1
Features
a174 ACPI-compliant day-of-month
alarm
a174 Y2K century bit
a174 Direct clock/calendar replace-
ment for IBM
®
AT-compatible
computers and other applications
a174 2 index shadow registers
a174 2.7–5.5V operation
a174 240 bytes of general nonvolatile
storage
a174 Dedicated 32.768kHz output pin
a174 System wake-up capability—
alarm interrupt output active in
battery-backup mode
a174 Less than 0.55µA load under bat-
tery operation
a174 Selectable Intel or Motorola bus
timing
a174 24-pin plastic SSOP
General Description
The CMOS bq3285LF is a low-power
microprocessor peripheral providing
a time-of-day clock and 100-year cal-
endar with alarm features and bat-
tery operation. The architecture is
based on the bq3285 RTC with added
features: century bit, low-voltage op-
eration, 32.768kHz output, 126 addi-
tional bytes of CMOS, two shadow
registers of last address used, and a
day-of-month alarm to be compliant
with the ACPI RTC specification.
A 32.768kHz output is available for
sustaining power-management ac-
tivities. The bq3285LF 32kHz out-
put is always on whenever VCC is
valid. In VCC standby mode, the
32kHz is active, and the bq3285LF
typically draws 100µA. Wake-up ca-
pability is provided by an alarm in-
terrupt, which is active in battery-
backup mode. In battery-backup
mode, current drain is less than
550nA.
The bq3285LF write-protects the
clock, calendar, and storage registers
during power failure. A backup
battery then maintains data and oper-
ates the clock and calendar.
The bq3285LF is a fully compatible
real-time clock for IBM AT-
compatible computers and other ap-
plications. The only external compo-
nents are a 32.768kHz crystal and a
backup battery.
The bq3285LF is intended for use in
3V systems; however, it may also op-
erate at 5V and then go into a 3V
power-down state, write-protecting
as if in a 3V system.
Y2K-Enhanced
Real-Time Clock
(
RTC
)
bq3285LF
1
PN3285ED/LD.eps
24-Pin SSOP
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
32k
EXTRAM
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 Pin Names
6/99 B
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
32K 32.768kHz output
EXTRAM Extended RAM enable
RCL RAM clear input
BC 3V backup cell input
X1–X2 Crystal inputs
VCC Supply voltage input
VSS Ground