Kicking The Watchdog

As has been explained, the watchdog will wait for the configured amount of time. If the watchdog determines that the program has crashed, it will trigger a Watchdog Interrupt (if EWDI is set) after the configured amount of time. 128 Instruction cycles later, if the program still appears to be crashed, it will reboot the system.

In order to prevent a watchdog reboot, your software must let the watchdog know that it is still alive. It does this by executing the following code:

MOV TA,#0AAh ;Execute the Timed Access Protection
MOV TA,#55h ;code to open the Timed Access window
SETB RWT ;Reset the watchdog timer by setting the RWT bit

Setting the RWT bit (WDCON.0) is how your program lets the watchdog know that everything is ok--that your program hasnt crashed. Doing so will reset the watchdog timer.

You may be asking, "What are those first two MOV instructions for?" The RWT bit is a "protected bit." You may not write to it without executing the two MOV instructions. Note that you must also execute those two same instructions before modifying WDCON.0, WDCON. 1, and WDCON.3. Please see the chapter regarding "Timed Access Protection" for a more full discussion of why this is necessary.