T89C51AC2-RLSI-M datasheet(70/117 Pages) ATMEL

70 / 117 page

Draft.A - March 30, 2001

Preview - Confidential

T89C51AC2

14. WatchDog Timer

14.1. Introduction

T89C51AC2 contains a powerful programmable hardware WatchDog Timer (WDT) that automatically resets the

chip if it software fails to reset the WDT before the selected time interval has elapsed. It permits large Time-Out

ranking from 16ms to 2s @Fosc = 12MHz.

This WDT consist of a 14-bit counter plus a 7-bit programmable counter, a WatchDog Timer reset register

(WDTRST) and a WatchDog Timer programming (WDTPRG) register. When exiting reset, the WDT is -by default-

disable. To enable the WDT, the user has to write the sequence 1EH and E1H into WDTRST register. When the

WatchDog Timer is enabled, it will increment every machine cycle while the oscillator is running and there is no

way to disable the WDT except through reset (either hardware reset or WDT overflow reset). When WDT overflows,

it will generate an output RESET pulse at the RST pin. The RESET pulse duration is 96xTOSC, where TOSC=1/

FOSC. To make the best use of the WDT, it should be serviced in those sections of code that will periodically be

executed within the time required to prevent a WDT reset.

Figure 61. WatchDog Timer

÷ 6

÷ PS

CPU and Peripheral

Clock

Fwd

CLOCK

RESET

Decoder

Control

WDTRST

Enable

14-bit COUNTER

7-bit COUNTER

Outputs

PERIPHERAL CLOCK

RESET

T89C51AC2-RLSI-M Datasheet(PDF) 70 Page - ATMEL Corporation