Datasheet

MAX793/MAX794/MAX795
Figure 18 shows maximum transient duration vs. reset-
comparator overdrive, for which reset pulses are not
generated. The graph was produced using negative-
going V
CC
pulses, starting at 3.3V and ending below
the reset threshold by the magnitude indicated (reset
comparator overdrive). The graph shows the maximum
pulse width a negative-going V
CC
transient can typically
have without causing a reset pulse to be issued. As
the amplitude of the transient increases (i.e., goes far-
ther below the reset threshold), the maximum allowable
pulse width decreases. Typically, a V
CC
transient that
goes 40mV below the reset threshold and lasts for 10µs
or less does not cause a reset pulse to be issued.
A 0.1µF bypass capacitor mounted close to the V
CC
pin provides additional transient immunity.
Watchdog Software Considerations
There is a way to help the watchdog timer monitor soft-
ware execution more closely, which 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, in which the watchdog timer would continue
to be reset within the loop, keeping the watchdog from
timing out. Figure 19 shows an example of a flow dia-
gram where the I/O driving the watchdog input is set
high at the beginning of the program, set low at the
beginning of every subroutine or loop, then set high
again when the program returns to the beginning. If the
program should hang in any subroutine, the problem
would quickly be corrected, since the I/O is continually
set low and the watchdog timer is allowed to time out,
causing a reset or interrupt to be issued.
3.0V/3.3V Adjustable Microprocessor
Supervisory Circuits
18 ______________________________________________________________________________________
100
0
10 20 30 100
10
20
30
80
90
MAX793-FIG 18
RESET COMPARATOR OVERDRIVE, V
RST
- V
CC
(mV)
MAXIMUM PULSE DURATION (µs)
40 50 60 70 80 90
60
40
50
70
Figure 18. Maximum Transient Duration without Causing a
Reset Pulse vs. Reset Comparator Overdrive
Figure 19. Watchdog Flow Diagram
START
SET WDI
HIGH
RETURN
PROGRAM
CODE
Subroutine or
Program Loop
SET WDI LOW