__________Applications Information

Selecting the Reset and

Watchdog Timeout Capacitor

The reset timeout period is adjustable to accommodate

a variety of µP applications. Adjust the reset timeout

the reset timeout capacitor as follows:

age (<10nA) type capacitor. Ceramic is recommended.

The watchdog timeout period is adjustable to accom-

modate a variety of µP applications. With this feature,

the watchdog timeout can be optimized for software

execution. The programmer can determine how often

the watchdog timer should be serviced. Adjust the

(Figure 3). For normal-mode operation, calculate the

watchdog timeout capacitor as follows:

low leakage (<10nA) type capacitor. Ceramic is recom-

mended.

The

monitoring other voltages is simple, and Figure 4 shows

shown in the

Reset Threshold section.

Wake-Up Timer

In some applications, it is advantageous to put a µP

into sleep mode, periodically “wake it up” to perform

checks and/or tasks, then put it back into sleep mode.

The MAX6301 family supervisors can easily accommo-

date this technique. Figure 5 illustrates an example

using the MAX6302 and an 80C51.

In Figure 5, just before the µC puts itself into sleep

mode, it pulls WDS high. The µC’s I/O pins maintain

their logic levels while in sleep mode and WDS remains

high. This places the MAX6302 in extended mode,

increasing the watchdog timeout 500 times. When the

watchdog timeout period ends, a reset is applied on

the 80C51, “waking it up” to perform tasks. While the µP

is performing tasks, the 80C51 pulls WDS low (select-

ing normal mode), and the MAX6302 monitors the µP

for hang-ups. When the µP finishes its tasks, it puts

itself back into sleep mode, drives WDS high, and

starts the cycle over again. This is a power-saving tech-

nique, since the µP is operating only part of the time

and the MAX6302 has very low quiescent current.

Adding a Manual Reset Function

A manual reset option can easily be implemented by

connecting a normally open momentary switch in paral-

lel with R2 (Figure 6). When the switch is closed, the

voltage on RESET IN goes to zero, initiating a reset.

When the switch is released, reset remains asserted for

the reset timeout period and then is cleared. The push-

button switch is effectively debounced by the reset

timer.

+5V, Low-Power µP Supervisory Circuits

with Adjustable Reset/Watchdog

8

_______________________________________________________________________________________

0.1µF

MAX6301

MAX6302

MAX6303

MAX6304

RESET IN

R2

R1

R2

MAX6302

RESET

WDI

WDS

I/O

I/O

I/O

GND

80C51

GND

RST

* THREE-STATE LEAKAGE MUST BE <10µA.

*

Figure 5. Wake-Up Timer