Not sure if you want to measure Pulse Width or Frquency, but here's a start.

If you need acurracy, you should use the Capture Mode of a CCP module.
First one has an Instant Interrupt example too. ;)
http://www.picbasic.co.uk/forum/showthread.php?p=23401 (Bruce)
http://www.picbasic.co.uk/forum/showthread.php?p=41621 (Bruce)

Or, you can get pretty good results with a Timer and interrupts ...
http://www.picbasic.co.uk/forum/showthread.php?t=1044 (mister_e)
http://www.picbasic.co.uk/forum/showthread.php?p=16636 (mister_e)
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