Interval Timer Problem

Do you use that timer only as a cycle counter or do you use it as the system timer for an OS or for any interrupt?

As I said in a previous post, the interval timer always counts the cycles of the system clock: if you reduce the timeout period you DON'T improve timer resolution; on the contrary, you shorten the measurable time, since it resets to zero in a shorter time.

If you use the code you posted above, you rather need a very long timeout period, greater than the maximum interval you are supposed to measure.