MAX791
close to SWT as possible. The sum of PC board leak-
age + SWT capacitor leakage must be small compared
to ±100nA.
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 17 shows an example flow diagram where the
I/O driving the watchdog input is set high at the begin-
ning of the program, set low at the beginning of every
subroutine or loop, then set high again when the pro-
gram 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.
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 associ-
ated 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 exponentially, which more than satisfies the
maximum fall-time requirement.
Microprocessor Supervisory Circuit
16 ______________________________________________________________________________________
