W79E217A Datasheet, PDF (101/207 Pages) Winbond – 8-bit Microcontroller

Preliminary W79E217A Data Sheet
When the selected time-out occurs, the watchdog interrupt flag WDIF (WDCON.3) is set. After
watchdog time-out, and if Watchdog Timer Reset EWT (WDCON.1) is enabled, the Watchdog Timer
will cause a reset 512 clocks later. This reset lasts two machine cycles, and the Watchdog Timer reset
flag WTRF (WDCON.2) is set, which indicates that the Watchdog Timer caused the reset. RWT can
be used to clear Watchdog timer before a time-out occurs.
The Watchdog Timer is disabled by a power-on/fail reset. The external reset and Watchdog Timer
reset can not disable Watchdog Timer, instead it only restart the Timer.
The control bits that support the Watchdog Timer are described as below:
Watchdog Timer Control (WDCON)
BIT NAME
FUNCTION
7
- Reserved.
6
POR
Power-on reset flag. The hardware sets this flag during power–up, and it can only
be cleared by software. This flag can also be written by software.
5-4
- Reserved.
Watchdog Timer Interrupt Flag. If the watchdog interrupt is enabled, the hardware
3
WDIF
sets this bit to indicate that the watchdog interrupt has occurred. If the interrupt is
not enabled, this bit indicates that the time-out period has elapsed. This bit must be
cleared by software.
Watchdog Timer Reset Flag. If EWT is 0, the Watchdog Timer has no affect on this
2
WTRF
bit. Otherwise, the hardware sets this bit when the Watchdog Timer causes a reset.
It can be cleared by software or a power-fail reset. It can be also read by software,
which helps determine the cause of a reset.
1
EWT
Enable Watchdog-Timer Reset. Set this bit to enable the Watchdog Timer Reset
function.
0
RWT
Reset Watchdog Timer. Set this bit to reset the Watchdog Timer before a time-out
occurs. This bit is automatically cleared by the hardware.
The POR, EWT, WDIF and RWT bits are protected by the Timed Access procedure. This procedure
prevents software, especially errant code, from accidentally enabling or disabling the Watchdog Timer.
An example is provided below.
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Publication Release Date: December 14, 2007
Revision A3.0