If you're going to use the loop time to keep track of elapsed time, you'll need to check the packet number, and account for missed packets (loops).
What happens is that if your code is too slow, it might not make it back in time to read the very next data packet, and thus might miss a loop.
What you need to do is to keep track of the packet number of the last data packet you read, and multiply the difference between that number and the current packet number by 26.2 before adding to your match timer.
BTW, DON'T multiply by 26.2 (a float). DO multiply by 262, and simply consider your match timer to be in units of 10ths of milliseconds instead of milliseconds. The same goes for any other floating point operations.
You need to realize that the PIC processor does not natively support floating point operations, and any floating point calculations you request will be performed in software -- which takes up more code space, AND takes MUCH longer. IF you are currently using floats, this COULD be why you are missing data packets, and why your timer was not accurate.
Better yet than multiplying by 262 is to not multiply at all! Just leave your timer in units of 26.2ms clock ticks. (You still need to account for missed packets.)
OK. this post is getting too long. If you need more details, just ask. (Or search. I'm pretty sure this has all been discussed before.
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