> Van: Daniel Watford <piclist@CCTEC.LTD.UK>
> Aan: PICLIST@MITVMA.MIT.EDU
> Onderwerp: Re: Newbie code doesn't seem to work - please take a look
> Datum: donderdag 8 juli 1999 12:05
>
> Hi again,

Hello Daniel,

> I've checked the watchdog timer, and indeed it was enabled.
> I've now disabled it and the circuit is working perfectly.
> What does the watchdog time do exactly?

Wachdog-timers are invented to mess-up programs of Newbies.  Really ! :-)

Ah, well.  Id see you don't beleive me.  O.k. Another explanation of their
existence:  It is possible that a controller will get stuck in some kind of
loop, due to bad programming (go kick the programmer :-) or due to power
spikes and the like.  When this happens, the circuit has to be reset to
enable it to work again.

So, the first controllers where build wid a LED attached, that blinked when
all was well.  If it stopped blinking, someone had to press the RESET
button.

After a while someone build a "missing puls" detector (a retriggerable
mono-flop) and connected that to the blinking led and the RESET button.
Now, whenever the led stopped blinking, the mono-flop would time-out, and
reset the system.  No human needed anymore !

Later they decided to build the mono-flop into the system.  That feature is
now known as a "WachDog".  It will simply count pulses (from and
independend internal source).  If the WachDog-counter overflows it will
either Reset the controller (or generate a Interrupt, depended on how it's
configured).  The blinking LED was replaced by an Instruction to re-set the
WachDog (CLRWDT -> Clear WachDog Timer).

So, when a WachDog is enabled and not re-set every once-in-a-while, it will
eventually reset the controller, creating strange effects for unsuspecting
Newbies .... (Sorry, could not let this one go ... :-)

Greetz,
   Rudy Wieser