1
Features
a174 Direct clock/calendar replace-
ment for IBM
®
AT-compatible
computers and other applications
a174 Functionally compatible with the
DS1285
- Closely matches MC146818A
pin configuration
a174 114 bytes of general nonvolatile
storage
a174 Automatic backup and write-
protect control to external SRAM
a174 160 ns cycle time allows fast bus
operation
a174 Less than 0.5 µA load under bat-
tery operation
a174 14 bytes for clock/calendar and
control
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 BCD or binary format for clock
and calendar data
a174 Programmable square wave out-
put
a174 Three individually maskable in-
terrupt event flags:
- Periodic rates from 122 µs to
500 ms
- Time-of-day alarm once per
second to once per day
- End-of-clock update cycle
a174 24-pin plastic DIP or SOIC
General Description
The CMOS bq4285 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 include
three maskable interrupt sources,
square wave output, and 114 bytes of
general nonvolatile storage.
The bq4285 write-protects the clock,
calendar, and storage registers dur-
ing power failure. A backup battery
then maintains data and operates
the clock and calendar.
The bq4285 is a fully compatible real-
time clock for IBM AT-compatible com-
puters and other applications. The only
external components are a 32.768kHz
crystal and a backup battery.
The bq4285 integrates a battery-
backup controller to make a standard
CMOS SRAM nonvolatile during
power-fail conditions. During power-
fail, the bq4285 automatically write-
protects the external SRAM and pro-
vides a VCC output sourced from the
clock backup battery.
28-Pin PLCC No longer available
Pin Names
AD0–AD7 Multiplexed address/data
input/output
MOT Bus type select input
CS Chip select input
AS Address strobe input
DS Data strobe input
R/W Read/write input
INT Interrupt request output
RST Reset input
SQW Square wave output
BC 3V backup cell input
X1–X2 Crystal inputs
NC No connect
CEIN RAM chip enable input
CEOUT RAM chip enable output
VOUT Supply output
VCC +5V supply
VSS Ground
SLUS002A JUNE 1991 - REVISED MAY 2004
1
PN428501.eps
24-Pin DIP or SOIC
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
BC
INT
RST
DS
V
SS
R/W
AS
CS
V
OUT
X
1
X
2
AD
0
AD
1
AD
2
AD
3
AD
4
AD
5
AD
6
AD
7
V
SS
CE
IN
Pin Connections
bq4285
Real-Time Clock
(
RTC
)
With NVRAM Control