1 of 2 020900
FEATURES
Drop-in replacement for IBM AT computer
clock/calendar
Pin compatible with the MC146818B and
DS1287A
Totally nonvolatile with over 10 years of
operation in the absence of power
Self-contained subsystem includes lithium,
quartz, and support circuitry.
Counts seconds, minutes, hours, days, da y 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– 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
– 15 bytes of clock and control registers
– 113 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 122 µs to 500 ms
– End of clock update cycle
Century register
PIN ASSIGNMENT
PIN DESCRIPTION
AD0-AD7 - Multiplexed Address/Data Bus
NC - No Connect
MOT - Bus Type Selection
CS - RTC Chip Select Input
AS - Address Strobe
R/W- Read/Write Input
DS - Data Strobe
RESET - Reset Input
IRQ - Interrupt Request Output
SQW - Square Wave Output
VCC - +5 Volt Main Supply
RCLR - RAM Clear
GND - Ground
DESCRIPTION
The DS12C887A Real Time Clock plus RAM is designed to be a direct upgrade replacement for the
DS12887A in existing IBM compatible personal computers to add hardware year 2000 compliance. A
century byte was added to memory location 50, 32h, as called out by the PC AT specification. The
DS12C887A is identical in form, fit, and function to the DS1287A, and has an additional 64 bytes of
general purpose RAM. Access to this additi onal RAM space i s dete rm ined by th e lo gic level p resent ed on
AD6 during the address port ion of an access c ycle. The RCLR pin is used to cl ear (set t o logic 1) all 113
bytes of general purpose RAM but does not affect the RAM associated with the real time clock. In order
DS12C887
A
Real Time Clock
www.dalsemi.com
MOT 1
2
3
4
5
6
7
8
9
10
11
12
23
DS12C887A 24-Pin
ENCAPSULATED PACKAGE
NC
AD1
AD3
AD4
AD5
AD6
AD7
GND
VCC
SQW
NC
NC
IRQ
RESET
DS
NC
R/W
A
S
CS
24
22
21
20
19
18
17
16
15
14
13
NC
AD0
AD2
RCLR
DS12C887A
2 of 2
to clear the R AM, RCLR must be forced to an input logic “0” (–0.3 to +0.8 volts) during batter y back-up
mode when V CC is not applied.
The RCLR function is designed to be used via human interface (shorting to ground manually or by
switch) and not to be driven with external buffers. All other operation, description and specification is
identical to the DS12C887.
DS12C887 REAL TIME CLOCK PLUS RAM
PKG 24-PIN
DIM MIN MAX
A IN
MM 1.320
33.53 1.335
33.91
B IN
MM 0.675
17.15 0.700
17.78
C IN
MM 0.345
8.76 0.370
9.40
D IN
MM 0.100
2.54 0.130
3.30
E IN
MM 0.015
0.38 0.030
0.76
F IN
MM 0.110
2.79 0.140
3.56
G IN
MM 0.090
2.29 0.110
2.79
H IN
MM 0.590
14.99 0.630
16.00
J IN
MM 0.008
0.20 0.012
0.30
K IN
MM 0.015
0.38 0.021
0.53
NOTE: Pins 2, 3, 16, 20 and 22 are missing by
design
NOTE: This device cannot be stored or
shipped in conductive material which will give a
continuity path between the RAM Clear pin and
ground.
