BQ4845 Datasheet, PDF (7/22 Pages) Texas Instruments – Parallel RTC With CPU Supervisor

bq4845/bq4845Y
Watchdog Timer
The watchdog monitors microprocessor activity through
the Watchdog input (WDI). To use the watchdog func-
tion, connect WDI to a bus line or a microprocessor I/O
line. If WDI remains high or low for longer than the
watchdog time-out period (1.5 seconds default), the
bq4845 asserts WDO and RST.
Watchdog Input
The bq4845 resets the watchdog timer if a change of
state (high to low, low to high, or a minimum 100ns
pulse) occurs at the Watchdog input (WDI) during the
watchdog period. The watchdog time-out is set by WD0-
WD2 in register B. The bq4845 maintains the watchdog
time-out programming through power cycles. The de-
fault state (no valid battery power) of WD0-WD2 is 000
or 1.5s on power-up. Table 3 shows the programmable
watchdog time-out rates. The watchdog time-out period
immediately after a reset is equal to the programmed
watchdog time-out.
To disable the watchdog function, leave WDI floating. An
internal resistor network (100kΩ equivalent impedance
at WDI) biases WDI to approximately 1.6V. Internal
comparators detect this level and disable the watchdog
timer. When VCC is below the power-fail threshold, the
bq4845 disables the watchdog function and disconnects
WDI from its internal resistor network, thus making it
high impedance.
Watchdog Output
The Watchdog output (WDO) remains high if there is a
transition or pulse at WDI during the watchdog time-
out period. The bq4845 disables the watchdog function
and WDO is a logic high when VCC is below the power
fail threshold, battery-backup mode is enabled, or WDI
is an open circuit. In watchdog mode, if no transition oc-
curs at WDI during the watchdog time-out period, the
bq4845 asserts RST for the reset time-out period t1.
WDO goes low and remains low until the next transition
at WDI. If WDI is held high or low indefinitely, RST will
generate pulses (t1 seconds wide) every t3 seconds. Fig-
ure 3 shows the watchdog timing.
Aug. 1995
Figure 3. Watchdog Time-out Period and Reset Active Time
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