> -----Original Message-----
> From: Larry Bradley [SMTP:lhbradley@IGS.NET]
> Sent: Tuesday, April 22, 2003 10:30 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      CCP compare function
>
> I'm trying to use the CCP module on a 628 to generate 10ms interrupts.
> I've
> done this before using timer1, preloading it with a value then reloading
> in
> the interrupt routine. It works, but I thought I would try it with the CCP
> module, since it isn't necessary to keep reloading the timer in the
> interrupt routine.
>
> Using a 20 mHz xtal, setting the CCP registers to 50,000 should generate
> an
> interrupt every 10 msec. I use the interrupt to accumulate hours, mins and
> seconds for  a time-of-day clocks. When I compare the time from the PIC
> with the time from my GPS, over about 14 hours, my time is about 7 seconds
> too fast. Changing the CCP value to 50007 makes the clock track the GPS
> time just fine.
>
> The xtal is a nominal 25 ppm device - this could account for 1 or 2 counts
> in 50000, but not 7, I wouldn't think
>
> Has anyone else played with timer1 and the CCP in this fashion?
>
The first thing I would suspect is that you don't have the correct
capacitors on each leg of the crystal and it's oscillating slightly off
frequency.  The only way you can reliably measure this (without potentialy
affecting xtal frequency) is by making a very simple main loop that simply
toggles an output every X instructions, and then measuring that output with
a frequency counter.

Regards

Mike


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