Hi Tony - tricky this electronic text communication isn't it?
I think that I simply didn't do a good job of asking the question.
Having reviewed a number of other designs for shift lights, I've seen code written to count pulses and divide by time, to do it twice in order to ignore any spurious results - and several other variations.
The question I was TRYING to ask ( :-) ) was: do you consider the interrupt method to be the most appropriate and accurate method for measuring pulse frequency ?
I was NOT querying the accuracy of your implementation - just looking for "reassurance" that this is, indeed, the way to go.
Obviously the rest of your code doesn't relate to my application, but the two interface suggestions will come in handy as well.
My thanks again.