
tPLBX BS1 Hold after PAGEL Low 67 - ns
tWLBX BS2/1 Hold after RDY/BSY high 67 - ns
ATmega328/P
Electrical Characteristics
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 374
Symbol Parameter Min. Max Units
tPLWL PAGEL Low to WR Low 67 - ns
tBVWL BS1 Valid to WR Low 67 - ns
tWLWH WR Pulse Width Low 150 - ns
tWLRL WR Low to RDY/BSY Low 0 1 μs
tWLRH WR Low to RDY/BSY High(1) 3.2 3.4 ms
tWLRH_CE WR Low to RDY/BSY High for Chip Erase(2) 9.8 10.5 ms
tXLOL XTAL1 Low to OE Low 0 - ns
tBVDV BS1 Valid to DATA valid 0 350 ns
tOLDV OE Low to DATA Valid - 350 ns
tOHDZ OE High to DATA Tri-stated - 250 ns
Note:
1. tWLRH is valid for the Write Flash, Write EEPROM, Write Fuse bits and Write Lock bits commands.
2. tWLRH_CE is valid for the Chip Erase command.
Figure 32-6. Parallel Programming Timing, Including some General Timing Requirements
Data & Contol
(DATA, XA0/1, BS1, BS2)
XTAL1
tXHXL
tWLWH
tDVXH tXLDX
tPLWL
tWLRH
WR
RDY/BSY
PAGEL
tPHPL
tPLBX
tBVPH
tXLWL
tWLBX
tBVWL
WLRL
Figure 32-7. Parallel Programming Timing, Loading Sequence with Timing Requirements
XTAL1
PAGEL
tPLXH
XLXH
ttXLPH
ADDR0 (Low Byte) DATA (Low Byte) DATA (High Byte) ADDR1 (Low Byte)
DATA
BS1
XA0
XA1
LOAD ADDRESS
(LOW BYTE)
LOAD DATA
(LOW BYTE)
LOAD DATA
(HIGH BYTE)
LOAD DATA
LOAD ADDRESS
(LOW BYTE)
ATmega328/P
Electrical Characteristics
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 375
Note: The timing requirements shown in Parallel Programming Characteristics (i.e., tDVXH, tXHXL, and
tXLDX) also apply to loading operation
Figure 32-8. Parallel Programming Timing, Reading Sequence (within the Same Page) with Timing
Requirements
XTAL1
OE
ADDR0 (Low Byte) DATA (Low Byte) DATA (High Byte) ADDR1 (Low Byte)
DATA
BS1
XA0
XA1
LOAD ADDRESS
(LOW BYTE)
READ DATA
(LOW BYTE)
READ DATA
(HIGH BYTE)
LOAD ADDRESS
(LOW BYTE)
tBVDV
tOLDV
tXLOL
tOHDZ
Note: The timing requirements shown in Parallel Programming Characteristics (i.e., tDVXH, tXHXL, and
tXLDX) also apply to reading operation.
ATmega328/P
Electrical Characteristics
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 376
33. Typical Characteristics (TA = -40°C to 85°C)
The following charts show typical behavior. These figures are not tested during manufacturing. All current
consumption measurements are performed with all I/O pins configured as inputs and with internal pull-
ups enabled. A sine wave generator with rail-to-rail output is used as clock source.
The power consumption in Power-down mode is independent of clock selection.
The current consumption is a function of several factors such as: operating voltage, operating frequency,
loading of I/O pins, switching rate of I/O pins, code executed and ambient temperature. The dominating
factors are operating voltage and frequency.
The current drawn from capacitive loaded pins may be estimated (for one pin) as CL*VCC*f where CL =
load capacitance, VCC = operating voltage and f = average switching frequency of I/O pin.
The parts are characterized at frequencies higher than test limits. Parts are not guaranteed to function
properly at frequencies higher than the ordering code indicates.
The difference between current consumption in Power-down mode with Watchdog Timer enabled and
Power-down mode with Watchdog Timer disabled represents the differential current drawn by the
Watchdog Timer.
33.1 ATmega328 Typical Characteristics
33.1.1 Active Supply Current
Figure 33-1. ATmega328: Active Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
5.5 V
5.0 V
4.5 V
4.0 V
3.3 V
2.7 V
1.8 V
0
0.2
0.4
0.6
0.8
1
1.2
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Frequency (MHz)
I
CC
(mA)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 377
Figure 33-2. ATmega328: Active Supply Current vs. Frequency (1MHz - 20MHz)
5.5V
5.0V
4.5V
0
2
4
6
8
10
12
14
0 2 4 6 8 10 12 14 16 18 20
Frequency (MHz)
I
CC
(mA)
1.8 V
2.7 V
3.3 V
4.0 V
Figure 33-3. Active Supply Current vs. VCC (Internal RC Oscillator, 128kHz)
85 °C
25 °C
-40 °C
0
0.04
0.08
0.12
0.16
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 378
Figure 33-4. Active Supply Current vs. VCC (Internal RC Oscillator, 1MHz)
85 °C
25 °C
-40 °C
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
ICC (mA)
Figure 33-5. Active Supply Current vs. VCC (Internal RC Oscillator, 8MHz)
85 °C
25 °C
-40 °C
0
1
2
3
4
5
6
7
8
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 379
33.1.2 Idle Supply Current
Figure 33-6. ATmega328: Idle Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
5.5 V
5.0 V
4.5 V
4.0 V
3.3 V
2.7 V
1.8 V
0
0.04
0.08
0.12
0.16
0.2
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Frequency (MHz)
I
CC
(mA)
Figure 33-7. ATmega328: Idle Supply Current vs. Frequency (1MHz - 20MHz)
5.5 V
5.0 V
4.5 V
0
0.5
1
1.5
2
2.5
3
3.5
4
0 2 4 6 8 10 12 14 16 18 20
Frequency (MHz)
I
CC
(mA)
1.8 V
2.7 V
3.3 V
4.0 V
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 380
Figure 33-8. ATmega328: Idle Supply Current vs. VCC (Internal RC Oscillator, 128kHz)
CC
85 °C
25 °C
-40 °C
0
0.01
0.02
0.03
0.04
0.05
0.06
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
Figure 33-9. ATmega328: Idle Supply Current vs. VCC (Internal RC Oscillator, 1MHz)
85 °C
25 °C
-40 °C
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 381
Figure 33-10. ATmega328: Idle Supply Current vs. VCC (Internal RC Oscillator, 8MHz)
85 °C
25 °C
-40 °C
0
0.4
0.8
1.2
1.6
2
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
33.1.3 Supply Current of IO Modules
The tables and formulas below can be used to calculate the additional current consumption for the
different I/O modules in Active and Idle mode. The enabling or disabling of the I/O modules are controlled
by the Power Reduction Register. See “Power Reduction Register” for details.
Table 33-1. ATmega328: Additional Current Consumption for the different I/O modules (absolute
values)
PRR bit Typical numbers (µA)
VCC = 2V, F = 1MHz VCC = 3V, F = 4MHz VCC = 5V, F = 8MHz
PRUSART0 3.20 22.17 100.25
PRTWI 7.34 46.55 199.25
PRTIM2 7.34 50.79 224.25
PRTIM1 6.19 41.25 176.25
PRTIM0 1.89 14.28 61.13
PRSPI 6.94 43.84 186.50
PRADC 8.66 61.80 295.38
Table 33-2. ATmega328: Additional Current Consumption (percentage) in Active and Idle mode
PRR bit Additional Current consumption
compared to Active with external clock
(see Figure 33-1 and Figure 33-2)
Additional Current consumption
compared to Idle with external clock (see
Figure 33-6 and Figure 33-7)
PRUSART0 1.4% 7.8%
PRTWI 3.0% 16.6%
PRTIM2 3.3% 17.8%
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 382
PRR bit Additional Current consumption
compared to Active with external clock
(see Figure 33-1 and Figure 33-2)
Additional Current consumption
compared to Idle with external clock (see
Figure 33-6 and Figure 33-7)
PRTIM1 2.7% 14.5%
PRTIM0 0.9% 4.8%
PRSPI 2.9% 15.7%
PRADC 4.1% 22.1%
It is possible to calculate the typical current consumption based on the numbers from the above table for
other VCC and frequency settings.
Related Links
PRR
33.1.3.1 Example
Calculate the expected current consumption in idle mode with TIMER1, ADC, and SPI enabled at VCC =
2.0V and F = 1MHz. From Table Additional Current Consumption (percentage) in Active and Idle mode in
the previous section, third column, we see that we need to add 14.5% for the TIMER1, 22.1% for the
ADC, and 15.7% for the SPI module. Reading from Figure Idle Supply Current vs. Low Frequency
(0.1-1.0MHz), we find that the idle current consumption is ~0.045mA at VCC = 2.0V and F = 1MHz. The
total current consumption in idle mode with TIMER1, ADC, and SPI enabled, gives:
ICCtotal 0.045mA(1+0.145+0.221+0.157) 0.069mA
33.1.4 Power-down Supply Current
Figure 33-11. ATmega328: Power-Down Supply Current vs. VCC (Watchdog Timer Disabled)
CC
85 °C
25 °C
-40 °C
0
0.2
0.4
0.6
0.8
1
1.2
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 383
Figure 33-12. ATmega328: Power-Down Supply Current vs. VCC (Watchdog Timer Enabled)
85 °C
25 °C
-40 °C
0
1
2
3
4
5
6
7
8
9
10
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
33.1.5 Power-save Supply Current
Figure 33-13. ATmega328: Power-Save Supply Current vs. VCC (Watchdog Timer Disabled and
32kHz Crystal Oscillator Running)
25 °C
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 384
33.1.6 Standby Supply Current
Figure 33-14. ATmega328: Standby Supply Current vs. Vcc (Watchdog Timer Disabled)
6MHz_xtal
6MHz _r es
4MHz_xtal
4MHz _r es
2MHz_xtal
2MHz _r es
1MHz_res
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
33.1.7 Pin Pull-Up
Figure 33-15. ATmega328: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 1.8V)
85 °C
25 °C
-40 °C
0
10
20
30
40
50
60
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
V
OP
(V)
I
OP
(uA)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 385
Figure 33-16. ATmega328: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 2.7V)
85 °C
25 °C
-40 °C
0
10
20
30
40
50
60
70
80
90
0 0.5 1 1.5 2 2.5 3
V
OP
(V)
IOP (uA)
Figure 33-17. ATmega328: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 5V)
0
20
40
60
80
100
120
140
160
0123456
V
OP
(V)
I
OP
(uA)
85 °C
25 °C
-40 °C
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 386
Figure 33-18. ATmega328: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 1.8V)
0
5
10
15
20
25
30
35
40
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
V
RESET
(V)
I
RESET
(uA)
85 °C
25 °C
-40 °C
Figure 33-19. ATmega328: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 2.7V)
0
10
20
30
40
50
60
70
0 0.5 1 1.5 2 2.5 3
V
RESET
(V)
I
RESET
(uA)
85 °C
25 °C
-40 °C
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 387
Figure 33-20. ATmega328: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 5V)
0
20
40
60
80
100
120
0123456
V
RESET
(V)
I
RESET
(uA)
85 °C
25 °C
-40 °C
33.1.8 Pin Driver Strength
Figure 33-21. I/O Pin Output Voltage vs. Sink Current (VCC = 3V)
85 °C
25 °C
-40 °C
0
0.2
0.4
0.6
0.8
1
0 5 10 15 20 25
I
OL
(mA)
V
OL
(V)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 388
Figure 33-22. I/O Pin Output Voltage vs. Sink Current (VCC = 5V)
85 °C
25 °C
-40 °C
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20 25
I
OL
(mA)
V
OL
(V)
Figure 33-23. I/O Pin Output Voltage vs. Source Current (Vcc = 3V)
85 °C
25 °C
-40 °C
0
0.5
1
1.5
2
2.5
3
3.5
0 5 10 15 20 25
I
OH
(mA)
VOH (V)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 389
Figure 33-24. I/O Pin Output Voltage vs. Source Current (VCC = 5V)
85 °C
25 °C
-40 °C
4.3
4.4
4.5
4.6
4.7
4.8
4.9
5
5.1
0 5 10 15 20 25
I
OH
(mA)
VOH (V)
33.1.9 Pin Threshold and Hysteresis
Figure 33-25. I/O Pin Input Threshold Voltage vs. VCC (VIH, I/O Pin read as ‘1’)
85 °C
25 °C
-40 °C
0
0.5
1
1.5
2
2.5
3
3.5
4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
Threshold (V)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 390
Figure 33-26. I/O Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’)
I/O PIN INPUT THRESHOLD VOLTAGE vs. VCC
VIL, IO PIN READ AS '0'
85 °C
25 °C
-40 °C
0
0.5
1
1.5
2
2.5
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
Threshold (V)
Figure 33-27. I/O Pin Input Hysteresis vs. VCC
85 °C
25 °C
-40 °C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Input Hysteresis (V)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 391
Figure 33-28. Reset Input Threshold Voltage vs. VCC (VIH, I/O Pin read as ‘1’)
85 °C
25 °C
-40 °C
0
0.5
1
1.5
2
2.5
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
Threshold (V)
Figure 33-29. Reset Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’)
85 °C
25 °C
-40 °C
0
0.5
1
1.5
2
2.5
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
Threshold (V)
Figure 33-30. Reset Pin Input Hysteresis vs. VCC
85 °C
25 °C
-40 °C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
Input Hysteresis (V)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 392
33.1.10 BOD Threshold
Figure 33-31. BOD Thresholds vs. Temperature (BODLEVEL is 1.8V)
1
0
1.75
1.77
1.79
1.81
1.83
1.85
-60 -40 -20 0 20 40 60 80 100
Temperature (°C)
Threshold (V)
Figure 33-32. BOD Thresholds vs. Temperature (BODLEVEL is 2.7V)
1
0
2.66
2.68
2.7
2.72
2.74
2.76
2.78
-60 -40 -20 0 20 40 60 80 100
Temperature (°C)
Threshold (V)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 393
Figure 33-33. BOD Thresholds vs. Temperature (BODLEVEL is 4.3V)
1
0
4.25
4.3
4.35
4.4
-60 -40 -20 0 20 40 60 80 100
Temperature (°C)
Threshold (V)
Figure 33-34. Calibrated Bandgap Voltage vs. Vcc
85 °C
25 °C
-40 °C
1.124
1.126
1.128
1.13
1.132
1.134
1.136
1.138
1.5 2 2.5 3 3.5 4 4.5 5 5.5
Vcc (V)
Bandgap Voltage (V)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 394
33.1.11 Internal Oscillator Speed
Figure 33-35. Watchdog Oscillator Frequency vs. Temperature
5.5 V
4.0 V
3.3 V
2.7 V
109
110
111
112
113
114
115
116
117
118
119
-60 -40 -20 0 20 40 60 80 100
Temperature (°C)
F
RC
(kHz)
Figure 33-36. Watchdog Oscillator Frequency vs. VCC
85 °C
25 °C
-40 °C
1.5 2 2.5 3 3.5 4 4.5 5 5.5
(V)
F
RC
(kHz)
108
110
112
114
116
118
120
V
CC
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 395
Figure 33-37. Calibrated 8 MHz RC Oscillator Frequency vs. VCC
85 °C
25 °C
-40 °C
7.4
7.6
7.8
8
8.2
8.4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
FRC (MHz)
Figure 33-38. Calibrated 8MHz RC Oscillator Frequency vs. Temperature
5.0 V
3.0 V
7.6
7.7
7.8
7.9
8
8.1
8.2
8.3
8.4
-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100
Temperature (°C)
FRC (MHz)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 396
Figure 33-39. Calibrated 8MHz RC Oscillator Frequency vs. OSCCAL Value
85 °C
25 °C
-40 °C
0
2
4
6
8
10
12
14
16
0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 256
OSCCAL (X1)
F
RC
(MHz)
33.1.12 Current Consumption of Peripheral Units
Figure 33-40. ADC Current vs. VCC (AREF = AVCC)
85 °C
25 °C
-40 °C
0
50
100
150
200
250
300
350
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 397
Figure 33-41. Analog Comparator Current vs. VCC
85 °C
25 °C
-40 °C
0
20
40
60
80
100
120
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
Figure 33-42. AREF External Reference Current vs. VCC
85 °C
25 °C
-40 °C
0
20
40
60
80
100
120
140
160
180
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 398
Figure 33-43. Brownout Detector Current vs. VCC
85 °C
25 °C
-40 °C
0
5
10
15
20
25
30
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(uA)
Figure 33-44. Programming Current vs. VCC
85 °C
25 °C
-40 °C
0
1
2
3
4
5
6
7
8
9
10
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 399
33.1.13 Current Consumption in Reset and Reset Pulsewidth
Figure 33-45. Reset Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
5.5 V
5.0 V
4.5 V
4.0 V
3.3 V
2.7 V
1.8 V
0
0.05
0.1
0.15
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Frequency (MHz)
I
CC
(mA)
Figure 33-46. Reset Supply Current vs. Frequency (1MHz - 20MHz)
5.5 V
5.0 V
4.5 V
0
0.5
1
1.5
2
2.5
3
0 2 4 6 8 10 12 14 16 18 20
Frequency (MHz)
I
CC
(mA)
1.8 V
2.7 V
3.3 V
4.0 V
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 400
Figure 33-47. Minimum Reset Pulse Width vs. Vcc
85 °C
25 °C
-40 °C
0
200
400
600
800
1000
1200
1400
1600
1800
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Pulsewidth (ns)
ATmega328/P
Typical Characteristics (TA = -40°C to 85°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 401
34. Typical Characteristics (TA = -40°C to 105°C)
The following charts show typical behavior. These figures are not tested during manufacturing. All current
consumption measurements are performed with all I/O pins configured as inputs and with internal pull-
ups enabled. A sine wave generator with rail-to-rail output is used as clock source.
The power consumption in Power-Down mode is independent of clock selection.
The current consumption is a function of several factors such as: operating voltage, operating frequency,
loading of I/O pins, switching rate of I/O pins, code executed and ambient temperature. The dominating
factors are operating voltage and frequency.
The current drawn from capacitive loaded pins may be estimated (for one pin) as CL*VCC*f where CL =
load capacitance, VCC = operating voltage and f = average switching frequency of I/O pin.
The parts are characterized at frequencies higher than test limits. Parts are not recommended to function
properly at frequencies higher than the ordering code indicates.
The difference between current consumption in Power-Down mode with watchdog timer enabled and
Power-Down mode with watchdog timer disabled represents the differential current drawn by the
watchdog timer.
34.1 ATmega328P Typical Characteristics
34.1.1 Active Supply Current
Figure 34-1. ATmega328P: Active Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
ATmega328P: Active Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
5.5V
5.0V
4.5V
4.0V
3.6V
2.7V
1.8V
0
0.2
0.4
0.6
0.8
1
1.2
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Frequency (MHz)
ICC (mA)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 402
Figure 34-2. ATmega328P: Active Supply Current vs. Frequency (1MHz - 20MHz)
ATmega328P: Active Supply Current vs. Frequency (1MHz - 20MHz)
5.5V
5.0V
4.5V
0
2
4
6
8
10
12
14
0 2 4 6 8 10 12 14 16 18 20
Frequency (MHz)
ICC (mA)
1.8V
2.7V
3.6V
4.0V
Figure 34-3. ATmega328P: Active Supply Current vs. VCC (Internal RC Oscillator, 128kHz)
ATmega328P: Active Supply Current vs. VCC (Internal RC Oscillator, 128kHz)
105°C
85°C
25°C
-40°C
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 403
Figure 34-4. ATmega328P: Active Supply Current vs. VCC (Internal RC Oscillator, 1MHz)
ATmega328P: Active Supply Current vs. VCC (Internal RC Oscillator, 1MHz)
105°C
85°C
25°C
-40°C
0.2
0.4
0.6
0.8
1
1.2
1.4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
Figure 34-5. ATmega328P: Active Supply Current vs. VCC (Internal RC Oscillator, 8MHz)
ATmega328P: Active Supply Current vs. VCC (Internal RC Oscillator, 8MHz)
105°C
85°C
25°C
-40°C
1
2
3
4
5
6
7
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 404
34.1.2 Idle Supply Current
Figure 34-6. ATmega328P: Idle Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
5.5V
5.0V
4.5V
4.0V
3.6V
2.7V
1.8V
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0 0.2 0.4 0.6 0.8 1
Frequency (MHz)
I
CC
(mA)
Figure 34-7. ATmega328P: Idle Supply Current vs. Frequency (1MHz - 20MHz)
5.5V
5.0V
4.5V
0
0.5
1
1.5
2
2.5
3
3.5
0 2 4 6 8 10 12 14 16 18 20
Frequency (MHz)
ICC (mA)
1.8V
2.7V
3.6V
4.0V
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 405
Figure 34-8. ATmega328P: Idle Supply Current vs. VCC (Internal RC Oscillator, 128kHz)
105°C
85°C
25°C
-40°C
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
Figure 34-9. ATmega328P: Idle Supply Current vs. VCC (Internal RC Oscillator, 1MHz)
105°C
85°C
25°C
-40°C
0.1
0.15
0.2
0.25
0.3
0.35
0.4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 406
Figure 34-10. ATmega328P: Idle Supply Current vs. VCC (Internal RC Oscillator, 8MHz)
105°C
85°C
25°C
-40°C
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(mA)
34.1.3 Supply Current of IO Modules
The tables and formulas below can be used to calculate the additional current consumption for the
different I/O modules in Active and Idle mode. The enabling or disabling of the I/O modules are controlled
by the Power Reduction Register. See “Power Reduction Register” for details.
Table 34-1. ATmega328P: Additional Current Consumption for the different I/O modules (absolute
values)
PRR bit Typical numbers (µA)
VCC = 2V, F = 1MHz VCC = 3V, F = 4MHz VCC = 5V, F = 8MHz
PRUSART0 3.20 22.17 100.25
PRTWI 7.34 46.55 199.25
PRTIM2 7.34 50.79 224.25
PRTIM1 6.19 41.25 176.25
PRTIM0 1.89 14.28 61.13
PRSPI 6.94 43.84 186.50
PRADC 8.66 61.80 295.38
Table 34-2. ATmega328P: Additional Current Consumption (percentage) in Active and Idle mode
PRR bit Additional Current consumption
compared to Active with external clock
(see Figure 34-1 and Figure 34-2)
Additional Current consumption
compared to Idle with external clock (see
Figure 34-6 and Figure 34-7)
PRUSART0 1.4% 7.8%
PRTWI 3.0% 16.6%
PRTIM2 3.3% 17.8%
PRTIM1 2.7% 14.5%
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 407
PRR bit Additional Current consumption
compared to Active with external clock
(see Figure 34-1 and Figure 34-2)
Additional Current consumption
compared to Idle with external clock (see
Figure 34-6 and Figure 34-7)
PRTIM0 0.9% 4.8%
PRSPI 2.9% 15.7%
PRADC 4.1% 22.1%
It is possible to calculate the typical current consumption based on the numbers from the above table for
other VCC and frequency settings.
Related Links
PRR
34.1.3.1 Example
Calculate the expected current consumption in idle mode with TIMER1, ADC, and SPI enabled at VCC =
2.0V and F = 1MHz. From Table Additional Current Consumption (percentage) in Active and Idle mode in
the previous section, third column, we see that we need to add 14.5% for the TIMER1, 22.1% for the
ADC, and 15.7% for the SPI module. Reading from Figure Idle Supply Current vs. Low Frequency
(0.1-1.0MHz), we find that the idle current consumption is ~0.045mA at VCC = 2.0V and F = 1MHz. The
total current consumption in idle mode with TIMER1, ADC, and SPI enabled, gives:
ICCtotal 0.045mA(1+0.145+0.221+0.157) 0.069mA
34.1.4 Power-down Supply Current
Figure 34-11. ATmega328P: Power-Down Supply Current vs. VCC (Watchdog Timer Disabled)
ATmega328P: Power-Down Supply Current vs. VCC (Watchdog Timer Disabled)
105°C
85°C
25°C
-40°C
0
0.5
1
1.5
2
2.5
3
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
I
CC
(µA)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 408
Figure 34-12. ATmega328P: Power-Down Supply Current vs. VCC (Watchdog Timer Enabled)
ATmega328P: Power-Down Supply Current vs. VCC (Watchdog Timer Enabled)
105°C
85°C
25°C
-40°C
2
3
4
5
6
7
8
9
10
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (µA)
34.1.5 Power-save Supply Current
Figure 34-13. ATmega328P: Power-Save Supply Current vs. VCC (Watchdog Timer Disabled and
32kHz Crystal Oscillator Running)
105°C
85°C
25°C
-40°C
0
0.5
1
1.5
2
2.5
3
3.5
4
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (µA)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 409
34.1.6 Standby Supply Current
Figure 34-14. ATmega328P: Standby Supply Current vs. VCC (Watchdog Timer Disabled)
ATmega328P: Standby Supply Current vs. Vcc (Watchdog Timer Disabled)
6 MHz_xtal
6 MHz_res
4 MHz_xtal
4 MHz_res
2 MHz_xtal
2 MHz_res
1 MHz_res
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.11
0.12
0.13
0.14
0.15
2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
34.1.7 Pin Pull-Up
Figure 34-15. ATmega328P: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 1.8V)
ATmega328P: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 1.8V)
105°C
85°C
25°C
-40°C
0
5
10
15
20
25
30
35
40
45
50
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
V
OP
(V)
I
OP
(µA)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 410
Figure 34-16. ATmega328P: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 2.7V)
ATmega328P: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 2.7V)
105°C
85°C
25°C
-40°C
0
10
20
30
40
50
60
70
80
0 0.5 1 1.5 2 2.5 3
VOP (V)
IOP (µA)
Figure 34-17. ATmega328P: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 5V)
ATmega328P: I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 5V)
0
20
40
60
80
100
120
140
160
0 1 2 3 4 5
V
OP
(V)
I
OP
(µA)
105°C
85°C
25°C
-40°C
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 411
Figure 34-18. ATmega328P: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 1.8V)
ATmega328P: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 1.8V)
105°C
85°C
25°C
-40°C
0
5
10
15
20
25
30
35
40
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
VRESET (V)
IRESET (µA)
Figure 34-19. ATmega328P: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 2.7V)
ATmega328P: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 2.7V)
0
10
20
30
40
50
60
0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
VRESET (V)
IRESET (µA)
105°C
85°C
25°C
-40°C
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 412
Figure 34-20. ATmega328P: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 5V)
ATmega328P: Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 5V)
0
20
40
60
80
100
120
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
VRESET (V)
IRESET (µA)
105°C
85°C
25°C
-40°C
34.1.8 Pin Driver Strength
Figure 34-21. ATmega328P: I/O Pin Output Voltage vs. Sink Current (VCC = 3V)
ATmega328P: I/O Pin Output Voltage vs. Sink Current (VCC = 3V)
105°C
85°C
25°C
-40°C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20
I
OL
(mA)
V
OL
(V)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 413
Figure 34-22. ATmega328P: I/O Pin Output Voltage vs. Sink Current (VCC = 5V)
ATmega328P: I/O Pin Output Voltage vs. Sink Current (VCC = 5V)
105°C
85°C
25°C
-40°C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 5 10 15 20
IOL (mA)
VOL (V)
Figure 34-23. ATmega328P: I/O Pin Output Voltage vs. Source Current (Vcc = 3V)
ATmega328P: I/O Pin Output Voltage vs. Source Current (Vcc = 3V)
105°C
85°C
25°C
-40°C
1.9
2.1
2.3
2.5
2.7
2.9
3.1
0 5 10 15 20
IOH (mA)
VOH (V)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 414
Figure 34-24. ATmega328P: I/O Pin Output Voltage vs. Source Current (VCC = 5V)
ATmega328P: I/O Pin Output Voltage vs. Source Current (VCC = 5V)
105°C
85°C
25°C
-40°C
4.3
4.4
4.5
4.6
4.7
4.8
4.9
5
5.1
0 5 10 15 20
IOH (mA)
VOH (V)
34.1.9 Pin Threshold and Hysteresis
Figure 34-25. ATmega328P: I/O Pin Input Threshold Voltage vs. VCC (VIH, I/O Pin read as ‘1’)
ATmega328P: I/O Pin Input Threshold Voltage vs. VCC (VIH, I/O Pin read as ‘1’)
105°C
85°C
25°C
-40°C
1
1.3
1.6
1.9
2.2
2.5
2.8
3.1
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Threshold (V)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 415
Figure 34-26. ATmega328P: I/O Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’
ATmega328P: I/O Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’
105°C
85°C
25°C
-40°C
0.2
0.5
0.8
1.1
1.4
1.7
2
2.3
2.6
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Threshold (V)
Figure 34-27. ATmega328P: I/O Pin Input Hysteresis vs. VCC
ATmega328P: I/O Pin Input Hysteresis vs. VCC
105°C
85°C
25°C
-40°C
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1.8 2.3 2.8 3.3 3.8 4.3 4.8 5.3
VCC (V)
Input Hysteresis (mV)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 416
Figure 34-28. ATmega328P: Reset Input Threshold Voltage vs. VCC (VIH, I/O Pin read as ‘1’)
ATmega328P: Reset Input Threshold Voltage vs. VCC (VIH, I/O Pin read as ‘1’)
105°C
85°C
25°C
-40°C
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Threshold (V)
Figure 34-29. ATmega328P: Reset Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’)
ATmega328P: Reset Input Threshold Voltage vs. VCC (VIL, I/O Pin read as ‘0’)
105°C
85°C
25°C
-40°C
0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
2.1
2.3
2.5
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Threshold (V)
Figure 34-30. ATmega328P: Reset Pin Input Hysteresis vs. VCC
ATmega328P: Reset Pin Input Hysteresis vs. VCC
105°C
85°C
25°C
-40°C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Input Hysteresis (V)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 417
34.1.10 BOD Threshold
Figure 34-31. ATmega328P: BOD Thresholds vs. Temperature (BODLEVEL is 1.8V)
ATmega328P: BOD Thresholds vs. Temperature (BODLEVEL is 1.8V)
Rising Vcc
Falling Vcc
1.78
1.79
1.8
1.81
1.82
1.83
1.84
-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110
Temperature (°C)
Threshold (V)
Figure 34-32. ATmega328P: BOD Thresholds vs. Temperature (BODLEVEL is 2.7V)
ATmega328P BOD Thresholds vs. Temperature (BODLEVEL is 2.7V)
Rising Vcc
Falling Vcc
2.66
2.67
2.68
2.69
2.7
2.71
2.72
2.73
2.74
2.75
2.76
2.77
2.78
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110
Temperature (°C)
Threshold (V)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 418
Figure 34-33. ATmega328P: BOD Thresholds vs. Temperature (BODLEVEL is 4.3V)
ATmega328P BOD Thresholds vs. Temperature (BODLEVEL is 4.3V)
Rising Vcc
Falling Vcc
4.24
4.26
4.28
4.3
4.32
4.34
4.36
4.38
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110
Temperature (°C)
Threshold (V)
Figure 34-34. ATmega328P: Calibrated Bandgap Voltage vs. VCC
ATmega328P: Calibrated Bandgap Voltage vs. Vcc
105°C
85°C
25°C
-40°C
1.121
1.124
1.127
1.13
1.133
1.136
1.139
1.5 2 2.5 3 3.5 4 4.5 5 5.5
Vcc (V)
Bandgap Voltage (V)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 419
34.1.11 Internal Oscillator Speed
Figure 34-35. ATmega328P: Watchdog Oscillator Frequency vs. Temperature
ATmega328P: Watchdog Oscillator Frequency vs. Temperature
5.5V
4.0V
3.6V
2.7V
106
108
110
112
114
116
118
120
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110
Temperature (°C)
F
RC
(kHz)
Figure 34-36. ATmega328P: Watchdog Oscillator Frequency vs. VCC
ATmega328P C Watchdog Oscillator Frequency vs. VCC
105°C
85°C
25°C
-40°C
106
108
110
112
114
116
118
120
2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
FRC (kHz)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 420
Figure 34-37. ATmega328P: Calibrated 8 MHz RC Oscillator Frequency vs. VCC
ATmega328P: Calibrated 8 MHz RC Oscillator Frequency vs. VCC
105°C
85°C
25°C
-40°C
7.4
7.6
7.8
8
8.2
8.4
8.6
2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
F
RC
(MHz)
Figure 34-38. ATmega328P: Calibrated 8MHz RC Oscillator Frequency vs. Temperature
ATmega328P: Calibrated 8MHz RC Oscillator Frequency vs. Temperature
5.5V
5.0V
4.5V
4.0V
3.6V
2.7V
1.8V
7.5
7.6
7.7
7.8
7.9
8
8.1
8.2
8.3
8.4
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110
Temperature (°C)
FRC (MHz)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 421
Figure 34-39. ATmega328P: Calibrated 8MHz RC Oscillator Frequency vs. OSCCAL Value
ATmega328P Calibrated 8MHz RC Oscillator Frequency vs. OSCCAL Value
105°C
85°C
25°C
-40°C
4
5
6
7
8
9
10
11
12
13
14
0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240
OSCCAL (X1)
FRC (MHz)
34.1.12 Current Consumption of Peripheral Units
Figure 34-40. ATmega328P: ADC Current vs. VCC (AREF = AVCC)
ATmega328P: ADC Current vs. VCC (AREF = AVCC)
105°C
85°C
25°C
-40°C
40
60
80
100
120
140
160
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (µA)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 422
Figure 34-41. ATmega328P: Analog Comparator Current vs. VCC
ATmega328P: Analog Comparator Current vs. VCC
105°C
85°C
25°C
-40°C
30
40
50
60
70
80
90
100
1.5 2 2.5 3 3.5 4 4.5 5 5.5
V
CC
(V)
ICC (µA)
Figure 34-42. ATmega328P: AREF External Reference Current vs. VCC
ATmega328P: AREF External Reference Current vs. VCC
105°C
85°C
25°C
-40°C
40
60
80
100
120
140
160
180
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (µA)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 423
Figure 34-43. ATmega328P: Brownout Detector Current vs. VCC
ATmega328P: Brownout Detector Current vs. VCC
105°C
85°C
25°C
-40°C
10
15
20
25
30
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (µA)
Figure 34-44. ATmega328P: Programming Current vs. VCC
ATmega328P: Programming Current vs. VCC
105°C
85°C
25°C
-40°C
1
2
3
4
5
6
7
8
9
10
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 424
34.1.13 Current Consumption in Reset and Reset Pulsewidth
Figure 34-45. ATmega328P: Reset Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
ATmega328P: Reset Supply Current vs. Low Frequency (0.1MHz - 1.0MHz)
5.5V
5.0V
4.5V
4.0V
3.6V
2.7V
1.8V
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Frequency (MHz)
ICC (mA)
Figure 34-46. ATmega328P: Reset Supply Current vs. Frequency (1MHz - 20MHz)
ATmega328P Reset Supply Current vs. Frequency (1MHz - 20MHz)
5.5V
5.0V
4.5V
0
0.5
1
1.5
2
2.5
3
0 2 4 6 8 10 12 14 16 18 20
Frequency (MHz)
I
CC
(mA)
4.0V
3.6V
2.7V
1.8V
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 425
Figure 34-47. ATmega328P: Minimum Reset Pulse Width vs. Vcc
ATmega328P: Minimum Reset Pulse Width vs. Vcc
105°C
85°C
25°C
-40°C
0
200
400
600
800
1000
1200
1400
1600
1800
1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Pulsewidth (ns)
ATmega328/P
Typical Characteristics (TA = -40°C to 105°C)
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 426
35. Register Summary
Offset Name Bit Pos.
0x23 PINB 7:0 PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0
0x24 DDRB 7:0 DDRB7 DDRB6 DDRB5 DDRB4 DDRB3 DDRB2 DDRB1 DDRB0
0x25 PORTB 7:0 PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0
0x26 PINC 7:0 PINC6 PINC5 PINC4 PINC3 PINC2 PINC1 PINC0
0x27 DDRC 7:0 DDRC6 DDRC5 DDRC4 DDRC3 DDRC2 DDRC1 DDRC0
0x28 PORTC 7:0 PORTC6 PORTC5 PORTC4 PORTC3 PORTC2 PORTC1 PORTC0
0x29 PIND 7:0 PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0
0x2A DDRD 7:0 DDRD7 DDRD6 DDRD5 DDRD4 DDRD3 DDRD2 DDRD1 DDRD0
0x2B PORTD 7:0 PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0
0x2C
...
0x34
Reserved
0x35 TIFR0 7:0 OCF0B OCF0A TOV0
0x36 TIFR1 7:0 ICF1 OCF1B OCF1A TOV1
0x37 TIFR2 7:0 OCF2B OCF2A TOV2
0x38
...
0x3A
Reserved
0x3B PCIFR 7:0 PCIF2 PCIF1 PCIF0
0x3C EIFR 7:0 INTF1 INTF0
0x3D EIMSK 7:0 INT1 INT0
0x3E GPIOR0 7:0 GPIOR0[7:0]
0x3F EECR 7:0 EEPM[1:0] EERIE EEMPE EEPE EERE
0x40 EEDR 7:0 EEDR[7:0]
0x41 EEARL and EEARH
7:0 EEAR[7:0]
15:8 EEAR[9:8]
0x43 GTCCR 7:0 TSM PSRASY PSRSYNC
0x44 TCCR0A 7:0 COM0A[1:0] COM0B[1:0] WGM0[1:0]
0x45 TCCR0B 7:0 FOC0A FOC0B WGM02 CS0[2:0]
0x46 TCNT0 7:0 TCNT0[7:0]
0x47 OCR0A 7:0 OCR0A[7:0]
0x48 OCR0B 7:0 OCR0B[7:0]
0x49 Reserved
0x4A GPIOR1 7:0 GPIOR1[7:0]
0x4B GPIOR2 7:0 GPIOR2[7:0]
0x4C SPCR0 7:0 SPIE0 SPE0 DORD0 MSTR0 CPOL0 CPHA0 SPR0[1:0]
0x4D SPSR0 7:0 SPIF0 WCOL0 SPI2X0
0x4E SPDR0 7:0 SPID[7:0]
0x4F Reserved
0x50 ACSR 7:0 ACD ACBG ACO ACI ACIE ACIC ACIS[1:0]
0x51 DWDR 7:0 DWDR[7:0]
0x52 Reserved
0x53 SMCR 7:0 SM[2:0] SE
ATmega328/P
Register Summary
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 427
Offset Name Bit Pos.
0x54 MCUSR 7:0 WDRF BORF EXTRF PORF
0x55 MCUCR 7:0 BODS BODSE PUD IVSEL IVCE
0x56 Reserved
0x57 SPMCSR 7:0 SPMIE RWWSB SIGRD RWWSRE BLBSET PGWRT PGERS SPMEN
0x58
...
0x5C
Reserved
0x5D SPL and SPH
7:0 SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0
15:8 SP11 SP10 SP9 SP8
0x5F SREG 7:0 I T H S V N Z C
0x60 WDTCSR 7:0 WDIF WDIE WDP[3] WDCE WDE WDP[2:0]
0x61 CLKPR 7:0 CLKPCE CLKPS[3:0]
0x62
...
0x63
Reserved
0x64 PRR 7:0 PRTWI0 PRTIM2 PRTIM0 PRTIM1 PRSPI0 PRUSART0 PRADC
0x65 Reserved
0x66 OSCCAL 7:0 CAL[7:0]
0x67 Reserved
0x68 PCICR 7:0 PCIE2 PCIE1 PCIE0
0x69 EICRA 7:0 ISC1[1:0] ISC0[1:0]
0x6A Reserved
0x6B PCMSK0 7:0 PCINT[7:0]
0x6C PCMSK1 7:0 PCINT[14:8]
0x6D PCMSK2 7:0 PCINT[23:16]
0x6E TIMSK0 7:0 OCIE0B OCIE0A TOIE0
0x6F TIMSK1 7:0 ICIE1 OCIE1B OCIE1A TOIE1
0x70 TIMSK2 7:0 OCIE2B OCIE2A TOIE2
0x71
...
0x77
Reserved
0x78 ADCL and ADCH
7:0 ADC[7:0]
15:8 ADC[9:8]
0x78 ADCL and ADCH
(ADLAR = 1)
7:0 ADC[1:0]
15:8 ADC[9:2]
0x7A ADCSRA 7:0 ADEN ADSC ADATE ADIF ADIE ADPS [2:0]
0x7B ADCSRB 7:0 ACME ADTS[2:0]
0x7C ADMUX 7:0 REFS[1:0] ADLAR MUX[3:0]
0x7D Reserved
0x7E DIDR0 7:0 ADC5D ADC4D ADC3D ADC2D ADC1D ADC0D
0x7F DIDR1 7:0 AIN1D AIN0D
0x80 TCCR1A 7:0 COM1A[1:0] COM1B[1:0] WGM1[1:0]
0x81 TCCR1B 7:0 ICNC1 ICES1 WGM13 WGM12 CS1[2:0]
0x82 TCCR1C 7:0 FOC1A FOC1B
0x83 Reserved
ATmega328/P
Register Summary
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 428
Offset Name Bit Pos.
0x84 TCNT1L and
TCNT1H
7:0 TCNT1[7:0]
15:8 TCNT1[15:8]
0x86 ICR1L and ICR1H
7:0 ICR1[7:0]
15:8 ICR1[15:8]
0x88 OCR1AL and
OCR1AH
7:0 OCR1A[7:0]
15:8 OCR1A[15:8]
0x8A OCR1BL and
OCR1BH
7:0 OCR1B[7:0]
15:8 OCR1B[15:8]
0x8C
...
0xAF
Reserved
0xB0 TCCR2A 7:0 COM2A[1:0] COM2B[1:0] WGM2[1:0]
0xB1 TCCR2B 7:0 FOC2A FOC2B WGM22 CS2[2:0]
0xB2 TCNT2 7:0 TCNT2[7:0]
0xB3 OCR2A 7:0 OCR2A[7:0]
0xB4 OCR2B 7:0 OCR2B[7:0]
0xB5 Reserved
0xB6 ASSR 7:0 EXCLK AS2 TCN2UB OCR2AUB OCR2BUB TCR2AUB TCR2BUB
0xB7 Reserved
0xB8 TWBR 7:0 TWBR [7:0]
0xB9 TWSR 7:0 TWS7 TWS6 TWS5 TWS4 TWS3 TWPS[1:0]
0xBA TWAR 7:0 TWA[6:0] TWGCE
0xBB TWDR 7:0 TWD[7:0]
0xBC TWCR 7:0 TWINT TWEA TWSTA TWSTO TWWC TWEN TWIE
0xBD TWAMR 7:0 TWAM6 TWAM5 TWAM4 TWAM3 TWAM2 TWAM1 TWAM0
0xBE
...
0xBF
Reserved
0xC0 UCSR0A 7:0 RXC0 TXC0 UDRE0 FE0 DOR0 UPE0 U2X0 MPCM0
0xC1 UCSR0B 7:0 RXCIE0 TXCIE0 UDRIE0 RXEN0 TXEN0 UCSZ02 RXB80 TXB80
0xC2 UCSR0C 7:0 UMSEL0 [1:0] UPM0 [1:0] USBS0 UCSZ01 /
UDORD0
UCSZ00 /
UCPHA0 UCPOL0
0xC3 Reserved
0xC4 UBRR0L and
UBRR0H
7:0 UBRR0[7:0]
15:8 UBRR0[11:8]
0xC6 UDR0 7:0 TXB / RXB[7:0]
35.1 Note
1. For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved
I/O memory addresses should never be written.
2. I/O registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI and CBI
instructions. In these registers, the value of single bits can be checked by using the SBIS and SBIC
instructions.
3. Some of the Status flags are cleared by writing a logical one to them. Note that, unlike most other
AVRs, the CBI and SBI instructions will only operate on the specified bit, and can therefore be used
ATmega328/P
Register Summary
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 429
on registers containing such Status flags. The CBI and SBI instructions work with registers 0x00 to
0x1F only.
4. When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used.
When addressing I/O registers as data space using LD and ST instructions, 0x20 must be added to
these addresses. The ATmega328/P is a complex microcontroller with more peripheral units than
can be supported within the 64 location reserved in Opcode for the IN and OUT instructions. For
the extended I/O space from 0x60 - 0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD
instructions can be used.
ATmega328/P
Register Summary
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 430
36. Instruction Set Summary
ARITHMETIC AND LOGIC INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
ADD Rd, Rr Add two Registers without Carry Rd ← Rd + Rr Z,C,N,V,H 1
ADC Rd, Rr Add two Registers with Carry Rd ← Rd + Rr + C Z,C,N,V,H 1
ADIW Rdl,K Add Immediate to Word Rdh:Rdl ← Rdh:Rdl + K Z,C,N,V,S 2
SUB Rd, Rr Subtract two Registers Rd ← Rd - Rr Z,C,N,V,H 1
SUBI Rd, K Subtract Constant from Register Rd ← Rd - K Z,C,N,V,H 1
SBC Rd, Rr Subtract two Registers with Carry Rd ← Rd - Rr - C Z,C,N,V,H 1
SBCI Rd, K Subtract Constant from Reg with Carry. Rd ← Rd - K - C Z,C,N,V,H 1
SBIW Rdl,K Subtract Immediate from Word Rdh:Rdl ← Rdh:Rdl - K Z,C,N,V,S 2
AND Rd, Rr Logical AND Registers Rd ← Rd · Rr Z,N,V 1
ANDI Rd, K Logical AND Register and Constant Rd ← Rd · K Z,N,V 1
OR Rd, Rr Logical OR Registers Rd ← Rd v Rr Z,N,V 1
ORI Rd, K Logical OR Register and Constant Rd ← Rd v K Z,N,V 1
EOR Rd, Rr Exclusive OR Registers Rd ← Rd ⊕ Rr Z,N,V 1
COM Rd One’s Complement Rd ← 0xFF - Rd Z,C,N,V 1
NEG Rd Two’s Complement Rd ← 0x00 - Rd Z,C,N,V,H 1
SBR Rd,K Set Bit(s) in Register Rd ← Rd v K Z,N,V 1
CBR Rd,K Clear Bit(s) in Register Rd ← Rd · (0xFF - K) Z,N,V 1
INC Rd Increment Rd ← Rd + 1 Z,N,V 1
DEC Rd Decrement Rd ← Rd - 1 Z,N,V 1
TST Rd Test for Zero or Minus Rd ← Rd · Rd Z,N,V 1
CLR Rd Clear Register Rd ← Rd ⊕ Rd Z,N,V 1
SER Rd Set Register Rd ← 0xFF None 1
MUL Rd, Rr Multiply Unsigned R1:R0 ← Rd x Rr Z,C 2
MULS Rd, Rr Multiply Signed R1:R0 ← Rd x Rr Z,C 2
MULSU Rd, Rr Multiply Signed with Unsigned R1:R0 ← Rd x Rr Z,C 2
FMUL Rd, Rr Fractional Multiply Unsigned R1:R0 ← (Rd x Rr) << 1 Z,C 2
FMULS Rd, Rr Fractional Multiply Signed R1:R0 ← (Rd x Rr) << 1 Z,C 2
FMULSU Rd, Rr Fractional Multiply Signed with Unsigned R1:R0 ← (Rd x Rr) << 1 Z,C 2
BRANCH INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
RJMP k Relative Jump PC ← PC + k + 1 None 2
IJMP Indirect Jump to (Z) PC ← Z None 2
JMP(1) k Direct Jump PC ← k None 3
ATmega328/P
Instruction Set Summary
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 431
BRANCH INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
RCALL k Relative Subroutine Call PC ← PC + k + 1 None 3
ICALL Indirect Call to (Z) PC ← Z None 3
CALL(1) k Direct Subroutine Call PC ← k None 4
RET Subroutine Return PC ← STACK None 4
RETI Interrupt Return PC ← STACK I 4
CPSE Rd,Rr Compare, Skip if Equal if (Rd = Rr) PC ← PC + 2 or 3 None 1/2/3
CP Rd,Rr Compare Rd - Rr Z, N,V,C,H 1
CPC Rd,Rr Compare with Carry Rd - Rr - C Z, N,V,C,H 1
CPI Rd,K Compare Register with Immediate Rd - K Z, N,V,C,H 1
SBRC Rr, b Skip if Bit in Register Cleared if (Rr(b)=0) PC ← PC + 2 or 3 None 1/2/3
SBRS Rr, b Skip if Bit in Register is Set if (Rr(b)=1) PC ← PC + 2 or 3 None 1/2/3
SBIC A, b Skip if Bit in I/O Register Cleared if (I/O(A,b)=1) PC ← PC + 2 or 3 None 1/2/3
SBIS A, b Skip if Bit in I/O Register is Set if (I/O(A,b)=1) PC ← PC + 2 or 3 None 1/2/3
BRBS s, k Branch if Status Flag Set if (SREG(s) = 1) then PC←PC+k + 1 None 1/2
BRBC s, k Branch if Status Flag Cleared if (SREG(s) = 0) then PC←PC+k + 1 None 1/2
BREQ k Branch if Equal if (Z = 1) then PC ← PC + k + 1 None 1/2
BRNE k Branch if Not Equal if (Z = 0) then PC ← PC + k + 1 None 1/2
BRCS k Branch if Carry Set if (C = 1) then PC ← PC + k + 1 None 1/2
BRCC k Branch if Carry Cleared if (C = 0) then PC ← PC + k + 1 None 1/2
BRSH k Branch if Same or Higher if (C = 0) then PC ← PC + k + 1 None 1/2
BRLO k Branch if Lower if (C = 1) then PC ← PC + k + 1 None 1/2
BRMI k Branch if Minus if (N = 1) then PC ← PC + k + 1 None 1/2
BRPL k Branch if Plus if (N = 0) then PC ← PC + k + 1 None 1/2
BRGE k Branch if Greater or Equal, Signed if (N ⊕ V= 0) then PC ← PC + k + 1 None 1/2
BRLT k Branch if Less Than Zero, Signed if (N ⊕ V= 1) then PC ← PC + k + 1 None 1/2
BRHS k Branch if Half Carry Flag Set if (H = 1) then PC ← PC + k + 1 None 1/2
BRHC k Branch if Half Carry Flag Cleared if (H = 0) then PC ← PC + k + 1 None 1/2
BRTS k Branch if T Flag Set if (T = 1) then PC ← PC + k + 1 None 1/2
BRTC k Branch if T Flag Cleared if (T = 0) then PC ← PC + k + 1 None 1/2
BRVS k Branch if Overflow Flag is Set if (V = 1) then PC ← PC + k + 1 None 1/2
BRVC k Branch if Overflow Flag is Cleared if (V = 0) then PC ← PC + k + 1 None 1/2
BRIE k Branch if Interrupt Enabled if ( I = 1) then PC ← PC + k + 1 None 1/2
BRID k Branch if Interrupt Disabled if ( I = 0) then PC ← PC + k + 1 None 1/2
ATmega328/P
Instruction Set Summary
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 432
BIT AND BIT-TEST INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
SBI P,b Set Bit in I/O Register I/O(P,b) ← 1 None 2
CBI P,b Clear Bit in I/O Register I/O(P,b) ← 0 None 2
LSL Rd Logical Shift Left Rd(n+1) ← Rd(n), Rd(0) ← 0 Z,C,N,V 1
LSR Rd Logical Shift Right Rd(n) ← Rd(n+1), Rd(7) ← 0 Z,C,N,V 1
ROL Rd Rotate Left Through Carry Rd(0)←C,Rd(n+1)← Rd(n),C¬Rd(7) Z,C,N,V 1
ROR Rd Rotate Right Through Carry Rd(7)←C,Rd(n)← Rd(n+1),C←Rd(0) Z,C,N,V 1
ASR Rd Arithmetic Shift Right Rd(n) ← Rd(n+1), n=0...6 Z,C,N,V 1
SWAP Rd Swap Nibbles Rd(3...0)←Rd(7...4),Rd(7...4)¬Rd(3...0) None 1
BSET s Flag Set SREG(s) ← 1 SREG(s) 1
BCLR s Flag Clear SREG(s) ← 0 SREG(s) 1
BST Rr, b Bit Store from Register to T T ← Rr(b) T 1
BLD Rd, b Bit load from T to Register Rd(b) ← T None 1
SEC Set Carry C ← 1 C 1
CLC Clear Carry C ← 0 C 1
SEN Set Negative Flag N ← 1 N 1
CLN Clear Negative Flag N ← 0 N 1
SEZ Set Zero Flag Z ← 1 Z 1
CLZ Clear Zero Flag Z ← 0 Z 1
SEI Global Interrupt Enable I ← 1 I 1
CLI Global Interrupt Disable I ← 0 I 1
SES Set Signed Test Flag S ← 1 S 1
CLS Clear Signed Test Flag S ← 0 S 1
SEV Set Two’s Complement Overflow. V ← 1 V 1
CLV Clear Two’s Complement Overflow V ← 0 V 1
SET Set T in SREG T ← 1 T 1
CLT Clear T in SREG T ← 0 T 1
SEH Set Half Carry Flag in SREG H ← 1 H 1
CLH Clear Half Carry Flag in SREG H ← 0 H 1
DATA TRANSFER INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
MOV Rd, Rr Move Between Registers Rd ← Rr None 1
MOVW Rd, Rr Copy Register Word Rd+1:Rd ← Rr+1:Rr None 1
LDI Rd, K Load Immediate Rd ← K None 1
LD Rd, X Load Indirect Rd ← (X) None 2
LD Rd, X+ Load Indirect and Post-Increment Rd ← (X), X ← X + 1 None 2
ATmega328/P
Instruction Set Summary
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 433
DATA TRANSFER INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
LD Rd, - X Load Indirect and Pre-Decrement X ← X - 1, Rd ← (X) None 2
LD Rd, Y Load Indirect Rd ← (Y) None 2
LD Rd, Y+ Load Indirect and Post-Increment Rd ← (Y), Y ← Y + 1 None 2
LD Rd, - Y Load Indirect and Pre-Decrement Y ← Y - 1, Rd ← (Y) None 2
LDD Rd,Y+q Load Indirect with Displacement Rd ← (Y + q) None 2
LD Rd, Z Load Indirect Rd ← (Z) None 2
LD Rd, Z+ Load Indirect and Post-Increment Rd ← (Z), Z ← Z+1 None 2
LD Rd, -Z Load Indirect and Pre-Decrement Z ← Z - 1, Rd ← (Z) None 2
LDD Rd, Z+q Load Indirect with Displacement Rd ← (Z + q) None 2
LDS Rd, k Load Direct from SRAM Rd ← (k) None 2
ST X, Rr Store Indirect (X) ← Rr None 2
ST X+, Rr Store Indirect and Post-Increment (X) ← Rr, X ← X + 1 None 2
ST - X, Rr Store Indirect and Pre-Decrement X ← X - 1, (X) ← Rr None 2
ST Y, Rr Store Indirect (Y) ← Rr None 2
ST Y+, Rr Store Indirect and Post-Increment (Y) ← Rr, Y ← Y + 1 None 2
ST - Y, Rr Store Indirect and Pre-Decrement Y ← Y - 1, (Y) ← Rr None 2
STD Y+q,Rr Store Indirect with Displacement (Y + q) ← Rr None 2
ST Z, Rr Store Indirect (Z) ← Rr None 2
ST Z+, Rr Store Indirect and Post-Increment (Z) ← Rr, Z ← Z + 1 None 2
ST -Z, Rr Store Indirect and Pre-Decrement Z ← Z - 1, (Z) ← Rr None 2
STD Z+q,Rr Store Indirect with Displacement (Z + q) ← Rr None 2
STS k, Rr Store Direct to SRAM (k) ← Rr None 2
LPM Load Program Memory R0 ← (Z) None 3
LPM Rd, Z Load Program Memory Rd ← (Z) None 3
LPM Rd, Z+ Load Program Memory and Post-Inc Rd ← (Z), Z ← Z+1 None 3
SPM Store Program Memory (Z) ← R1:R0 None -
IN Rd, A In from I/O Location Rd ← I/O (A) None 1
OUT A, Rr Out to I/O Location I/O (A) ← Rr None 1
PUSH Rr Push Register on Stack STACK ← Rr None 2
POP Rd Pop Register from Stack Rd ← STACK None 2
MCU CONTROL INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
NOP No Operation No Operation None 1
SLEEP Sleep (see specific descr. for Sleep function) None 1
ATmega328/P
Instruction Set Summary
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 434
MCU CONTROL INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
WDR Watchdog Reset (see specific descr. for WDR/timer) None 1
BREAK Break For On-chip Debug Only None N/A
ATmega328/P
Instruction Set Summary
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 435
37. Packaging Information
37.1 32-pin 32A
Note:
Note: For the most current package drawings, see the Microchip Packaging Specification located at http://
www.microchip.com/packaging
TITLE DRAWING NO. REV.
32A, 32-lead, 7 x 7mm body size, 1.0mm body thickness,
0.8mm lead pitch, thin profile plastic quad flat package (TQFP) C
32A
2010-10-20
PIN 1 IDENTIFIER
0°~7°
PIN 1
L
C
A1 A2 A
D1
D
eE1 E
B
Notes:
1. This package conforms to JEDEC reference MS-026, Variation ABA.
2. Dimensions D1 and E1 do not include mold protrusion. Allowable
protrusion is 0.25mm per side. Dimensions D1 and E1 are maximum
plastic body size dimensions including mold mismatch.
3. Lead coplanarity is 0.10mm maximum.
A – – 1.20
A1 0.05 – 0.15
A2 0.95 1.00 1.05
D 8.75 9.00 9.25
D1 6.90 7.00 7.10 Note 2
E 8.75 9.00 9.25
E1 6.90 7.00 7.10 Note 2
B 0.30 – 0.45
C 0.09 – 0.20
L 0.45 – 0.75
e 0.80 TYP
COMMON DIMENSIONS
(Unit of measure = mm)
SYMBOL MIN NOM MAX NOTE
ATmega328/P
Packaging Information
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 436
37.2 32-pin 32M1-A
Note:
Note: For the most current package drawings, see the Microchip Packaging Specification located at http://
www.microchip.com/packaging
COMMON DIMENSIONS
(Unit of Measure = mm)
SYMBOL MIN NOM MAX NOTE
D1
D
E1 E
e
b
A3
A2
A1
A
D2
E2
0.08 C
L
1
2
3
P
P
0
1
2
3
A 0.80 0.90 1.00
A1 – 0.02 0.05
A2 – 0.65 1.00
A3 0.20 REF
b 0.18 0.23 0.30
D
D1
D2 2.95 3.10 3.25
4.90 5.00 5.10
4.70 4.75 4.80
4.70 4.75 4.80
4.90 5.00 5.10
E
E1
E2 2.95 3.10 3.25
e 0.50 BSC
L 0.30 0.40 0.50
P – – 0.60
– – 12o
Note: JEDEC Standard MO-220, Fig. 2 (Anvil Singulation), VHHD-2.
TOP VIEW
SIDE VIEW
BOTTOM VIEW
0
Pin 1 ID
Pin #1 Notch
(0.20 R)
K 0.20 – –
K
K
32M1-A , 32-pad, 5 x 5 x 1.0mm Body, Lead Pitch 0.50mm,
3.10mm Exposed Pad, Micro Lead Frame Package (MLF) 32M1-A
03/14/2014
F
ATmega328/P
Packaging Information
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 437
37.3 28-pin 28M1
Note:
For the most current package drawings, see the Microchip Packaging Specification located at http://
www.microchip.com/packaging
TITLE DRAWING NO.GPC REV.
Package Drawing Contact:
packagedrawings@atmel.com 28M1ZBV B
28M1,28-pad, 4 x 4 x 1.0mm Body, Lead Pitch 0.45mm,
2.4 x 2.4mm Exposed Pad, Thermally Enhanced
Plastic Very Thin Quad Flat No Lead Package (VQFN)
10/24/08
SIDE VIEW
Pin 1 ID
BOTTOM VIEW
TOP VIEW
Note: The ter mi nal #1 ID is a Laser -ma r ked Feat ur e .
D
E
e
K
A1
C
A
D2
E2
y
L
1
2
3
b
1
2
3
0.45 COMMON DIMENSIONS
(Unit of Measure = mm)
SYMBOL MIN NOM MAX NOTE
R 0.20
0.4 Ref
(4x)
A
A1
b
C
D
D2
E
E2
e
L
y
K
0.80 0.90 1.00
0.00 0.02 0.05
0.17 0.22 0.27
0.20 REF
3.95 4.00 4.05
2.35 2.40 2.45
3.95 4.00 4.05
2.35 2.40 2.45
0.45
0.35 0.40 0.45
0.00 – 0.08
0.20 – –
37.4 28-pin 28P3
Note:
ATmega328/P
Packaging Information
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 438
Note: For the most current package drawings, see the Microchip Packaging Specification located at http://
www.microchip.com/packaging
2325 Orchard Parkway
San Jose, CA 95131
TITLE DRAWING NO. REV.
28P3 , 28-lead (0.300"/7.62mm Wide) Plastic Dual
Inline Package (PDIP) B
28P3
09/28/01
PIN
1
E1
A1
B
REF
E
B1
C
L
SEATING PLANE
A
0º ~ 15º
D
e
eB
B2
(4 PLACES)
COMMON DIMENSIONS
(Unit of Measure = mm)
SYMBOL MIN NOM MAX NOTE
A – – 4.5724
A1 0.508 – –
D 34.544 – 34.798 Note 1
E 7.620 – 8.255
E1 7.112 – 7.493 Note 1
B 0.381 – 0.533
B1 1.143 – 1.397
B2 0.762 – 1.143
L 3.175 – 3.429
C 0.203 – 0.356
eB – – 10.160
e 2.540 TYP
Note: 1. Dimensions D and E1 do not include mold Flash or Protrusion.
Mold Flash or Protrusion shall not exceed 0.25mm (0.010").
ATmega328/P
Packaging Information
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 439
38. Errata
38.1 Errata ATmega328/P
The revision letter in this section refers to the revision of the ATmega328/P device.
38.1.1 Rev. D
1 – Analog MUX can be turned off when setting ACME bit
If the ACME (Analog Comparator Multiplexer Enabled) bit in ADCSRB is set
while MUX3 in ADMUX is '1' (ADMUX[3:0]=1xxx), all MUXes are turned off
until the ACME bit is cleared.
Fix/Workaround:
Clear the MUX3 bit before setting the ACME bit.
2 – TWI Data setup time can be too short
When running the device as a TWI slave with a system clock above 2MHz,
the data setup time for the first bit after ACK may in some cases be too
short. This may cause a false start or stop condition on the TWI line.
Fix/Workaround:
Insert a delay between setting TWDR and TWCR.
38.1.2 Rev. C
Not sampled.
38.1.3 Rev. B
1 – Analog MUX can be turned off when setting ACME bit
If the ACME (Analog Comparator Multiplexer Enabled) bit in ADCSRB is set
while MUX3 in ADMUX is '1' (ADMUX[3:0]=1xxx), all MUXes are turned off
until the ACME bit is cleared.
Fix/Workaround:
Clear the MUX3 bit before setting the ACME bit.
2 – Unstable 32kHz Oscillator
The 32kHz oscillator does not work as system clock. The 32kHz oscillator
used as asynchronous timer is inaccurate.
Fix/Workaround:
None.
38.1.4 Rev. A
1 – Unstable 32kHz Oscillator
The 32kHz oscillator does not work as system clock. The 32kHz oscillator
used as asynchronous timer is inaccurate.
Fix/Workaround:
None.
ATmega328/P
Errata
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 440
39. Datasheet Revision History
Please note that the referring page numbers in this section are referred to this document. The referring
revision in this section are referring to the document revision.
39.1 Rev. A – 2/2018
Section Changes
Full data sheet • Change of document style
• New Microchip document number DS40001984A replaces Atmel
42735B
• Added bit numbering in registers where this was missing
SPL and SPH • Added SP11
System Clock and Clock
Options
• Added input to the Clock Multiplexer from the Watchdog Oscillator in
Figure 13-1
• Added warning not to use Watchdog Oscillator (128 kHz Internal
Oscillator) as both Watchdog Timer and System Oscillator at the same
time
• Corrected capacitances in Table 13-8
39.2 Pre Microchip Revisions
39.2.1 Rev. B – 11/2016
1. Update I/O Multiplexing
2. Errata section updated
– ATmega328P: Removed die revision E to K:
• Die revision E to J was not sampled.
• Die revision K was not released to production.
– ATmega328: Removed die revision E to K:
• Die revision E to J was not sampled.
• Die revision K was not released to production.
39.2.2 Rev. A – 06/2016
Initial document release.
ATmega328/P
Datasheet Revision History
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 441
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• Microchip is willing to work with the customer who is concerned about the integrity of their code.
ATmega328/P
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 442
• Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their
code. Code protection does not mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the
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All other trademarks mentioned herein are property of their respective companies.
ATmega328/P
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 443
© 2018, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved.
ISBN: 978-1-5224-2686-8
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ATmega328/P
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 444
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Tel: 40-21-407-87-50
Spain - Madrid
Tel: 34-91-708-08-90
Fax: 34-91-708-08-91
Sweden - Gothenberg
Tel: 46-31-704-60-40
Sweden - Stockholm
Tel: 46-8-5090-4654
UK - Wokingham
Tel: 44-118-921-5800
Fax: 44-118-921-5820
Worldwide Sales and Service
© 2018 Microchip Technology Inc. Datasheet Complete DS40001984A-page 445
