DS2438AZ/T&R - Maxim Integrated

AVAILABLE

Functional Diagrams

Pin Configurations appear at end of data sheet.

Functional Diagrams continued at end of data sheet.

UCSP is a trademark of Maxim Integrated Products, Inc.

For pricing, delivery, and ordering information, please contact Maxim Direct

at 1-888-629-4642, or visit Maxim’s website at www.maximintegrated.com.

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070605

FEATURES

§ Unique 1-Wire® interface requires only one

port pin for communication

§ Provides unique 64-bit serial number

§ Eliminates thermistors by sensing battery

temperature on-chip

§ On-board A/D converter allows monitoring

of battery voltage for end-of-charge and end-

of-discharge determination

§ On-board integrated current accumulator

facilitates fuel gauging

§ Elapsed time meter in binary format

§ 40-byte nonvolatile user memory available

for storage of battery-specific data

§ Reverts to low-power sleep mode on battery

pack disconnect (feature disabled on

DS2438AZ)

§ Operating range -40ºC to +85ºC

§ Applications include portable computers,

portable/cellular phones, consumer

electronics, and handheld instrumentation

PIN ASSIGNMENT

PIN DESCRIPTION

DQ - Data In/Out

VAD - General A/D input

VSENS+ - Battery current monitor input (+)

VSENS- - Battery current monitor input (-)

VDD - Power Supply (2.4V to 10.0V)

GND - Ground

NC - No connect

DESCRIPTION

The DS2438 Smart Battery Monitor provides several functions that are desirable to carry in a battery

pack: a means of tagging a battery pack with a unique serial number, a direct-to-digital temperature

sensor which eliminates the need for thermistors in the battery pack, an A/D converter which measures

the battery voltage and current, an integrated current accumulator which keeps a running total of all

current going into and out of the battery, an elapsed time meter, and 40 bytes of nonvolatile EEPROM

memory for storage of important parameters such as battery chemistry, battery capacity, charging

methodology and assembly date. Information is sent to/from the DS2438 over a 1-Wire interface, so that

only one wire (and ground) needs to be connected from a central microprocessor to a DS2438. This

means that battery packs need only have three output connectors: battery power, ground, and the 1-Wire

interface.

Because each DS2438 contains a unique silicon serial number, multiple DS2438s can exist on the same

1-Wire bus. This allows multiple battery packs to be charged or used in the system simultaneously.

Applications for the smart battery monitor include portable computers, cellular telephones, and handheld

instrumentation battery packs in which it is critical to monitor real-time battery performance. Used in

conjunction with a microcontroller in the host system, the DS2438 provides a complete smart battery

pack solution that is fully chemistry-independent. The customization for a particular battery chemistry

and capacity is realized in the code programmed into the microcontroller and DS2438 EEPROM, and

only a software revision is necessary should a designer wish to change battery pack chemistry.

DS2438

Smart Battery Monito

GND

VSENS+

VSENS-

VDD

DS2438Z, DS2438AZ

8-Pin SOIC (150-mil)

DS2438

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ORDERING INFORMATION

PART MARKING PACKAGE INFORMATION

DS2438Z+ DS2438 8-Pin SOIC

DS2438Z+T&R DS2438 DS2438Z+ on Tape-and-Reel

DS2438AZ+ DS2438A 8-Pin SOIC

DS2438AZ+T&R DS2438A DS2438AZ+ on Tape-and-Reel

DS2438Z DS2438 8-Pin SOIC

DS2438Z/T&R DS2438 DS2438Z on Tape-and-Reel

DS2438AZ DS2438A 8-Pin SOIC

DS2438AZ/T&R DS2438A DS2438AZ on Tape-and-Reel

+ Denotes lead-free package.

DETAILED PIN DESCRIPTION

PIN SYMBOL DESCRIPTION

1 GND

Ground

2 VSENS+ Battery Input: connection for battery current to be monitored (see text)

3 VSENS- Battery Input: connection for battery current to be monitored (see text)

4 VAD ADC Input: input for general purpose A/D

5 VDD VDD Pin: input supply voltage

6, 7 NC No Connect

8 DQ Data Input/Out: for 1-Wire operation: Open drain

OVERVIEW

The block diagram of Figure 1 shows the seven major components of the DS2438:

1. 64-bit lasered ROM

2. temperature sensor

3. battery voltage A/D

4. battery current A/D

5. current accumulators

6. elapsed time meter

7. 40-byte nonvolatile user-memory

Each DS2438 contains a unique 64-bit lasered ROM serial number so that several battery packs can be

charged/monitored by the same host system. Furthermore, other Dallas products featuring the same

1-Wire bus architecture with a 64-bit ROM can reside on the same bus; refer to the Dallas Automatic

Identification Data book for the specifications of these products.

Communication to the DS2438 is via a 1-Wire port. With the 1-Wire port, the memory and control

functions will not be available until the ROM function protocol has been established. The master must

first provide one of four ROM function commands: 1) Read ROM, 2) Match ROM, 3) Search ROM, or 4)

Skip ROM. These commands operate on the 64-bit lasered ROM portion of each device and can singulate

a specific device if many are present on the 1-Wire line as well as to indicate to the bus master how many

and what types of devices are present. After a ROM function sequence has been successfully executed,

the memory and control functions are accessible and the master may then provide any one of the six

memory and control function commands.

DS2438

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Control function commands may be issued which instruct the DS2438 to perform a temperature

measurement or battery voltage A/D conversion. The result of these measurements will be placed in the

DS2438’s memory map, and may be read by issuing a memory function command which reads the

contents of the temperature and voltage registers. Additionally, the charging/discharging battery current is

measured without user intervention, and again, the last completed result is stored in DS2438 memory

space. The DS2438 uses these current measurements to update three current accumulators; the first stores

net charge for fuel gauge calculations, the second accumulates the total charging current over the life of

the battery, and the remaining accumulator tallies battery discharge current. The elapsed time meter data,

which can be used in calculating battery self-discharge or time-related charge termination limits, also

resides in the DS2438 memory map and can be extracted with a memory function command. The

nonvolatile user memory of the DS2438 consists of 40 bytes of EEPROM. These locations may be used

to store any data the user wishes and are written to using a memory function command. All data and

commands are read and written least significant bit first.

PARASITE POWER

The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” power

whenever the DQ pin is high. DQ will provide sufficient power as long as the specified timing and

voltage requirements are met (see the section titled “1-Wire Bus System”). The advantage of parasite

power is that the ROM may be read in absence of normal power, i.e., if the battery pack is completely

discharged.

DS2438 BLOCK DIAGRAM Figure 1

DS2438

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OPERATION-MEASURING TEMPERATURE

The DS2438 measures temperatures through the use of an on-board temperature measurement technique.

The temperature reading is provided in a 13-bit, two’s complement format, which provides 0.03125°C of

resolution. Table 1 describes the exact relationship of output data to measured temperature. The data is

transmitted serially over the 1-Wire interface. The DS2438 can measure temperature over the range of

-55°C to +125°C in 0.03125°C increments. For Fahrenheit usage, a lookup table or conversion factor

must be used.

Note that temperature is represented in the DS2438 in terms of a 0.03125°C LSb, yielding the following

13-bit format. The 3 least significant bits of the Temperature Register will always be 0. The remaining

13 bits contain the two’s complement representation of the temperature in °C, with the MSb holding the

sign (S) bit. See “Memory Map” section for the Temperature Register address location.

Temperature Register Format Table 1

2-1 2

-2 2

-3 2

-4 2

-5 0 0 0 LSB

MSb (unit = °C) LSb

S 26 2

5 2

4 2

3 2

2 2

1 2

0 MSB

TEMPERATURE DIGITAL OUTPUT (Binary) DIGITAL OUTPUT (Hex)

+125°C 01111101 00000000 7D00h

+25.0625°C 00011001 00010000 1910h

+0.5°C 00000000 10000000 0080h

0°C 00000000 00000000 0000h

-0.5°C 11111111 10000000 FF80h

-25.0625°C 11100110 11110000 E6F0h

-55°C 11001001 00000000 C900h

OPERATION-MEASURING BATTERY VOLTAGE

The on-board analog-to-digital converter (ADC) has 10 bits of resolution and will perform a conversion

when the DS2438 receives a command protocol (Convert V) instructing it to do so. The result of this

measurement is placed in the 2-byte Voltage Register. The range for the DS2438 ADC is 0 volt to

10 volt; this range is suitable for NiCd or NiMH battery packs up to six cells, and for lithium ion battery

packs of two cells. The full-scale range of the ADC is scaled to 10.23 volt, resulting in a resolution of

10 mV.

While the ADC has a range that extends down to 0 volt, it is important to note that the battery voltage can

also be the supply voltage to the DS2438. As such, the accuracy of the ADC begins to degrade below

battery voltages of 2.4 volt, and the ability to make conversions is limited by the operating voltage range

of the DS2438.

Voltage is expressed in this register in scaled binary format, as outlined in Table 2. Note that while codes

exist for values below 2.4 volt, accuracy of the ADC and the limitation on the DS2438’s supply voltage

make it unlikely that these values would be used in actual practice. See “Memory Map” section for the

Voltage Register address location.

DS2438

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VOLTAGE REGISTER FORMAT Table 2

27 2

6 2

5 2

4 2

3 2

2 2

1 2

0 LSB

MSb (unit = 10 mV) LSb

0 0 0 0 0 0 29 2

8 MSB

BATTERY

VOLTAGE

DIGITAL OUTPUT (Binary) DIGITAL OUTPUT (Hex)

0.05V 0000 0000 0000 0101 0005h

2.7V 0000 0001 0000 1110 010Eh

3.6V 0000 0001 0110 1000 0168h

5V 0000 0001 1111 0100 01F4h

7.2V 0000 0010 1101 0000 02D0h

9.99V 0000 0011 1110 0111 03E7h

10V 0000 0011 1110 1000 03E8H

For applications requiring a general purpose voltage A/D converter, the DS2438 can be configured so that

the result of a Convert V command will place the scaled binary representation of the voltage on the VAD

input (as opposed to the VDD input) into the Voltage Register in the same format described in Table 2.

Depending upon the state of the Status/Configuration Register, either (but not both) the VDD or VAD

voltage will be stored in the Voltage Register upon receipt of the Convert V command. Refer to the

description of the Status/Configuration Register in the Memory Map section for details. If the VAD input

is used as the voltage input, the A/D will be accurate for 1.5V < VAD < 2VDD over the range 2.4V < VDD <

5.0V. This feature gives the user the ability to have a voltage A/D that meets spec accuracy for inputs

over the entire range of 1.5V < VAD < 10V for VDD = 5.0V.

OPERATION - MEASURING BATTERY CURRENT

The DS2438 features an A/D converter that effectively measures the current flow into and out of the

battery pack by measuring the voltage across an external sense resistor. It does so in the background at a

rate of 36.41 measurements/sec; thus, no command is required to initiate current flow measurements.

However, the DS2438 will only perform current A/D measurements if the IAD bit is set to “1” in the

Status/Configuration Register. The DS2438 measures current flow in and out of the battery through the

VSENS pins; the voltage from the VSENS+ pin to the VSENS- pin is considered to be the voltage across the

current sense resistor, RSENS. The VSENS+ terminal may be tied directly to the RSENS resistor, however, for

VSENS-, we recommend use of an RC low pass filter between it and the GND end of RSENS (see the block

diagram in Figure 1). Using a 100 kW (min) resistor (RF) and a 0.1 mF tantalum capacitor (CF), the filter

cutoff is approximately 15.9 Hz. The current A/D measures at a rate of 36.41 times per second, or once

every 27.46 ms. This filter will capture the effect of most current spikes, and will thus allow the current

accumulators to accurately reflect the total charge which has gone into or out of the battery.

The voltage across current sense resistor RSENS is measured by the ADC and the result is placed in the

Current Register in two’s complement format. The sign (S) of the result, indicating charge or discharge,

resides in the most significant bit of the Current Register, as shown in Table 3. See “Memory Map” in

Figure 7 for the Current Register address location.

DS2438

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CURRENT REGISTER FORMAT Table 3

(This register actually stores the voltage measured across current sense resistor RSENS.

This value can be used to calculate battery pack current using the equation below.)

27 2

6 2

5 2

4 2

3 2

2 2

1 2

0 LSB

MSb (unit = 0.2441mV) LSb

S S S S S S 29 2

8 MSB

The battery pack current is calculated from the Current Register value using the equation:

= Current Register / (4096 * RSENS) (where RSENS is in W)

For example, if 1.25A is flowing into the pack, and the pack uses a 0.025W sense resistor, the DS2438

will write the value 12810 to the Current Register. From this value, battery pack current can be calculated

to be:

I = 128 / ( 4096 * 0.025) = 1.25A

Because small current ADC offset errors can have a large cumulative effect when current is integrated

over time, the DS2438 provides a method for canceling offset errors in the current ADC. After each

current measurement is completed, the measured value is added to the contents of the Offset Register and

the result is then stored in the Current Register. The Offset Register is a two-byte nonvolatile read/write

offset, as shown in Table 4.

OFFSET REGISTER FORMAT Table 4

24 2

3 2

2 2

1 2

0 0 0 0 LSB

MSb (unit = 0.2441 mV) LSb

X X X S 28 2

7 2

6 2

5 MSB

The following process can be used to calibrate the current ADC:

1. Write all zeroes to the Offset Register

2. Force zero current through RSENS

3. Read the Current Register value

4. Disable the current ADC by setting the IAD bit in the Status/Configuration Register to “0”

5. Change the sign of the previously-read Current Register value by performing the two’s complement

and write the result to the Offset Register

6. Enable the current ADC by setting the IAD bit in the Status/Configuration Register to “1”

NOTE:

When writing to the Offset Register, current measurement MUST be disabled (IAD bit set to “0”).

The current ADC calibration process is done for each DS2438 device prior to shipment. However, for

best results, battery pack manufacturers should calibrate the current ADC during initial battery pack

testing, and the host system should calibrate whenever possible (during battery charging, for example).

DS2438

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OPERATION - CURRENT ACCUMULATORS

The DS2438 tracks the remaining capacity of a battery using the Integrated Current Accumulator (ICA).

The ICA maintains a net accumulated total of current flowing into and out of the battery; therefore, the

value stored in this register is an indication of the remaining capacity in a battery and may be used in

performing fuel gauge functions. In addition, the DS2438 has another register that accumulates only

charging (positive) current (CCA) and one that accumulates only discharging (negative) current (DCA).

The CCA and DCA give the host system the information needed to determine the end of life of a

rechargeable battery, based on total charge/discharge current over its lifetime.

The current measurement described above yields the voltage across sense resistor RSENS measured every

27.46 ms. This value is then used to increment or decrement the ICA register, increment the CCA (if

current is positive), or increment the DCA (if current is negative). The ICA is a scaled 8-bit volatile

binary counter that integrates the voltage across RSENS over time. The ICA is only

incremented/decremented if the IAD bit is set to 1 in the Status/Configuration Register. Table 5 illustrates

the contents of the ICA. See Memory Map section for the address location of the ICA.

ICA REGISTER FORMAT Table 5

(This register accumulates the voltage measured across current sense resistor RSENS. This

value can be used to calculate remaining battery capacity using the equation below.)

27 2

6 2

5 2

4 2

3 2

2 2

1 2

MSb (unit = 0.4882 mVhr) LSb

Remaining battery capacity is calculated from the ICA value using the equation:

Remaining Capacity

= ICA / (2048 * RSENS) (where RSENS is in W)

For example, if a battery pack has 0.625 Ahr of remaining capacity, and the pack uses a 0.025W sense

resistor, the ICA will contain the value 32. From this value, remaining capacity can be calculated to be:

Remaining Capacity = 32 / ( 2048 * 0.025) = 0.625 Ahr

Since the accuracy of the current ADC is +2 LSb, measurements of very small currents can be inaccurate

by a high percentage. Because these inaccuracies can turn into large ICA errors when accumulated over a

long period of time, the DS2438 provides a method for filtering out potentially erroneous small signals so

that they are not accumulated. The DS2438’s Threshold Register specifies a current measurement

magnitude (after offset cancellation) above which the measurement is accumulated in the ICA, CCA and

DCA and below which it is not accumulated. The format of the Threshold Register is shown in Table 6.

The power-on default Threshold Register value is 00h (no threshold).

NOTE:

When writing to the Threshold Register, current measurement must be disabled (IAD bit set to “0”).

DS2438

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THRESHOLD REGISTER FORMAT Table 6

TH2 TH1 0 0 0 0 0 0

MSb LSb

TH2 TH1 THRESHOLD

0 0 None (default)

0 1 ±2 LSB

1 0 ±4 LSB

1 1 ±8 LSB

The Charging Current Accumulator (CCA) is a two-byte nonvolatile read/write counter which represents

the total charging current the battery has encountered in its lifetime. It is only updated when current

through RSENS, is positive; i.e., when the battery is being charged. Similarly, the Discharge Current

Accumulator (DCA) is a two-byte nonvolatile counter which represents the total discharging current the

battery has encountered over its lifetime.

The CCA and DCA can be configured to function in any of three modes: disabled, enabled with shadow-

to-EEPROM, and enabled without shadow-to-EEPROM. When the CCA and DCA are disabled (by

setting either the IAD bit or the CA bit in the Status/Configuration Register to “0”), the memory in page

07h is free for general purpose data storage. When the CCA and DCA are enabled (by setting both IAD

and CA to “1”), page 07h is reserved for these registers, and none of the bytes in page 07h should be

written to via the 1-Wire bus. When the CCA and DCA are enabled, their values are automatically

shadowed to EEPROM memory by setting the EE bit in the Status/Configuration Register to “1”. When

these registers are configured to shadow to EEPROM, the information will accumulate over the lifetime

of the battery pack and will not be lost when the battery becomes discharged. Shadow-to-EEPROM is

disabled when the EE bit is “0”. Table 7 illustrates the format of the CCA and DCA registers. Table 8

summarizes the modes of operation for ICA, CCA and DCA.

CCA/DCA REGISTER FORMAT Table 7

27 2

6 2

5 2

4 2

3 2

2 2

1 2

0 LSB

MSb (unit = 15.625 mVHr) LSb

215 2

14 2

13 2

12 2

11 2

10 2

9 2

8 MSB

ICA/CCA/DCA MODES OF OPERATION Table 8

IAD Bit CA Bit EE Bit ICA CCA/DCA CCA/DCA Copy-

to-EEPROM

0 X X Inactive Inactive Inactive

1 0 X Active Inactive Inactive

1 1 0 Active Active Inactive

1 1 1 Active Active Active

DS2438

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Figure 2 illustrates the activity of the ICA, CCA, and DCA over a sample charge/discharge cycle of a

battery pack, assuming the DS2438 is configured for the ICA to function and the CCA/DCA to function

and shadow data to EEPROM. To simplify the illustration of the accumulators, they are treated as analog

values, although they are digital counters in the DS2438. Note that when the battery becomes fully

discharged, i.e., the ICA value reaches 0, the CCA and DCA register values are maintained.

CURRENT ACCUMULATOR ACTIVITY Figure 2

SENSE RESISTOR SELECTION

The selection of RSENS involves a tradeoff. On the one hand, the impedance of RSENS must be minimized

to avoid excessive voltage drop during peak current demands. On the other hand, the impedance of RSENS

should be maximized to achieve the finest resolution for current measurement and accumulation. Table 9

below lists several example RSENS values, the LSb of the current calculation ( 1/(4096 * RSENS) ) and the

LSb of the remaining capacity calculation ( 1/(2048 * RSENS) ). The user should carefully consider

voltage drop at maximum current and required current measurement/accumulation resolution when

selecting RSENS.

SENSE RESISTOR TRADEOFFS Table 9

SENSE RESISTOR

VALUE (RSENS)

CURRENT lsb

REMAINING

CAPACITY lsb

MAX REMAINING

CAPACITY VALUE

25 mW 9.76 mA 19.53 mAHr 5000 mAhr

50 mW 4.88 mA 9.76 mAHr 2500 mAhr

100 mW 2.44 mA 4.88 mAHr 1250 mAhr

200 mW 1.22 mA 2.44 mAHr 625 mAhr

OPERATION - ELAPSED TIME METER

An internal oscillator is used as the timebase for the timekeeping functions. The elapsed time functions

are double buffered, allowing the master to read elapsed time without the data changing while it is being

read. To accomplish this, a snapshot of the counter data is transferred to holding registers which the user

accesses. This occurs after the 8th bit of the Recall Memory command.

The elapsed time meter (ETM) is a 4-byte binary counter with 1-second resolution. The ETM can

accumulate 136 years of seconds before rolling over. Time/date is represented by the number of seconds

since a reference point, which is determined by the user. For example, 12:00 A.M., January 1, 1970 could

be used as a reference point.

DS2438

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Two other time-related functions are available. The first is the Disconnect Timestamp, which is written to

by the DS2438 whenever it senses that the DQ line has been low for approximately 2 seconds. This

condition would signal that the battery pack has been removed from the system; the time when that

occurs is written into the Disconnect Timestamp register, so that upon replacement into the system, the

system can determine how long the device has been in storage, to facilitate self-discharge corrections to

the remaining battery capacity. After the disconnect has been detected, the DS2438 reverts to a sleep

mode, during which nothing is active except the real time clock. Some applications may prefer that the

data converters and current accumulators continue operation following a pack disconnect. Thus, a

version of the DS2438 (part number DS2438A) is offered for those applications. Other than not reverting

to a low-power sleep mode following disconnect, there are no specification differences between the

DS2438 and the DS2438A.

The other timestamp is the End-of-Charge timestamp, which is written to by the DS2438 whenever it

senses that charging is finished (when current changes direction). This timestamp allows the user to

calculate the amount of time the battery has been in a discharge or storage state, again to facilitate self-

discharge calculations.

The format of the ETM, Disconnect, and End-of-Charge registers are as shown in Table 10. Refer to the

“Memory Map” section for the address location of the time-related registers.

TIME REGISTER FORMAT Table 10

27 2

6 2

5 2

4 2

3 2

2 2

1 2

0 LSB

MSb (unit = 1s) LSb

215 2

14 2

13 2

12 2

11 2

10 2

9 2

MSb (unit = 1s) LSb

223 2

22 2

21 2

20 2

19 2

18 2

17 2

MSb (unit = 1s) LSb

231 2

30 2

29 2

28 2

27 2

26 2

25 2

24 MSB

64-BIT LASERED ROM

Each DS2438 contains a unique ROM code that is 64 bits long. The first 8 bits are a 1-Wire family code

(DS2438 code is 26h). The next 48 bits are a unique serial number. The last 8 bits are a CRC of the first

56 bits. (See Figure 3.) The 64-bit ROM and ROM Function Control section allow the DS2438 to operate

as a 1-Wire device and follow the 1-Wire protocol detailed in the section “1-Wire Bus System.” The

functions required to control sections of the DS2438 are not accessible until the ROM function protocol

has been satisfied. This protocol is described in the ROM function protocol flow chart (Figure 5). The

1-Wire bus master must first provide one of four ROM function commands: 1) Read ROM, 2) Match

ROM, 3) Search ROM, or 4) Skip ROM. After a ROM function sequence has been successfully executed,

the functions specific to the DS2438 are accessible and the bus master may then provide any one of the

six memory and control function commands.

DS2438

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64-BIT LASERED ROM FORMAT Figure 3

8-BIT CRC CODE 48-BIT SERIAL NUMBER 8-BIT FAMILY CODE (26h)

MSb LSb MSb LSb MSb LSb

CRC Generation

The DS2438 has an 8-bit CRC stored in the most significant byte of the 64-bit ROM. The bus master can

compute a CRC value from the first 56 bits of the 64-bit ROM and compare it to the value stored within

the DS2438 to determine if the ROM data has been received error-free by the bus master. The equivalent

polynomial function of this CRC is:

CRC = X8 + X5 + X4 +1

The DS2438 also generates an 8-bit CRC value using the same polynomial function shown above and

provides this value to the bus master to validate the transfer of data bytes. In each case where a CRC is

used for data transfer validation, the bus master must calculate a CRC value using the polynomial

function given above and compare the calculated value to either the 8-bit CRC value stored in the 64-bit

ROM portion of the DS2438 (for ROM reads) or the 8-bit CRC value computed within the DS2438

(which is read as a 9th byte when a scratchpad is read). The comparison of CRC values and decision to

continue with an operation are determined entirely by the bus master. There is no circuitry inside the

DS2438 that prevents a command sequence from proceeding if the CRC stored in or calculated by the

DS2438 does not match the value generated by the bus master. Proper use of the CRC as outlined in the

flowchart of Figure 6 can result in a communication channel with a very high level of integrity.

The 1-Wire CRC can be generated using a polynomial generator consisting of a shift register and XOR

gates as shown in Figure 4. Additional information about the Dallas 1-Wire Cyclic Redundancy Check is

available in Application Note 27 entitled “Understanding and Using Cyclic Redundancy Checks with

Dallas Semiconductor Touch Memory Products.”

The shift register bits are initialized to 0. Then starting with the least significant bit of the family code,

1 bit at a time is shifted in. After the 8th bit of the family code has been entered, the serial number is

entered. After the 48th bit of the serial number has been entered, the shift register contains the CRC

value.

1-WIRE CRC CODE Figure 4

DS2438

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ROM FUNCTIONS FLOWCHART Figure 5

DS2438

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MEMORY/CONTROL FUNCTIONS FLOWCHART Figure 6

YES

DS2438

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MEMORY/CONTROL FUNCTIONS FLOWCHART Figure 6 (continued)

MEMORY MAP

The DS2438’s memory is organized as shown in Figure 7. The memory consists of a scratchpad RAM

and storage SRAM/EEPROM. The scratchpad helps insure data integrity when communicating over the

1-Wire bus. Data is first written to the scratchpad where it can be read back. After the data has been

verified, a copy scratchpad command will transfer the data to the appropriate page in memory (pages 0-2

are primarily volatile SRAM, pages 3-7 are EEPROM). This process insures data integrity when

modifying the memory.

The DS2438’s memory is organized as 64 bytes of memory, in eight 8-byte pages. Each page has its own

scratchpad space, organized as 8 bytes of memory. When reading a scratchpad, there is a 9th byte which

may be read with a Read Scratchpad command. This byte contains a cyclic redundancy check (CRC)

byte, which is the CRC over all of the 8 bytes in the currently selected scratchpad. This CRC is

implemented in the fashion described in the section titled “CRC Generation.”

DS2438

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Page 0 (00h)

The first page contains the most frequently accessed information of the DS2438, and most locations are

volatile read-only bytes with the exception of the Status/Configuration Register (Byte 0) and the

Threshold Register (Byte 7). The Status/Configuration Register is a nonvolatile read/write byte which

defines which features of the DS2438 are enabled and how they will function. The register is formatted as

follows:

X ADB NVB TB AD EE CA IAD

MSb LSb

IAD = Current A/D Control Bit. “1” = the current A/D and the ICA are enabled, and current

measurements will be taken at the rate of 36.41 Hz; “0” = the current A/D and the ICA have been

disabled. The default value of this bit is a “1” (current A/D and ICA are enabled).

CA = Current Accumulator Configuration. “1” = CCA/DCA is enabled, and data will be stored and can

be retrieved from page 7, bytes 4-7; “0” = CCA/DCA is disabled, and page 7 can be used for general

EEPROM storage. The default value of this bit is a “1” (current CCA/DCA are enabled).

EE = Current Accumulator Shadow Selector bit. “1” = CCA/DCA counter data will be shadowed to

EEPROM each time the respective register is incremented; “0”= CCA/DCA counter data will not be

shadowed to EEPROM. The CCA/DCA could be lost as the battery pack becomes discharged. If the CA

bit in the status/configuration register is set to “0”, the EE bit will have no effect on the DS2438

functionality. The default value of this bit is a “1” (current CCA/DCA data shadowed to EEPROM).

AD = Voltage A/D Input Select Bit. “1” = the battery input (VDD) is selected as the input for the

DS2438 voltage A/D converter; “0” = the general purpose A/D input (VAD) is selected as the voltage

A/D input. For either setting, a Convert V command will initialize a voltage A/D conversion. The default

value of this bit is a “1” (VDD is the input to the A/D converter).

TB = Temperature Busy Flag. “1” = temperature conversion in progress; “0” = temperature conversion

complete.

NVB = Nonvolatile Memory Busy Flag. “1” = Copy from Scratchpad to EEPROM in progress; “0” =

Nonvolatile memory not busy. A copy to EEPROM may take from 2 ms to 10 ms (taking longer at lower

supply voltages).

ADB = A/D Converter Busy Flag. “1” = A/D conversion in progress on battery voltage; “0” = conversion

complete, or no measurement being made. An A/D conversion takes approximately 10 ms.

X = Don’t care

Bytes 1 and 2 of page 0 contain the last completed temperature conversion in the format described in the

“Operation - Measuring Temperature” section. Bytes 3-4 contain the last completed voltage A/D

conversion result and bytes 5-6 contain the instantaneous current data. Byte 7 contains the Threshold

NOTE:

The data in the scratchpad of the status and threshold register will determine the operation of the device.

DS2438

16 of 29

Page 1 (01h)

The second page, Page 1, contains the ICA, elasped time meter, and current offset data. Both the ETM

and ICA are volatile read/write locations so that they may be set, changed, or cleared by the host

software. Bytes 0-3 contain the ETM data, formatted as described in the “Operation - Elapsed Time

Meter” section. Byte 4 contains the 8-bit ICA. Bytes 5 and 6 contain the Offset Register data. Byte 7 is

reserved and will read out as all “1”s.

Page 2 (02h)

The third page of memory (Page 2) contains the Disconnect (first 4 bytes) and End of Charge (remaining

4 bytes) timestamps. This page is volatile and read/write. Refer to the “Operation – Elapsed Time Meter”

section for the formatting of these locations.

Pages 3-7 (03h - 07h)

The remainder of the memory in the DS2438 (Pages 3 through 7) is backed with EEPROM. This memory

provides 40 bytes of user memory which may be used to carry any information the user wishes to store.

Additionally, the CCA/DCA information is stored in bytes 4-7 of page 7 if the DS2438 is configured

appropriately. If the CCA/DCA is used, page 7 should not be written to or current accumulator data will

be overwritten. See “Operation-Current Accumulators” for details.

MEMORY MAP Figure 7

PAGE BYTE CONTENTS R/W NV PAGE BYTE CONTENTS R/W NV

0 STATUS/

CONFIGURATION

R/W YES 0 USER BYTE R/W YES

1 TEMPERATURE

LSB

R NO 1 USER BYTE R/W YES

2 TEMPERATURE

MSB

R NO 2 USER BYTE R/W YES

0 3 VOLTAGE LSB R NO 3 3 USER BYTE R/W YES

4 VOLTAGE MSB R NO 4 USER BYTE R/W YES

5 CURRENT LSB R NO 5 USER BYTE R/W YES

6 CURRENT MSB R NO 6 USER BYTE R/W YES

7 THRESHOLD R/W YES 7 USER BYTE R/W YES

0 ETM BYTE 0 R/W NO 0 USER BYTE R/W YES

1 ETM BYTE 1 R/W NO 1 USER BYTE R/W YES

2 ETM BYTE 2 R/W NO 2 USER BYTE R/W YES

1 3 ETM BYTE 3 R/W NO 3 USER BYTE R/W YES

4 ICA R/W NO 4 4 USER BYTE R/W YES

5 OFFSET LSB R/W YES 5 USER BYTE R/W YES

6 OFFSET MSB R/W YES 6 USER BYTE R/W YES

7 RESERVED 7 USER BYTE R/W YES

R/W NO

0 DISCONNECT

BYTE 0

R/W NO · · · · ·

1 DISCONNECT

BYTE 1

R/W NO · · · · ·

2 DISCONNECT

BYTE 2

R/W NO · · · · ·

2 3 DISCONNECT

BYTE 3

R/W NO 0 USER BYTE R/W YES

4 END OF CHARGE

BYTE 0

R/W NO 1 USER BYTE R/W YES

5 END OF CHARGE

BYTE 1

R/W NO 2 USER BYTE R/W YES

6 END OF CHARGE

BYTE 2

R/W NO 7 3 USER BYTE R/W YES

7 END OF CHARGE

BYTE 3

R/W NO 4 USER BYTE/

CCA LSB

R/W YES

5 USER BYTE/

CCA MSB

R/W YES

6 USER BYTE/

DCA LSB

R/W YES

7 USER BYTE/

DCA MSB

R/W YES

DS2438

17 of 29

1-Wire BUS SYSTEM

The 1-Wire bus is a system which has a single bus master and one or more slaves. The DS2438 behaves

as a slave. The discussion of this bus system is broken down into three topics: hardware configuration,

transaction sequence, and 1-Wire signaling (signal types and timing).

HARDWARE CONFIGURATION

The 1-Wire bus has only a single line by definition; it is important that each device on the bus be able to

drive it at the appropriate time. To facilitate this, each device attached to the 1-Wire bus must have open

drain or 3-state outputs. The 1-Wire port of the DS2438 (DQ pin) is open drain with an internal circuit

equivalent to that shown in Figure 8. A multidrop bus consists of a 1-Wire bus with multiple slaves

attached. The 1-Wire bus requires a pull-up resistor of approximately 5 kW.

HARDWARE CONFIGURATION Figure 8

The idle state for the 1 wire bus is high. If for any reason a transaction needs to be suspended, the bus

MUST be left in the idle state if the transaction is to resume. Infinite recovery time can occur between

bits so long as the 1-Wire bus is in the inactive (high) state during the recovery period. If this does not

occur and the bus is left low, all components on the bus will be reset. See Wire-1 Reset Pulse Timing

(Figure 9).

TRANSACTION SEQUENCE

The protocol for accessing the DS2438 via the 1-Wire port is as follows:

§ Initialization

§ ROM Function Command

§ Memory Function Command

§ Transaction/Data

INITIALIZATION

All transactions on the 1-Wire bus begin with an initialization sequence. The initialization sequence

consists of a reset pulse transmitted by the bus master followed by presence pulse(s) transmitted by the

slave(s). The presence pulse lets the bus master know that the DS2438 is on the bus and is ready to

operate. For more details, see the “1-Wire Signaling” section.

DS2438

18 of 29

ROM FUNCTION COMMANDS

Once the bus master has detected a presence, it can issue one of the four ROM function commands. All

ROM function commands are 8-bits long. A list of these commands follows (refer to flowchart in

Figure 5):

Read ROM [33h]

This command allows the bus master to read the DS2438’s 8-bit family code (26h), unique 48-bit serial

number, and 8-bit CRC. This command can only be used if there is a single DS2438 on the bus. If more

than one slave is present on the bus, a data collision will occur when all slaves try to transmit at the same

time (open-drain will produce a wired-AND result).

Match ROM [55h]

The Match ROM command, followed by a 64-bit ROM sequence, allows the bus master to address a

specific DS2438 on a multidrop bus. Only the DS2438 that exactly matches the 64-bit ROM sequence

will respond to the following memory function command. All slaves that do not match the 64-bit ROM

sequence will wait for a reset pulse. This command can be used with a single or multiple devices on the

bus.

Skip ROM [CCh]

This command can save time in a single-drop bus system by allowing the bus master to access the

memory functions without providing the 64-bit ROM code. If more than one slave is present on the bus

and a read command is issued following the Skip ROM command, data collision will occur on the bus as

multiple slaves transmit simultaneously (open drain pull-downs will produce a wired-AND result).

Search ROM [F0h]

When a system is initially brought up, the bus master might not know the number of devices on the 1-

Wire bus or their 64-bit ROM codes. The search ROM command allows the bus master to use a process

of elimination to identify the 64-bit ROM codes of all slave devices on the bus.

Example of a ROM Search

The ROM search process is the repetition of a simple three-step routine: read a bit, read the complement

of the bit, then write the desired value of that bit. The bus master performs this simple, three-step routine

on each bit of the ROM. After one complete pass, the bus master knows the contents of the ROM in one

device. The remaining number of devices and their ROM codes may be identified by additional passes.

The following example of the ROM search process assumes four different devices are connected to the

same 1-Wire bus. The ROM data of the four devices is as shown (LSb first):

ROM1 = 00110101...

ROM2 = 10101010...

ROM3 = 11110101...

ROM4 = 00010001...

The search process is as follows:

1. The bus master begins the initialization sequence by issuing a reset pulse. The slave devices respond

by issuing simultaneous presence pulses.

2. The bus master will then issue the search ROM command on the 1-Wire bus (F0h).

DS2438

19 of 29

3. The bus master reads a bit from the 1-Wire bus. Each device will respond by placing the value of the

first bit of their respective ROM data onto the 1-Wire bus. ROM1 and ROM4 will place a 0 onto the

1-Wire bus, i.e., pull it low. ROM2 and ROM3 will place a 1 onto the 1-Wire bus by allowing the line

to stay high. The result is the logical AND of all devices on the line; therefore the bus master sees a 0.

The bus master reads another bit. Since the Search ROM data command is being executed, all of the

devices on the 1-Wire bus respond to this second read by placing the complement of the first bit of

their respective ROM data onto the 1-Wire bus. ROM1 and ROM4 will place a 1 onto the 1-wire,

allowing the line to stay high. ROM2 and ROM3 will place a 0 onto the 1-wire, thus it will be pulled

low. The bus master again observes a 0 for the complement of the first ROM data bit. The bus master

has determined that there are some devices on the 1-Wire bus that have a 0 in the first position and

others that have a 1.

The data obtained from the two reads of the three-step routine have the following interpretations:

00 - There are still devices attached which have conflicting bits in this position.

01 - All devices still coupled have a 0 bit in this bit position.

10 - All devices still coupled have a 1 bit in this bit position.

11 - There are no devices attached to the 1-Wire bus.

4. The bus master writes a 0. This deselects ROM2 and ROM3 for the remainder of this search pass,

leaving only ROM1 and ROM4 connected to the 1-Wire bus.

5. The bus master performs two more reads and receives a 0 bit followed by a 1 bit. This indicates that

all devices still coupled to the bus have 0s as their second ROM data bit.

6. The bus master then writes a 0 to keep both ROM1 and ROM4 coupled.

7. The bus master executes two reads and receives two 0 bits. This indicates that both 1 bits and 0 bits

exist as the 3rd bit of the ROM data of the attached devices.

8. The bus master writes a 0 bit. This deselects ROM1, leaving ROM4 as the only device still connected.

9. The bus master reads the remainder of the ROM bits for ROM4 and continues to access the part if

desired. This completes the first pass and uniquely identifies one part on the 1-Wire bus.

10. The bus master starts a new ROM search sequence by repeating steps 1 through 7.

11. The bus master writes a 1 bit. This decouples ROM4, leaving only ROM1 still coupled.

12. The bus master reads the remainder of the ROM bits for ROM1 and communicates to the underlying

logic if desired. This completes the second ROM search pass, in which another of the ROMs was

found.

13. The bus master starts a new ROM search by repeating steps 1 through 3.

14. The bus master writes a 1 bit. This deselects ROM1 and ROM4 for the remainder of this search pass,

leaving only ROM2 and ROM3 coupled to the system.

15. The bus master executes two read time slots and receives two 0s.

DS2438

20 of 29

16. The bus master writes a 0 bit. This decouples ROM3, and leaving only ROM2.

17. The bus master reads the remainder of the ROM bits for ROM2 and communicates to the underlying

logic if desired. This completes the third ROM search pass, in which another of the ROMs was found.

18. The bus master starts a new ROM search by repeating steps 13 through 15.

19. The bus master writes a 1 bit. This decouples ROM2, leaving only ROM3.

20. The bus master reads the remainder of the ROM bits for ROM3 and communicates to the underlying

logic if desired. This completes the fourth ROM search pass, in which another of the ROMs was

found.

Note that the bus master learns the unique ID number (ROM data pattern) of one 1-Wire device on each

ROM Search operation. The time required to derive the part’s unique ROM code is:

960 ms + (8 + 3 x 64) 61 ms = 13.16 ms

The bus master is therefore capable of identifying 75 different 1-Wire devices per second.

MEMORY COMMAND FUNCTIONS

The following command protocols are summarized in Table 11, and by the flowchart of Figure 6.

Write Scratchpad [4Ehxxh]

This command writes to the scratchpad page xxh of the DS2438. The entire 8-byte scratchpad space may

be written, but all writing begins with the byte present at address 0 of the selected scratchpad. After

issuing this command, the user must send the page number of the scratchpad to be written; then the user

may begin writing data to the DS2438 scratchpad. Writing may be terminated at any point by issuing a

reset. Valid page numbers for writing are 00h-07h.

Read Scratchpad [BEhxxh]

This command reads the contents of the scratchpad page xxh on the DS2438. After issuing this command,

the user must send the page number of the scratchpad to be read, and then may begin reading the data,

always beginning at address 0 of the selected scratchpad. The user may read through the end of the

scratchpad space (byte 07h), with any reserved data bits reading all logic 1s, then read the CRC of the

data, and after which the data read will be all logic 1s. If not all locations are to be read, the master may

issue a reset to terminate reading at any time. Valid page numbers are 00h - 07h.

Copy Scratchpad [48hxxh]

This command copies the scratchpad page xxh into the EEPROM / SRAM memory page xxh of the

DS2438. After issuing this command, the user must write a page number to direct which page of memory

the scratchpad is to be copied. Valid page numbers are 00h - 07h. During the copy function, the NVB bit

in the Status/Configuration register will be set to a “l”. When the copy is complete, this bit will reset to

“0”. If the bus master issues read time slots following this command, the DS2438 will output “0” on the

bus as long as it is busy copying the scratchpad to SRAM/EEPROM; it will return a “1” when the copy

process is complete.

DS2438

21 of 29

Recall Memory [B8hxxh]

This command recalls the stored values in EEPROM / SRAM page xxh to the scratchpad page xxh. This

command must proceed a Read SPxx command in order to read any page of memory on the DS2438.

Valid page numbers are 00h - 07h.

Convert T [44h]

This command begins a temperature conversion. No further data is required. The temperature conversion

will be performed, setting the TB flag in the Status/Configuration register to a “1” during conversion.

When the temperature conversion is done, the TB flag will clear to a “0”. If the bus master issues read

time slots following this command, the DS2438 will output “0” on the bus as long as it is busy making a

temperature conversion; it will return a “1” when the temperature conversion is complete.

Convert V [B4h]

This command instructs the DS2438 to initiate a voltage analog-to-digital conversion cycle. This sets the

ADB flag (see Status/Configuration register discussion in the Memory Map section). The voltage supply

that is measured is defined by the AD bit of the Status/Configuration register. When the A/D conversion

is done, the ADB flag is cleared and the current voltage value is placed in the VOLTAGE REGISTER of

page 00h. While an A/D conversion is taking place, all other memory functions are still available for use.

If the bus master issues read time slots following this command, the DS2438 will output “0” on the bus as

long as it is busy making a voltage measurement; it will return a “1” when the conversion is complete.

DS2438 COMMAND SET Table 11

INSTRUCTION DESCRIPTION PROTOCOL 1-WIRE BUS

MASTER STATUS

AFTER ISSUING

PROTOCOL

1-WIRE BUS

DATA AFTER

ISSUING

PROTOCOL

MEMORY COMMANDS

Read Scratchpad Reads bytes from

DS2438 Scratchpad

page xxh

BEh <page

00h-07h>

Rx <read up to nine

bytes of data>

Write Scratchpad Writes bytes to

DS2438

Scratchpad page xxh

4Eh <page

00h-07h>

Tx <write up to eight

bytes of data>

Copy Scratchpad Copies entire contents

of Scratchpad page

xxh

to 8-byte EEPROM/

SRAM page xxh

48h<page

00h-07h>

Idle or Rx of NVB bit {NVB bit in Status

copy complete (2-

10 ms, typ)}

Recall Memory Copies entire contents

of EEPROM/SRAM

page xxh to

Scratchpad

page xxh

B8h<page

00h-07h>

Idle Idle

Convert T Initiates temperature

conversion

44h Idle or Rx of TB bit {TB bit in Status

conversion

complete}

Convert V Initiates voltage A/D

conversion

B4h Idle or Rx of ADB bit {ADB bit in Status

conversion

complete}

DS2438

22 of 29

NOTES:

1. Temperature conversion takes up to 10 ms.

2. A/D conversion takes up to 4 ms.

3. EEPROM writes take up to 10 ms.

SAMPLE COMMAND SEQUENCE Table 12

Example: Bus Master enables the ICA, CCA, and DCA on a single DS2438 and configures it such that

the CCA/DCA information is shadowed to EEPROM. The voltage A/D is configured such that the

DS2438 will perform voltage measurements on the battery (VDD) voltage.

MASTER MODE DATA (LSB FIRST) COMMENTS

TX Reset Read pulse

RX Presence Presence pulse

TX CCh Skip ROM

TX 4Eh00h Issue Write SP 00h command

TX 0Fh Sets ICA, CA, EE, AD Bits active

TX Reset Reset pulse

RX Presence Presence pulse

TX CCh Skip ROM

TX BEh00h Issue Read SP 00h command

RX <9 data bytes> Read scratchpad data and CRC

TX Reset Reset pulse

RX Presence Presence pulse

TX CCh Skip ROM

TX 48h00h Issue Copy SP 00h command

RX Read Slots DS2438 returns a “1” when Copy SP is complete

TX Reset Reset pulse

RX Presence Presence Pulse, done

DS2438

23 of 29

SAMPLE COMMAND SEQUENCE Table 13

Example: Bus Master issues a temperature and voltage conversion, then reads the temperature, battery

voltage, battery current, all on a single DS2438.

MASTER MODE DATA (LSB FIRST) COMMENTS

TX Reset Reset pulse

RX Presence Presence pulse

TX CCh Skip ROM

TX 44h Issue Convert Temperature command, Read Slots

TX Reset Reset pulse

RX Presence Presence pulse

TX CCh Skip ROM

TX B4h Issue Convert Voltage command, Read Slots

TX Reset Reset pulse

RX Presence Presence pulse

TX CCh Skip ROM

TX B8h00h Issue Recall Memory page 00h command

TX Reset Reset pulse

RX Presence Presence pulse

TX CCh Skip ROM

TX BEh00h Issue Read SP 00h command

RX <9 data bytes> Read scratchpad data and CRC. This page contains

temperature, voltage, and current measurements.

TX Reset Reset pulse

RX Presence Presence pulse, done

DS2438

24 of 29

SAMPLE COMMAND SEQUENCE Table 14

Example: Assuming a single DS2438 is configured for its current accumulators to function, this sequence

allows the Bus Master to read the three current accumulators.

MASTER MODE DATA (LSB FIRST) COMMENTS

TX Reset Reset pulse

RX Presence Presence pulse

TX CCh Skip ROM

TX B8h01h Issue Recall Memory page 01h command

TX Reset Reset pulse

RX Presence Presence pulse

TX CCh Skip ROM

TX BEh01h Issue Read SP 01h command

RX <9 data bytes> Read scratchpad data and CRC. The ICA is

located in byte 04h

TX Reset Reset pulse

RX Presence Presence pulse

TX CCh Skip ROM

TX B8h07h Issue Recall Memory page 07h command

TX Reset Reset pulse

RX Presence Presence pulse

TX CCh Skip ROM

TX B8h07h Issue Read SP 07h command

RX <9 data bytes> Read scratchpad data and CRC. The CCA is

located in bytes 04h-05h and the DCA is located in

bytes 06h-07h.

TX Reset Reset pulse

RX Presence Presence pulse, done

I/O SIGNALING

The DS2438 requires strict protocols to insure data integrity. The protocol consists of several types of

signaling on one line: reset pulse, presence pulse, write 0, write 1, read 0, and read 1. All of these signals,

with the exception of the presence pulse, are initiated by the bus master.

The initialization sequence required to begin any communication with the DS2438 is shown in Figure 9.

A reset pulse followed by a presence pulse indicates the DS2438 is ready to send or receive data given the

correct ROM command and memory function command. The bus master transmits (Tx) a reset pulse (a

low signal for a minimum of 480 ms). The bus master then releases the line and goes into a receive

mode(Rx). The 1-Wire bus is pulled to a high state via the 5 kW pull-up resistor. After detecting the

rising edge on the I/O pin, the DS2438 waits 15-60 ms and then transmits the presence pulse (a low signal

for 60-240 ms). DS2438 data is read and written through the use of time slots to manipulate bits and a

command word to specify the transaction.

DS2438

25 of 29

INITIALIZATION PROCEDURE “RESET AND PRESENCE PULSES” Figure 9

480 ms £ tRSTL < ¥ *

480 ms £ tRSTH < ¥ (includes recovery time)

15 ms £ tPDH < 60 ms

60 ms £ tPDL < 240 ms

Write Time Slots

A write time slot is initiated when the host pulls the data line from a high (inactive) logic level to a low

logic level. There are two types of write time slots: Write 1 time slots and Write 0 time slots. All write

time slots must be a minimum of 60 ms in duration with a minimum of a 1 ms recovery time between

individual write cycles.

The DS2438 samples the I/O line in a window of 15 ms to 60 ms after the I/O line falls. If the line is high,

a Write 1 occurs. If the line is low, a Write 0 occurs (See Figure 10).

For the host to generate a Write 1 time slot, the data line must be pulled to a logic low level and then

released, allowing the data line to pull up to a high level within 15 microseconds after the start of the

write time slot. For the host to generate a Write 0 time slot, the data line must be pulled to a logic low

level and remain low for the duration of the write time slot.

Read Time Slots

The host generates read time slots when data is to be read from the DS2438. A read time slot is initiated

when the host pulls the data line from a logic high level to logic low level. The data line must remain at a

low logic level for a minimum of 1 ms; output data from the DS2438 is then valid within the next 14 ms

maximum.

The host therefore must stop driving the I/O pin low in order to read its state 15 ms from the start of the

read slot. (see Figure 10). By the end of the read time slot, the I/O pin will pull back high via the external

pull-up resistor. All read time slots must be a minimum of 60 ms in duration with a minimum of a 1 ms

recovery time between individual read slots.

RESISTOR

MASTER

DS2438

26 of 29

READ / WRITE TIMING DIAGRAM Figure 10

Write-One Time Slot

DS2438

SAMPLING WINDOW

60 ms £ tSLOT < 120 ms

1 ms £ tLOW1 < 15 ms

1 ms £ tREC < ¥

Write-Zero Time Slot

DS2438

60 ms £ tLOW0 < tSLOT < 120 ms

1 ms £ tREC < ¥

Read-Data Time Slot

60 ms £ tSLOT < 120 ms

1 ms £ tLOWR < 15 ms

0 £ tRELEASE < 45 ms

1 ms £ tREC < ¥

tRDV = 15 ms

SU < 1ms

RESISTOR

MASTER

RESISTOR

MASTER

DS2438

27 of 29

ABSOLUTE MAXIMUM RATINGS*

Voltage on VDD and VAD, Relative to Ground -0.3V to + 12V

Voltage on VSENS+, VSENS-, Relative to Ground <±300 mV

Voltage on Any Other Pin Relative to Ground -0.3V to + 7.0V

Operating Temperature -40°C to +85°C

Storage Temperature -55°C to +125°C

Soldering Temperature See J-STD-020A specification

* This is a stress rating only and functional operation of the device at these or any other conditions above

those indicated in the operation sections of this specification is not implied. Exposure to absolute

maximum rating conditions for extended periods of time may affect reliability.

RECOMMENDED DC OPERATING CONDITIONS

(-40°C to +85°C; 2.4V £ VDD £ 10.0V)

PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES

Supply Voltage VDD 2.4 10.0 V 1

Data Pin DQ -0.3 +5.5 V 1

DQ Pull-up Voltage 2.4 5.5

DC ELECTRICAL CHARACTERISTICS

(-40°C to +85°C; 2.4V £ VDD £ 10.0V)

PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES

Input Logic High VIH 2.0 V 1

Input Logic Low VIL -0.3 0.5 V 1

Shutdown Current IDD1 DQ=0, RTC

Active

25 mA

Active Current IDD DQ=1, ICA

Active or

Temperature or

Voltage

Conversions or

EEPROM write

in progress

50 100

Input Resistance RI DQ 500

kW 2

DS2438

28 of 29

ELECTRICAL CHARACTERISTICS: DIGITAL THERMOMETER

(-40°C to +85°C; 2.4V £ VDD £ 10.0V)

PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES

Thermometer Error (TACTUAL-

TMEASURED)

TERR ±2

°C

Conversion Time tCONVT 3 10 ms

ELECTRICAL CHARACTERISTICS: VOLTAGE A/D CONVERTER

(-40°C to +85°C; 2.4V £ VDD £ 10.0V)

PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES

A/D Error VDDERR

VADERR

VDD input

VAD input

±10

±25

±50

±75

VAD Input Range VADR 1.5V 10.0 V 1

VDD Input Range VDDR 2.4 10.0 V 1

Conversion Time tCONVV 3 10 ms

No Missing Code

Temperature Range

-40 +85

°C

ELECTRICAL CHARACTERISTICS: CURRENT A/D CONVERTER

(-40°C to +85°C; 2.4V £ VDD £ 10.0V)

PARAMETER SYMBOL CONDITION MIN TYP MAX UNI

NOTES

Current Measurement Input

Range (VSENS+ - VSENS-)

IADRANGE 250 mV

Current Measurement Error IADERR |VSENS+ - VSENS-|

£ 125 mV

±2

LSB

ELECTRICAL CHARACTERISTICS: RTC COUNTER

(-40°C to +85°C; 2.4V £ VDD £ 10.0V)

PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES

Clock Error RTCERR 0°C to +70°C 1 3 %

Resolution 1 sec

DS2438

AC ELECTRICAL CHARACTERISTICS: NV MEMORY

(-40°C to +85°C; 2.4V £ VDD £ 10.0V)

PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES

NV Write Cycle Time tWR 2 10 ms

EEPROM Writes (copy

scratchpad command)

NEEWR -20ºC to +55ºC 50k writes

EEPROM data retention tEEDR -20ºC to +55ºC 10 years

AC ELECTRICAL CHARACTERISTICS: 1-WIRE INTERFACE

(-40°C to +85°C; 2.4V £ VDD £ 10.0V)

PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES

Time Slot tSLOT 60 120

Recovery Time tREC 1

Write 0 Low Time tLOW0 60 120

Write 1 Low Time tLOW1 1 15

Read Data Valid tRDV 15

Reset Time High tRSTH 480

Reset Time Low tRSTL 480 980

Presence Detect High tPDH 15 60

Presence Detect Low tPDL 60 240

DQ Capacitance CDQ 25 pF

NOTES:

1. All voltages are referenced to GND.

2. Input load is to GND.

3. Current measurement accuracy is ±2 LSb or 1%, whichever is greater.

Maxim Integrated 160 Rio Robles, San Jose, CA 95134 USA 1-408-601-1000

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied.

Maxim reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits) shown in the Electrical

Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.