On May 11, 2005 11:32 am, peiserma@ridgid.com wrote:
> I need a sanity check on WDT wake-up from sleep. I thought I
> understand it, but MPSIM is giving different results.
>
> I'm working with a 12F675. I want it to power down via the sleep
> instruction, but periodically wake up, perform some work, and then go
> back to sleep. The exact timing between wake-ups is not critical.
>
> I interpret table 10-6 in conjunction with the STATUS register to say
> if WDT occurs after the SLEEP instruction was issued, then both /TO
> and /PD bits should be cleared (***). In the simulator, the /TO bit
> is actually set after the WDT, not cleared as I interpreted from the
> datasheet.

TO should be "1" after power-up, CLRWDT or Sleep.
TO is a poor indicator since you do not know if it was a sleep or reset 
that triggered it. Therefore leave TO out of your program.

If you let your program run-away, then the WDT is going to reset your 
micro and PD would be 1. If your micro was powered-up, then PD is 1.
If you performed a Sleep, then PD is 0.

Since you are doing a sleep, then use PD to differentiate between a 
reset-power-up and a sleep-power-up. If your program is overly 
complicated, you may want to insert a pair of checksum bytes in RAM, 
let's say AAh:55h and if you didn't find those there, then you are 
fairly sure you are dealing with a power-up and not a WDT type of 
reset.
If your program is fairly simple and rather impossible to get stuck in a 
forever-loop of any sort, then just testing PD should be adequate.
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist