On Tue, 24 Jun 1997, Tom Coonan wrote:

> Hey y'all,
>
> I've been off the list for a year, and didn't find the following in any
 FAQs...
>
> I am implementing a PIC "power manager" that will sleep for very long
> intervals.  It will use the WDT to awake every 2.3 seconds, decrement
> a counter, and see if the very long interval (e.g. say, a day or even a week)
> has expired.  I have already done this.  The problem, is that the WDT interval
> of 2.3 seconds is dictated by an on-chip RC that is highly temperature
> sensitive.  I am considering schemes where the PIC occasionally wakes
> up and times its own WDT interval while awake (when it has the more
> accurate crystal timebase) hence the "Self-calibration".
> Theoretically, I think this can work.  I have some formulas,
> etc. but I sure would love to learn of anyone else who has done this
> successfully.  I think there may be a more elegant approach than
> some of my current scribbles.  It's an interesting puzzle, ain't it?
>
> Thanx in advance
>
> Thomas A. Coonan
> tcoonan@mindspring.com
> (...need a freeware synthesized Verilog ASIC PIC core?...its rough, but its
> free ;-).... )
> http://www.mindspring.com/~tcoonan
>
Just a suggestion but if room permits, try adding a small 32 KHz crystal
and initialise the real time clock.  This is a real time clock which will
keep running even during sleep mode. The app. notes at microchip give an
example of it's use.