Rainbow Electronics T89C51RD2 - Manual (Page 43)

Rev. F - 15 February, 2001 43
T89C51RD2
6.10. 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.
6.10.1. 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
OSC
, where T
OSC
= 1/F
OSC
. 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 2
7
counter has been added to extend the Time-out capability, ranking from
16ms to 2s @ F
OSC
= 12MHz. To manage this feature, refer to WDTPRG register description, Table 22. (SFR0A7h).
Table 21. WDTRST Register
WDTRST Address (0A6h)
Write only, this SFR is used to reset/enable the WDT by writing 01EH then 0E1H in sequence.
7 6 5 4 3 2 1
Reset value X X X X X X X