bq4847/bq4847Y
RTC Module With CPU Supervisor
tolerance to allow for power supply
General Description
Features
and processor stabilization.
➤ Real-Time Clock counts seconds
The bq4847 Real-Time Clock Mod-
Th e bq4847 a ls o h a s a bu ilt -in
ule is a low-power microprocessor
through years in BCD format
watchdog timer to monitor processor
peripheral that integrates a time-
➤ Integrated battery and crystal
operation. If the microprocessor does
of-day clock, a 100-year calendar, a
not toggle the watchdog input (WDI)
CPU supervisor, a battery, and a crys-
➤ On-chip battery-backup switch-
over circuit with nonvolatile con-
trol for an external SRAM
within the programmed time-out,
tal in a 28-pin DIP module. The part
the bq4847 asserts WDO and RST.
is ideal for fax machines, copiers, in-
WD I u n con n e ct e d d is a b le s t h e
dustrial control systems, point-of-sale
watchdog timer.
➤ 130mAh battery capacity
terminals, data loggers, and comput-
ers.
The bq4847 can generate other in-
➤ ±1 minute per month clock accu-
terrupts based on a clock alarm con-
Th e bq4847 con t a in s a n in t er n a l
racy
dition or a periodic setting. The
battery and crystal. Through the use
➤ Less than 500nA of clock opera-
alarm interrupt can be set to occur
of the conditional chip enable output
tion current in backup mode
from once per second to once per
(CEOUT) and battery voltage output
month. The alarm can be made ac-
(VOUT) pin s, t h e bq4847 can write-
➤ Microprocessor reset valid to
tive in the battery-backup mode to
protect and make n on vola t ile a n
VCC = VSS
serve as a system wake-up call. For
ext er n a l SRAM. Th e ba cku p cell
interrupts at a rate beyond once per
p ower s t h e r ea l-t im e clock a n d
➤ I n d ep en d en t wa t ch d og t im er
with a programmable time-out
period
second, the periodic interrupt can be
programmed with periods of 30.5µs
to 500ms.
m a in t a in s SRAM in for m a t ion in
the absence of system voltage.
➤ Power-fail interrupt warning
T h e
b q 4 8 4 7
con t a i n s
a
t em per a t u r e-com pen sa t ed r efer-
ence and comparator circuit that
monitors the status of its voltage
su pply. Wh en a n ou t -of-t oler a n ce
condition is detected, the bq4847
gen er a t es a n in t er r u pt wa r n in g
a n d su bsequ en t ly a m icr opr oces-
sor r eset . Th e r eset st a ys a ct ive
for 200m s a ft er VCC r ises wit h in
➤ Programmable clock alarm inter-
r u pt a ct ive in ba t t er y-ba cku p
mode
Caution:
Take care to avoid inadvertent dis-
charge through VOUT and CEOUT
after battery isolation has been
broken.
➤ Programmable periodic interrupt
➤ Battery-low warning
Pin Connections
Pin Names
A0–A3
Clock/Control address
inputs
NC
No connect
V
28
1
V
OUT
NC
NC
CC
WE
27
26
2
3
VOUT
INT
Back-up battery output
Interrupt output
Microprocessor reset
Watchdog input
Watchdog output
+5V supply
CE
CE
NC
IN
DQ0–DQ7 Data inputs/outputs
4
WDO
INT
25
24
23
22
21
20
19
18
17
16
15
OUT
5
WE
OE
Write enable
RST
6
WDI
OE
CS
RST
WDI
WDO
VCC
VSS
A
3
7
Output enable
Chip select input
A
A
A
8
9
2
1
0
NC
10
11
12
13
14
DQ
7
DQ
DQ
DQ
DQ
CS
DQ
0
1
2
6
DQ
DQ
5
4
3
CEIN
External RAM chip
enable
V
SS
Ground
CEOUT
Conditional RAM chip
enable
28-Pin DIP Module
PN484701.eps
Sept. 1996
1