www.maxim-ic.com
DS12885/DS12885Q/DS12885T
Real-Time Clock
PIN ASSIGNMENT (Top View)
DS12885, 24 DIP
DS12885S, 24 SO 300mil
www.maxim-ic.com
FEATURES
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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
DS12885Q
28 PLCC
DS12885T
32 TQFP
Package Dimension Information
http://www.maxim-ic.com/TechSupport/DallasPackInfo.htm
Note:
Some revisions 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.
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072902
DS12885/DS12885Q/DS12885T
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,
V
BAT
, and
RCLR
, see the DS12887 data sheet.
TYPICAL OPERATING CIRCUIT
ORDERING INFORMATION
PART
DS12885
DS12885N
DS12885S
DS12885N
DS12885Q
DS12885QN
DS12885Q/T&R
DS12885T
DS12885TN
DS12885T/T&R
PIN-PACKAGE
24 DIP
24 DIP
24 SO
24 SO
28 PLCC
28 PLCC
28 PLCC/Tape and Reel
32 TQFP
32 TQFP
32 TQFP/Tape and Reel
TEMP RANGE
0°C to +70°C
-40°C to +85°C
0°C to +70°C
-40°C to +85°C
0°C to +70°C
-40°C to +85°C
0°C to +70°C
0°C to +70°C
-40°C to +85°C
0°C to +70°C
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DS12885/DS12885Q/DS12885T
PIN DESCRIPTION
AD0–AD7
N.C.
MOT
CS
AS
R/
W
DS
RESET
– Multiplexed Address/Data Bus
– No Connection
– Bus Type Selection
– Chip Select
– Address Strobe
– Read/Write Input
– Data Strobe
– Reset Input
IRQ
SQW
V
CC
GND
X1, X2
V
BAT
RCLR
– Interrupt Request Output
(Open Drain)
– Square-Wave Output
– +5V Supply
– Ground
– 32.768kHz Crystal
Connections
– +3V Battery Input
– 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 (C
L
) 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 crystal 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.
V
BAT
–
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 V
BAT
pin. A diode must not be placed in series with the
battery to the V
BAT
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 V
CC
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.
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DS12885/DS12885Q/DS12885T
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 freque ncy 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
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