bq3285
Real-Time Clock (RTC)
General Description
Features
➤ Calendar in day of the week, day
of the month, months, and years,
with automatic leap-year adjust-
ment
➤ Direct clock/calendar replace-
ment for IBM® AT-compatible
computers and other applications
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.
➤ Time of day in seconds, minutes,
➤ Functionally compatible with the
and hours
DS1285
- 12- or 24-hour format
- Optional daylight saving
- Closely matches MC146818A
pin configuration
adjustment
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.
➤ 114 bytes of general nonvolatile
➤ Programmable square wave out-
storage
put
➤ 160ns cycle time allows fast bus
➤ Three individually maskable in-
operation
terrupt event flags:
➤ Selectable Intel or Motorola bus
- Periodic rates from 122µs to
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
timing
500ms
➤ Less than 0.5µA load under bat-
- Time-of-day alarm once per
tery operation
second to once per day
- End-of-clock update cycle
➤ 24-pin plastic DIP or SOIC
➤ 14 bytes for clock/calendar and
control
➤ BCD or binary format for clock
and calendar data
Pin Names
Pin Connections
AD0–AD7
Multiplexed address/data
input/output
MOT
CS
Bus type select input
Chip select input
Address strobe input
Data strobe input
Read/write input
MOT
24
1
V
CC
SQW
NC
X1
X2
23
22
2
3
AS
4
RCL
BC
DS
AD
AD
AD
AD
AD
AD
AD
21
20
19
18
17
16
15
14
13
0
1
2
3
4
5
6
5
6
AD
AD
25
24
RCL
BC
0
1
5
R/W
INT
6
INT
RST
DS
7
8
9
10
11
Interrupt request
output
AD
AD
AD
AD
23
22
21
20
19
INT
RST
DS
2
3
4
5
7
8
9
RST
SQW
RCL
BC
Reset input
V
SS
V
NC
R/W
SS
Square wave output
RAM clear input
3V backup cell input
Crystal inputs
No connect
10
11
12
R/W
AS
CS
AD
7
V
SS
X1–X2
NC
28-Pin PLCC
PN328501.eps
24-Pin DIP or SOIC
PN328501.eps
VCC
VSS
+5V supply
Ground
Jan. 1999 E
1