Resets are issued to the µP during power-up, power-

down, and brownout conditions. The Typical Operating

Characteristics show a graph of the MAX6821–

Threshold Overdrive, for which reset pulses are not

generated. The graph was produced using negative-

voltage and ending below the reset threshold by the

magnitude indicated (reset threshold overdrive). The

graph shows the maximum pulse width that a negative-

ing a reset pulse. As the amplitude of the transient

increases (i.e., goes farther below the reset threshold),

the maximum allowable pulse width decreases.

reset threshold and lasts for 20µs or less will not trigger

a reset pulse.

Watchdog Software Considerations

One way to help the watchdog timer monitor software

execution more closely is to set and reset the watch-

dog 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 inside the

loop, keeping the watchdog from timing out.

Figure 5 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. As described

in the Watchdog Input Current section, this scheme

results in higher time average WDI input current than

does leaving WDI low for the majority of the timeout

period and periodically pulsing it low-high-low.

Low-Voltage SOT23 µP Supervisors with

Manual Reset and Watchdog Timer

_______________________________________________________________________________________

7

Figure 3. Watchdog Timing Relationship

Figure 4. Interfacing Open-Drain RESET to µPs with

Bidirectional Reset I/O

WDI

RESET*

*RESET IS THE INVERSE OF RESET.

MAX6822

RESET

GENERATOR

GND

GND

RESET

RESET

Figure 5. Watchdog Flow Diagram

START

SET WDI

HIGH

PROGRAM

CODE

SUBROUTINE OR

PROGRAM LOOP

SET WDI LOW

RETURN

MAX6823

µP

GND

MANUAL

RESET

RESET

I/O

RESET

MR

WDI

Typical Operating Circuit