Hi All.

I'm observing some behavior on the 16F877 that doesn't jive with the
manual. Can someone shed some light on this?

Application: WDT enabled, interrupts enabled, interrupts on RBO/INT
enabled via INTE. No other interrupts enabled, no other peripherals
in use.

Behavior: The PIC goes to sleep just fine, and wakes up each time the
WDT expires (roughly every 16ms). It also wakes up upon a change on
the RB0/INT pin, and processes the interrupt properly.

Problem: Regardless of the source of wake-up (WDT or RBO/INT), the
-TO bit is always cleared (0) after waking up. My reads of the manual
suggest that this is NOT expected behavior -- the -TO bit should be
cleared after waking up only if the reason for waking up was the WDT.

Relevant code looks like this:

   for(;;) {
     ...
     CLRWDT();
     SLEEP();
     if ( TO == 0 )
       DoTimerFn();
     ...
   }

The compiler has compiled the if() statement properly. I "trapped" T0
== 1 and it never happens. I can view on the logic analyzer that no
matter when the RB0/INT interrupt occurs relative to the periodic
wake-up due to WDT, DoTmerFn() is _always_ called. I want it to be
called only when the wakeup is due to WDT expiring.

I've also noticed one other thing - if I set a breakpoint in the ISR,
and then run from there, the -TO bit is correct when tested. But if I
let it run free, -TO is always 0.

Any ideas? Any example code of wake-from WDT and at least one other source?
--

  ______________________________________
   Andrew E. Kalman, Ph.D.   aek@pumpkininc.com

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