Hi
Measuring pulse-width is a part of frequency measurement, which has been discussed recently on this forum. You can have a look at that for some good guidelines on frequency measurement. Also search the rest of the site, as its been discussed before a number of times here.

Basically I'd say use an edge-triggered external interrupt to start/stop a timer to determine the pulse duration... only edge-triggered interrupts are prone to noise. If you are sure that the input is going to be of a fixed duty cycle, you can use a timer in gated mode, i.e. the timer runs only when signal on pin is high.

On the other hand if you are going to get high frequency pulses, you can simply count the number of pulses within a certain fixed interval (the longer the interval, the more the resolution, but also control action will be more delayed)

Kundi