skimask your last description is exactly what I am trying to do. My code essentially does just that over a 200 cycle period but not very gracefully as written, the start and stop of Timer1 I don't thinks is well done as I am using software to do it rather than hardware. In my attempt to measure the frequency fast, the physics of the problem limits how fast the measurement can be done for a given accuracy, but when comparing to a frequency counter approach I would need a 10 second period for 0.1Hz resolution, where by measuring the period as you point out can provide great resolution fast relative to a frequency counter, I guess I should have been more clear in my original posting, because my target is ~100 msecs for the measurement time. After doing more research it looks like the approach of using the capture/compare module with Timer1 as you point out Don is the way to go. I have not used the capture mode in the PIC's in an interrupt mode, are there any good code examples.
Terry