Dear Charles,

That was a good diagram for teach how a frequency doubler works.

I have the following comments :

1. Setting a gate width and capturing interrupts can limit your resolution. Like if it set at 1mS, then any pulse width below 1mS cannot be measured. In such a case there has to be a fall back algorithm to auomatically switch to the ext lower level of gate width.

2. It is assumed that the incoming pulse is "clean". But believe me, sorting the spike from the pulse IS the challenge in any pulse width measuring circuit. Or so I believe after my last project to find out the phase difference between two AC waveforms.

Raghu