Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s
1693L/AX80M
Ma x im u m V
Fa ll Tim e
CC
The VCC fall time is limited by the propagation delay of
the b a tte ry s witc hove r c omp a ra tor a nd s hould 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
VCC fall rate is just the inverse or 1A/100µF = 0.01V/µs.
The VCC fall rate decreases with time as VCC falls expo-
nentially, which more than satisfies the maximum fall-time
requirement.
START
SET
WDI
LOW
Wa t c h d o g S o ft w a re Co n s id e ra t io n s
A way to help the watchdog timer keep a closer watch
on software execution involves setting and resetting the
wa tc hd og inp ut a t d iffe re nt p oints in the p rog ra m,
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.
SUBROUTINE
OR PROGRAM LOOP
SET WDI
HIGH
RETURN
END
Figure 14. Watchdog Flow Diagram
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
