© 2002 Microchip Technology Inc.

DS21402B-page 5

TC32M

3.0

DETAILED DESCRIPTION

The TC32M provides three important functions to

safeguard stable processor operation: precision pro-

cessor monitor, watchdog sanity timer and external

override reset control.

3.1

Processor Monitor

below the nominal threshold voltage. As a result, this

pin is LOW when power is initially applied, holding the

processor in its reset state. RS remains low for a

allow the power supply and processor to stabilize.

3.2

Watchdog Timer

The processor drives the RS pin with an input/output

(I/O) line in series with an resistor voltage divider to

results in an internal voltage change (strobe) sufficient

threshold of the processor RESET pin. The processor

must continuously apply strobes in this manner within

a set period to verify proper software execution. A

momentary reset (500msec minimum) is generated by

the TC32M if a hardware or software failure keeps RS

from being strobed within the watchdog timeout period.

This action typically initiates the processor's power-up

routine. If the interruption persists, new reset pulses

are generated each timeout period until RS is strobed.

This timeout period is typically 700msec.

FIGURE 3-1:

WATCHDOG STROBE

The software routine that drives the RS strobe must be

in a section of the program that executes frequently

enough so the time between toggles is less than one

watchdog timeout period. The strobe signal can be

derived from microprocessor address, data and/or

control signals. Typical circuit examples are shown in

Figure 3-4.

3.3

Resistor Value Selection

The values of R1 and R2 must be chosen to ensure a

I/O line is low. The use of 10k

Ω, ±5% tolerance

resistors are recommended. These values result in a

nominal strobe level of 2.5 on RS (min/max of 2.13V/

values can be used, so long as the additive tolerances

of the power supply and resistor values result in a

tolerance conditions.

RS

will keep the watchdog timer from forcing RS LOW.