ST7L15 datasheet(50/138 Pages) STMICROELECTRONICS

50 / 138 page

ST7L15, ST7L19

50/138

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 upon expiration of a

programmed time period, unless the program re-

freshes the counter’s contents before the T6 bit is

cleared.

11.1.2 Main Features

■ Programmable free-running downcounter (64

increments of 16000 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

T[6:0]) is decremented every 16000 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

30µs.

Figure 31. Watchdog Block Diagram

RESET

WDGA

7-bit DOWNCOUNTER

fCPU

CLOCK DIVIDER

WATCHDOG CONTROL REGISTER (CR)

÷16000

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