T89C51AC2-RLSI-M datasheet(72/117 Pages) ATMEL

72 / 117 page

Draft.A - March 30, 2001

Preview - Confidential

T89C51AC2

14.3. WatchDog Timer during Power down mode and Idle

In Power Down mode the oscillator stops, which means the WDT also stops. While in Power Down mode the

user does not need to service the WDT. There are 2 methods of exiting Power Down mode: by a hardware reset

or via a level activated external interrupt which is enabled prior to entering Power Down mode. When Power

Down is exited with hardware reset, servicing the WDT should occur as it normally does whenever T89C51AC2

is reset. Exiting Power Down with an interrupt is significantly different. The interrupt is held low long enough for

the oscillator to stabilize. When the interrupt is brought high, the interrupt is serviced. To prevent the WDT from

resetting the device while the interrupt pin is held low, the WDT is not started until the interrupt is pulled high.

It is suggested that the WDT be reset during the interrupt service for the interrupt used to exit Power Down.

To ensure that the WDT does not overflow within a few states of exiting of powerdown, it is best to reset the

WDT just before entering powerdown.

In the Idle mode, the oscillator continues to run. To prevent the WDT from resetting T89C51AC2 while in Idle

mode, the user should always set up a timer that will periodically exit Idle, service the WDT, and re-enter Idle mode.

T89C51AC2-RLSI-M 数据表(PDF) 72 Page - ATMEL Corporation