Features
a174 Real-Time Clock counts seconds
through years in BCD format
a174 Integrated battery and crystal
a174 On-chip battery-backup switch-
over circuit with nonvolatile con-
trol for an external SRAM
a174 130mAh battery capacity
a174 ±1 minute per month clock accu-
racy
a174 Less than 500nA of clock opera-
tion current in backup mode
a174 Microprocessor reset valid to
VCC =VSS
a174 Independent watchdog timer
with a programmable time-out
period
a174 Power-fail interrupt warning
a174 Programmable clock alarm inter-
rupt active in battery-backup
mode
a174 Programmable periodic interrupt
a174 Battery-low warning
General Description
The bq4847 Real-Time Clock Mod-
ule is a low-power microprocessor
peripheral that integrates a time-
of-day clock, a 100-year calendar, a
CPU supervisor, a battery, and a crys-
tal in a 28-pin DIP module. The part
is ideal for fax machines, copiers, in-
dustrial control systems, point-of-sale
terminals, data loggers, and comput-
ers.
The bq4847 contains an internal
battery and crystal. Through the use
of the conditional chip enable output
(CEOUT) and battery voltage output
(VOUT) pins, the bq4847 can write-
protect and make nonvolatile an
external SRAM. The backup cell
powers the real-time clock and
maintains SRAM information in
the absence of system voltage.
The bq4847 contains a
temperature-compensated refer-
ence and comparator circuit that
monitors the status of its voltage
supply. When an out-of-tolerance
condition is detected, the bq4847
generates an interrupt warning
and subsequently a microproces-
sor reset. The reset stays active
for 200ms after VCC rises within
tolerance to allow for power supply
and processor stabilization.
The bq4847 also has a built-in
watchdog timer to monitor processor
operation. If the microprocessor does
not toggle the watchdog input (WDI)
within the programmed time-out,
the bq4847 asserts WDO and RST.
WDI unconnected disables the
watchdog timer.
The bq4847 can generate other in-
terrupts based on a clock alarm con-
dition or a periodic setting. The
alarm interrupt can be set to occur
from once per second to once per
month. The alarm can be made ac-
tive in the battery-backup mode to
serve as a system wake-up call. For
interrupts at a rate beyond once per
second, the periodic interrupt can be
programmed with periods of 30.5µs
to 500ms.
1
bq4847/bq4847Y
1
PN484701.eps
28-Pin DIP Module
2
3
4
5
6
7
8
28
27
26
25
24
23
22
21
9
10
20
19
11
12
18
17
13
14
16
15
V
CC
WE
CE
IN
CE
OUT
NC
WDI
OE
CS
NC
DQ
7
DQ
6
DQ
5
DQ
4
DQ
3
V
OUT
NC
NC
WDO
INT
RST
A
3
A
2
A
1
A
0
DQ
0
DQ
1
DQ
2
V
SS
Pin Connections
A0–A3 Clock/Control address
inputs
DQ0–DQ7 Data inputs/outputs
WE Write enable
OE Output enable
CS Chip select input
CEIN External RAM chip
enable
CEOUT Conditional RAM chip
enable
Pin Names
NC No connect
VOUT Back-up battery output
INT Interrupt output
RST Microprocessor reset
WDI Watchdog input
WDO Watchdog output
VCC +5V supply
VSS Ground
RTC Module With CPU Supervisor
Caution:
Take care to avoid inadvertent dis-
charge through V
OUT
and CE
OUT
after battery isolation has been
broken.
Sept. 1996