Ensuring a Valid

RESET/RESET Output

ing (sourcing) capabilities decline drastically. In the

case of the MAX6303, high-impedance CMOS-logic

inputs connected to

RESET can drift to undetermined

voltages. This presents no problem in most applica-

tions, since most µPs and other circuitry do not operate

In those applications where

RESET must be valid down

to 0V, adding a pull-down resistor between

RESET and

ground sinks any stray leakage currents, holding

RESET low (Figure 9). The value of the pull-down resis-

tor is not critical; 100k

Ω is large enough not to load

RESET and small enough to pull RESET to ground. For

applications using the MAX6304, a 100k

Ω pull-up resis-

Watchdog-Software Considerations

To help the watchdog timer monitor software execution

more closely, set and reset the watchdog input at differ-

ent 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, keep-

ing the watchdog from timing out.

Figure 11 shows an example of a flow diagram 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. When using extended mode,

as described in the

Watchdog Input Current section,

this scheme does result in higher average WDI input

current than does the method of leaving WDI low for the

majority of the timeout period and periodically pulsing it

low-high-low.

+5V, Low-Power µP Supervisory Circuits

with Adjustable Reset/Watchdog

10

______________________________________________________________________________________

MAX6303

RESET

GND

0.1µF

100k

Figure 9. Ensuring

MAX6304

RESET

GND

100k

0.1µF

START

SET WDI

LOW

SUBROUTINE OR

PROGRAM LOOP

SET WDI HIGH

RETURN

END

Figure 11. Watchdog Flow Diagram