ST72C254G2 datasheet(39/135 Pages) STMICROELECTRONICS

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ST72104G, ST72215G, ST72216G, ST72254G

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11 ON-CHIP PERIPHERALS

11.1 WATCHDOG TIMER (WDG)

11.1.1 Introduction

The Watchdog timer is used to detect the occur-

rence of a software fault, usually generated by ex-

ternal interference or by unforeseen logical condi-

tions, which causes the application program to

abandon its normal sequence. The Watchdog cir-

cuit generates an MCU reset on expiry of a pro-

grammed time period, unless the program refresh-

es the counter’s contents before the T6 bit be-

comes cleared.

11.1.2 Main Features

Programmable timer (64 increments of 12288

CPU cycles)

Programmable reset

Reset (if watchdog activated) when the T6 bit

reaches zero

Optional

reset

HALT

instruction

(configurable by option byte)

Hardware Watchdog selectable by option byte.

11.1.3 Functional Description

The counter value stored in the CR register (bits

T6:T0), is decremented every 12,288 machine cy-

cles, and the length of the timeout period can be

programmed by the user in 64 increments.

If the watchdog is activated (the WDGA bit is set)

and when the 7-bit timer (bits T6:T0) rolls over

from 40h to 3Fh (T6 becomes cleared), it initiates

a reset cycle pulling low the reset pin for typically

500ns.

The application program must write in the CR reg-

ister at regular intervals during normal operation to

prevent an MCU reset. The value to be stored in

the CR register must be between FFh and C0h

(see Table 11 . Watchdog Timing (fCPU = 8

MHz)):

– The WDGA bit is set (watchdog enabled)

– The T6 bit is set to prevent generating an imme-

diate reset

– The T5:T0 bits contain the number of increments

which represents the time delay before the

watchdog produces a reset.

Figure 25. Watchdog Block Diagram

RESET

WDGA

7-BIT DOWNCOUNTER

fCPU

CLOCK DIVIDER

WATCHDOG CONTROL REGISTER (CR)

÷12288

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