ST72104GX datasheet(39/141 Pages) STMICROELECTRONICS

39 / 141 page

ST72104Gx, ST72215Gx, ST72216Gx, ST72254Gx

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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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