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 160ns cycle time allows fast bus
operation
a174 Selectable Intel or Motorola bus
timing
a174 Less than 0.5µA load under bat-
tery operation
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 adjust-
ment
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 122µs to
500ms
- 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 bq3285 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 bq3285 write-protects the clock,
calendar, and storage registers during
power failure. A backup battery
then maintains data and operates the
clock and calendar.
The bq3285 is a fully compatible
real-time clock for IBM AT com-
patible computers and other appli-
cations. The only external compo-
nents are a 32.768kHz crystal and
a backup battery
1
bq3285
Real-Time Clock (RTC)
PN328501.eps
28-Pin PLCC
5
6
7
8
9
10
11
25
24
23
22
21
20
19
4 3 2 1
28 27 26
12 13 14 15 16 17 18
AD
0
AD
1
AD
2
AD
3
AD
4
AD
5
NC
AD
6
NC
AD
7
V
SS CS AS NC
RCL
BC
INT
RST
DS
V
SS
R/ W
X2 X1 MOT NC V
CC
SQW NC
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
RCL RAM clear input
BC 3V backup cell input
X1–X2 Crystal inputs
NC No connect
VCC +5V supply
VSS Ground
1
PN328501.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
NC
BC
INT
RST
DS
V
SS
R/W
AS
CS
MOT
X1
X2
AD
0
AD
1
AD
2
AD
3
AD
4
AD
5
AD
6
AD
7
V
SS
RCL
Pin Connections
Jan. 1999 E