User Manual

T89C51AC2

Rev. B – 19-Dec-01

14.2 WatchDog Timer

during Power down

mode and Idle

In Power Down mode the oscillator stops, whi c h means the WDT also s t ops . While in

Power Down mode, the user does not need to s erv ice the WDT. There are 2 methods of

exiting Pow er Down mo de: by a hardware reset or via a level activated external interrupt

which is enabled prior to ent ering Po w er Down mode. When P ower Down is exited with

hardware reset, t he watch dog is disabled. Exiting Power Down with an interrupt is signif-

icantly d ifferent. The interrupt shall be held low long enough for the oscillator to stab ilize.

When the interru pt is brought high, the in t errupt is serviced. To prev ent the WDT f r om

res etting the devic e while th e int errupt pin is hel d low, the W DT is no t started until th e

interrupt is pulled high. I t is s uggested that the WDT be reset during the i nterrup t service

for the interrupt used t o exit Power Down.

To ensure that the WDT does not ov erf low wi thin a few states of exit ing powerdown, it is

best to reset the WDT just before entering powerdown.

In the I dle m ode, the oscillator continues to run. To pr event the WDT f rom resetting

T89C51A C2 wh ile in Idle mode, the user should always set up a t im er that will periodi-

cally exit Idle, service the WDT , and re-enter Idle mode.

14.2.1 Register Table 34. WDTPRG Re gister

WDTPRG (S:A7h)

WatchDog Timer D uration

Programming regis ter

Rese t Va lue = XXXX X000 b

76543210

- - - - - S2 S1 S0

Bit

Number

Bit

Mnemonic Description

Reserved