Andy,
Typically period measurements are used when pulse by pulse data is needed. You could either measure the width of a pulse or the period of a cycle.
In either case you need to define a minimum speed, or maximum pulse width or period you want to detect. When the shaft slows down the time between pulses or edges gets longer and longer. Make sure your period measurement process has some kind of timeout or maximum time limit so that it does not hang up waiting for another edge onyour signal.
Lynn
