This watchdog circuitry continuously monitors a pin on the

µP that generates a positive going pulse during normal op-

eration. The period of this pulse is typically on the order of

milliseconds and the pulse width is typically on the order of

10’s of microseconds. If this pulse ever disappears, the

watchdog circuitry will time out and a RESET low will be sent

to the µP. The time out period is determined by two external

components, R

T

The width of the RESET pulse is set by C

resistor according to the following:

RESET

A square wave signal can also be monitored for errors by fil-

tering the C

signal are detected.

Figure 2 is a schematic diagram of a

typical circuit used to differentiate the input signal. Resistor

R

wave and create a short positive pulse suitable for the µP

monitor input. If the incoming signal continues in a high state

or in a low state for too long a period of time, a RESET low

will be generated.

The threshold voltage and input characteristics of this pin are

compatible with nearly all logic families.

There is a limit on the width of a pulse that can be reliably de-

tected by the watchdog circuit. This is due to the output re-

sistance of the transistor which discharges C

state is detected at the input. The minimum detectable pulse

width can be determined by the following formula:

PW

DS011252-14

FIGURE 2. Monitoring Square Wave µP Signals

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