AT89C51ED2-RDTUM datasheet(84/137 Pages) ATMEL

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4235K–8051–05/08

AT89C51RD2/ED2

19. Hardware Watchdog Timer

The WDT is intended as a recovery method in situations where the CPU may be subjected to

software upset. The WDT consists of a 14-bit counter and the Watchdog Timer ReSeT

(WDTRST) SFR. The WDT is by default disabled from exiting reset. To enable the WDT, user

must write 01EH and 0E1H in sequence to the WDTRST, SFR location 0A6H. When WDT 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 drive an output RESET HIGH pulse at the RST-pin.

19.1

Using the WDT

To enable the WDT, user must write 01EH and 0E1H in sequence to the WDTRST, SFR loca-

tion 0A6H. When WDT is enabled, the user needs to service it by writing to 01EH and 0E1H to

WDTRST to avoid WDT overflow. The 14-bit counter overflows when it reaches 16383 (3FFFH)

and this will reset the device. When WDT is enabled, it will increment every machine cycle while

the oscillator is running. This means the user must reset the WDT at least every 16383 machine

cycle. To reset the WDT the user must write 01EH and 0E1H to WDTRST. WDTRST is a write

only register. The WDT counter cannot be read or written. When WDT overflows, it will generate

an output RESET pulse at the RST-pin. The RESET pulse duration is 96 x TCLK PERIPH, where

TCLK PERIPH= 1/FCLK PERIPH. To make the best use of the WDT, it should be serviced in those sec-

tions of code that will periodically be executed within the time required to prevent a WDT reset.

To have a more powerful WDT, a 27 counter has been added to extend the Time-out capability,

ranking from 16 ms to 2s @ FOSCA = 12 MHz. To manage this feature, refer to WDTPRG register

description, Table 19-1. The WDTPRG register should be configured before the WDT activation

sequence, and can not be modified until next reset.

Table 19-1.

WDTRST Register

WDTRST - Watchdog Reset Register (0A6h)

Reset Value = XXXX XXXXb

Write only, this SFR is used to reset/enable the WDT by writing 01EH then 0E1H in sequence.

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