Darrel,
This may be stating the obvious, but either:
1) Both timer routines are accurate (Definitely not true)
2) T0 is accurate, T1 is not
3) T1 is accurate, T0 is not
4) Neither timer routine is accurate (if your Xtal is within specs and running fast, T0 runs fast and T1 runs slow, this could explain the results you have shown)
The Xtal error needs to be eliminated to see what’s happening with the last three. One suggestion may work. If you have a RTC handy that will output the 32768Hz, use this as an input to the timers. Then, you can compare the PIC clocks against RTC, all of which would be running at the same exact freq. Some tweeking of the code will be required for this. Also, This may lead to changes in the code which would mask the errors occurring with the 4Mhz Xtal, but it may be worth a try.
I have actually done this with a timer routine similar to the Elapsed timer, and it works fine. I will add it to this post when I get back to my home computer (Found it! Elapsed_Timer_INT_32.bas.txt). It has some significant differences though. It never stops the timer, only adds to the TMRxH byte, and counts 128 "ticks" per second. (edit: I also count DOWN. I think this added some efficiency with the ASM, but can't remember off the top of my head. It's run on an 18F4620)
The other is to get a measure of the actually freq of your crystal, then factor this into the timer errors. And I am not sure how to do this to the accuracy needed to find the problem.
Intriguing problem,
Steve
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