Features
a174 Direct clock/calendar replace-
ment for IBM®
AT-compatible computers and
other applications
a174 Functionally compatible with the
DS1287/DS1287A and
MC146818A
a174 114 bytes of general nonvolatile
storage
a174 Automatic backup supply and
write-protection to make external
SRAM nonvolatile
a174 Integral lithium cell and crystal
a174 160 ns cycle time allows fast bus
operation
a174 Intel bus timing
a174 14 bytes for clock/calendar and
control
a174 BCD or binary format for clock
and calendar data
a174 Calendar in day of the week, day of
the month, months, and years with
automatic leap-year adjustment
a174 Time of day in seconds, minutes,
and hours
- 12- or 24-hour format
- Optional daylight saving
adjustment
a174 Programmable square wave out-
put
a174 Three individually maskable in-
terrupt event flags:
- Periodic rates from 122 µsto
500 ms
- Time-of-day alarm once per
second to once per day
- End-of-clock update cycle
a174 Better than one minute per
month clock accuracy
General Description
The CMOS bq4287 is a low-power
microprocessor peripheral providing
a time-of-day clock and 100-year cal-
endar with alarm features and bat-
tery operation. Other features in-
clude three maskable interrupt
sources, square wave output, and
114 bytes of general nonvolatile
storage.
The bq4287 write-protects the clock,
calendar, and storage registers dur-
ing power failure. The integral
backup energy source then main-
tains data and operates the clock
and calendar.
The bq4287 uses its integral battery-
backup controller and battery to make
a standard CMOS SRAM nonvolatile
during power-fail conditions. During
power-fail, the bq4287 automatically
write-protects the external SRAM and
provides a V
CC output sourced from its
internal battery.
The bq4287 is a fully compatible
real-time clock for IBM AT-compatible
computers and other applications.
As shipped from Benchmarq, the
backup cell is electrically isolated
from the memory. Following the first
application of VCC, this isolation is
broken, and the backup cell provides
data retention to the clock, internal
RAM, VOUT, and CEOUT on subse-
quent power-downs.
The bq4287 is functionally equiva-
lent to the bq4285, except that the
battery (16, 20) and crystal pins (2,
3) are not accessible. These pins are
connected internally to a coin cell
and quartz crystal. The coin cell pro-
vides 130mAh of capacity. For a
complete description of features, op-
erating conditions, electrical charac-
teristics, bus timing, and pin de-
scriptions, see the bq4285 data
sheet.
1
Pin Names
AD0–AD7 Multiplexed address/data
input/output
CS Chip select input
ALE Address strobe input
RD Data strobe input
WR Read/write input
INT Interrupt request output
RST Reset input
SQW Square wave output
CEIN RAM chip enable input
CEOUT RAM chip enable output
NC No connect
VOUT Supply output
VCC +5V supply
VSS Ground
1
PN428701.eps
24-Pin DIP Module
2
3
4
5
6
7
8
24
23
22
21
20
19
18
17
9
10
16
15
11
12
14
13
V
CC
SQW
CE
OUT
NC
INT
RST
RD
NC
WR
ALE
CS
V
OUT
NC
NC
AD
0
AD
1
AD
2
AD
3
AD
4
AD
5
AD
6
AD
7
V
SS
CE
IN
bq4287
Pin Connections
Real-Time Clock Module With NVRAM Control
Caution:
Take care to avoid inadvertent dis-
charge through V
OUT
and CE
OUT
after battery isolation has been
broken.
Nov. 1993 C