Datasheet

MAX691A/MAX693A/MAX800L/MAX800M
Microprocessor Supervisory Circuits
______________________________________________________________________________________ 15
Maximum V
CC
Fall Time
The V
CC
fall time is limited by the propagation delay of
the battery switchover comparator and should not
exceed 0.03V/µs. A standard rule of thumb for filter
capacitance on most regulators is on the order of 100µF
per amp of current. When the power supply is shut off or
the main battery is disconnected, the associated initial
V
CC
fall rate is just the inverse or 1A/100µF = 0.01V/µs.
The V
CC
fall rate decreases with time as V
CC
falls expo-
nentially, which more than satisfies the maximum fall-time
requirement.
Watchdog Software Considerations
A way to help the watchdog timer keep a closer watch
on software execution involves setting and resetting the
watchdog input at different points in the program,
rather than “pulsing” the watchdog input high-low-high
or low-high-low. This technique avoids a “stuck” loop
where the watchdog timer continues to be reset within
the loop, keeping the watchdog from timing out. Figure
14 shows an example flow diagram where the I/O dri-
ving the watchdog input is set high at the beginning of
the program, set low at the beginning of every subrou-
tine or loop, then set high again when the program
returns to the beginning. If the program should “hang”
in any subroutine, the I/O is continually set low and the
watchdog timer is allowed to time out, causing a reset
or interrupt to be issued.
START
SET
WDI
LOW
RETURN
END
SUBROUTINE
OR PROGRAM LOOP
SET WDI
HIGH
Figure 14. Watchdog Flow Diagram