Do you take care of accounting for the latency of the interrupt when you reload the timer value?

If you do not then this will cause the timer to run slower than you had expected. The timer interrupt occurs when the timer has counted 0xFFFF to 0x0000. Any time spent in the process of getting into the timer interrupt routine and setting up to re-program the timer reload value gets added onto the actual timer period.

You should search this forum with the subject keywords of timer interrupt latency. This will lead you to quite a number of threads where this issue has been discussed before. You will even find suggestions from previous posters on how to specifically adjust the timer reload value in order to take the latency into account!!

The SEARCH feature is your friend !!!

Michael Karas