ST7SCR datasheet(40/102 Pages) STMICROELECTRONICS

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ST7SCR

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12 ON-CHIP PERIPHERALS

12.1 WATCHDOG TIMER (WDG)

12.1.1 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.

12.1.2 Main Features

Programmable timer (64 increments of 65536

CPU cycles)

Programmable reset

Reset (if watchdog activated) when the T6 bit

reaches zero

Hardware Watchdog selectable by option byte

Watchdog Reset indicated by status flag

12.1.3 Functional Description

The counter value stored in the CR register (bits

T[6:0]), is decremented every 65,536 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 T[6:0]) 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 14):

– The WDGA bit is set (watchdog enabled)

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

diate reset

– The T[5:0] bits contain the number of increments

which represents the time delay before the

watchdog produces a reset.

Table 14.Watchdog Timing (fCPU = 8 MHz)

Figure 25. Watchdog Block Diagram

CR Register

initial value

WDG timeout period

(ms)

Max

FFh

524.288

Min

C0h

8.192

RESET

WDGA

7-BIT DOWNCOUNTER

fCPU

CLOCK DIVIDER

WATCHDOG CONTROL REGISTER (CR)

÷65536

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