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Note:
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of this device may incorporate deviations from published specifications known as errata. Multiple revisions of any device
may be simultaneously available through various sales channels. For information about device errata, click here: http://www.maxim-ic.com/errata.
FEATURES
§ Drop-in replacement for IBM AT computer
clock/calendar
§ Pin configuration closely matches
MC146818B and DS1285
§ Counts seconds, minutes, hours, days, day of
the week, date, month, and year with leap-
year compensation valid up to 2100
§ Binary or BCD representation of time,
calendar, and alarm
§ 12-hour or 24-hour clock with AM and PM in
12-hour mode
§ Daylight Savings Time option
§ Selectable between Motorola and Intel bus
timing
§ Multiplex bus for pin efficiency
§ Interfaced with software as 128 RAM
locations
§ 14 bytes of clock and control registers
§ 114 bytes of general purpose RAM
§ Programmable square-wave output signal
bus-compatible interrupt signals (IRQ )
§ Three interrupts are separately software-
maskable and testable
§ Time-of-day alarm once/second to once/day
§ Periodic rates from 122µs to 500ms
§ End-of-clock update cycle
§ Optional 28-pin PLCC surface mount package
or 32-pin TQFP
§ Optional industrial temperature range
available
§ Underwriters Laboratory (UL) recognized
PIN ASSIGNMENT (Top View)
Package Dimension Information
http://www.maxim-ic.com/TechSupport/DallasPackInfo.htm
DS12885/DS12885Q/DS12885T
Real-
Time Clock
www
maxim
ic
.com
www.maxim
-
ic.com
DS12885Q
28 PLCC
DS12885T
32 TQFP
DS12885, 24 DIP
DS12885S, 24 SO 300mil
DS12885/DS12885Q/DS12885T
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DESCRIPTION
The DS12885 real-time clock plus RAM is designed to be a direct replacement for the DS1285. The
DS12885 is identical in form, fit, and function to the DS1285, and has an additional 64 bytes of general
purpose RAM. Access to this additional RAM space is determined by the logic level presented on AD6
during the address portion of an access cycle. An external crystal and battery are the only components
required to maintain time-of-day and memory status in the absence of power. For a complete description
of operating conditions, electrical characteristics, bus timing, and pin descriptions other than X1, X2,
VBAT, and RCLR , see the DS12887 data sheet.
TYPICAL OPERATING CIRCUIT
ORDERING INFORMATION
PART PIN-PACKAGE TEMP RANGE
DS12885 24 DIP 0°C to +70°C
DS12885N 24 DIP -40°C to +85°C
DS12885S 24 SO 0°C to +70°C
DS12885N 24 SO -40°C to +85°C
DS12885Q 28 PLCC 0°C to +70°C
DS12885QN 28 PLCC -40°C to +85°C
DS12885Q/T&R 28 PLCC/Tape and Reel 0°C to +70°C
DS12885T 32 TQFP 0°C to +70°C
DS12885TN 32 TQFP -40°C to +85°C
DS12885T/T&R 32 TQFP/Tape and Reel 0°C to +70°C
DS12885/DS12885Q/DS12885T
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PIN DESCRIPTION
AD0–AD7 – Multiplexed Address/Data Bus
N.C. – No Connection
MOT – Bus Type Selection
CS – Chip Select
AS – Address Strobe
R/ W – Read/Write Input
DS – Data Strobe
RESET – Reset Input
IRQ – Interrupt Request Output
(Open Drain)
SQW – Square-Wave Output
VCC – +5V Supply
GND – Ground
X1, X2 – 32.768kHz Crystal
Connections
VBAT – +3V Battery Input
RCLR – RAM Clear
PIN DESCRIPTION
X1, X2 – Connections for a standard 32.768kHz quartz crystal. The internal oscillator circuitry is
designed for operation with a crystal having a specified load capacitance (CL) of 6pF. The crystal is
connected directly to the X1 and X2 pins. There is no need for external capacitors or resistors. Note: X1
and X2 are very high-impedance nodes. It is recommended that they and the crystal be guard-ringed with
ground and that high-frequency signals be kept away from the crystal area. For more information about
crystal selection and cr ystal layout considerations, refer to Application Note 58 “Crystal Considerations
with Dallas Real Time Clocks.”
Oscillator startup times are highly dependent upon crystal characteristics and layout. High ESR and
escessive capactitive loads are the major contributors to long startup times. A circuit using a crystal with
the recommended characteristics and following the recommended layout usually start within one second.
VBAT – Battery input for any standard 3V lithium cell or other energy source. Battery voltage must be
held between 2.5V and 4V for proper operation. A maximum load of 0.5µA at +25°C in the absence of
power should be used to size the external energy source. Maximum load is measured using a
recommended crystal type connected to X1 and X2.
The battery should be connected directly to the VBAT pin. A diode must not be placed in series with the
battery to the VBAT pin. Furthermore, a diode is not necessary because reverse charging-current protection
circuitry is provided internally to the device and has passed the requirements of Underwriters
Laboratories for UL listing.
See “Conditions of Acceptability” at http://www.maxim-ic.com/TechSupport/QA/ntrl.htm.
RCLR – The RCLR pin is used to clear (set to logic 1) all 114 bytes of general purpose RAM but does
not affect the RAM associated with the real-time clock. In order to clear the RAM, RCLR must be forced
to an input logic “0” (-0.3V to +0.8V) during battery-backup mode when VCC is not applied. The RCLR
function is designed to be used by human interface (shorting to ground manually or by switch) and not to
be driven with external buffers. This pin is internally pulled up. Do not use an external pullup resistor on
this pin.
DS12885/DS12885Q/DS12885T
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CLOCK ACCURACY
The accuracy of the clock is dependent upon the accuracy of the crystal and the accuracy of the match
between the capacitive load of the oscillator circuit and the capacitive load for which the crystal was
trimmed. Additional error is added by crystal frequency drift caused by temperature shifts. External
circuit noise coupled into the oscillator circuit can result in the clock running fast. Refer to Application
Note 58 “Crystal Considerations with Dallas Real-Time Clocks” for detailed information.
RECOMMENDED LAYOUT FOR CRYSTAL