T89C51AC2-SLSIM datasheet(71/121 Pages) ATMEL

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A/T89C51AC2

4127G–8051–05/06

Watchdog Timer

A/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 in X1 mode.

This WDT consists of a 14-bit counter plus a 7-bit programmable counter, a Watchdog

Timer reset register (WDTRST) and a Watchdog Timer programming (WDTPRG) regis-

ter. 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

ment 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

Note:

When the Watchdog is enable it is impossible to change its period.

Figure 38. Watchdog Timer

WDTPRG

RESET

Decoder

Control

WDTRST

Enable

14-bit COUNTER

7-bit COUNTER

Outputs

RESET

Fwd Clock

T89C51AC2-SLSIM Datasheet(PDF) 71 Page - ATMEL Corporation