At 01:57 PM 6/21/02 -0400, Bob Ammerman wrote:
>How accurate does the 300 seconds have to be?  If you can tolerate some
>variation, I would consider using the watchdog timer.  You will have to
>calibrate the watchdog on a periodic basis - this is easily done by setting
>the watchdog to no pre-scaler, running the PIC, then timing how long it for
>the watchdog to trigger.  Use that measured time as the basis for setting
>how many watchdog timeouts you need to achieve 300 seconds.
>
>... This will be highly temperature dependent.

Depends upon how often you calibrate the timer.  I'm assuming that the
temperature would not change significantly within, say, 10 minutes.  Or 20
minutes.  In other words, it is application specific and original poster
has to decide how much variation he tolerate and how much the temperature
can be expected to change over what time period.  Then calibrate the
watchdog timer at that interval.

However, it has been pointed out that the watchdog consumes significant
current compared to just the quiescent sleep current.  The data sheet bears
this out - 5 uA (max) vs 25 uA (max).  Of course, if one used typical
values, it looks less bad.  But we don't do that, do we ;)

So next he has to decide if the extra 20 uA consumed by the watchdog is any
better or worse than other techniques.

It all depends upon the intended usage.  I would have NO hesitation using
the watchdog as a power-up timer for a freezer alarm.  Check for unsafe
temperature every 5 minutes or 4 minutes or 6 minutes - who cares?  Just
not important.

dwayne

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Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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