The resolution of PULSIN is dependent upon the oscillator frequency. If
a 4MHz oscillator is used, the pulse width is returned in 10us increments.
If a 20MHz oscillator is used, the pulse width will have a 2us resolution.
Defining an OSC value has no effect on PULSIN. The resolution always
changes with the actual oscillator speed.
PULSIN normally waits a maximum of 65535 counts before it determines
there is no pulse.
With a 4MHz clock that works out to be 655mS (or 131mS at 20MHz).