Interfacing to µPs with

Bidirectional Reset Pins

Since the RESET output on the MAX6324 is open-drain,

this device easily interfaces with µPs that have bidirec-

tional reset pins, such as the Motorola 68HC11.

Connecting the µP supervisor’s RESET output directly

to the microcontroller’s (µC’s) RESET pin with a single

pullup resistor allows either device to assert reset

(Figure 7).

MAX6324 Open-Drain

RESET Output

Allows Use with Multiple Supplies

Generally, the pullup resistor connected to the

MAX6324 will connect to the supply voltage that is

systems may use the open-drain output to level-shift

from the monitored supply to reset circuitry powered by

some other supply (Figure 8). Keep in mind that as the

IC’s ability to sink current at RESET. Also, with any pull-

toward 0. The voltage where this occurs depends on

the pullup resistor value and the voltage to which it is

connected.

Watchdog Software Considerations

To help the watchdog timer monitor software execution

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

ferent 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

time would continue to be reset within the loop, keeping

the watchdog from timing out.

Figure 9 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 be quickly

corrected, since the I/O is continually set low and the

watchdog time is allowed to time out, causing a reset or

interrupt to be issued.

µP Supervisory Circuits with Windowed

(Min/Max) Watchdog and Manual Reset

_______________________________________________________________________________________

9

MAX6323

GND

RESET

R1

100k

MAX6324

GND

GND

µP

RESET

RESET

INPUT

Figure 7. Interfacing to µPs with Bidirectional Reset Pins

MAX6324

GND

GND

+3.3V

+5.0V

5V SYSTEM

RESET

RESET

INPUT

Figure 8. MAX6324 Open-Drain RESET Output Allows Use with

Multiple Supplies