Figure 18 shows maximum transient duration vs. reset-

comparator overdrive, for which reset pulses are not

generated. The graph was produced using negative-

the reset threshold by the magnitude indicated (reset

comparator overdrive). The graph shows the maximum

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

goes 40mV below the reset threshold and lasts for 10µs

or less does not cause a reset pulse to be issued.

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

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