Measure pulse-width using external interrupt.

The pulse width is the length of time between the rising edge of a signal, and the corresponding falling edge (or vice versa):

     __________
____|          |______
    ^          ^
    Rising     Falling
    <--------->
    Pulse width

You need to use an interrupt to detect the rising edge change, start a timer of some kind, and stop it on the falling edge detect interrupt.