AT89C51ID2-RDTIM datasheet(108/159 Pages) ATMEL

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108

AT89C51ID2

4289A–8051–09/03

Hardware Watchdog

Timer

The WDT is intended as a recovery method in situations where the CPU may be sub-

jected 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.

Using the WDT

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

location 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 T

CLK PERIPH, where TCLK PERIPH= 1/FCLK

PERIPH. 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.

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

capability, ranking from 16ms to 2s @ F

OSCA = 12MHz. To manage this feature, refer to

WDTPRG register description, Table 81.

Table 81. 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.

7654

3210

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AT89C51ID2-RDTIM Datasheet(PDF) 108 Page - ATMEL Corporation