> this is exactly the type of info that is not in any book. thanks for this.

You're welcome. What will best help you learn is a project that you
design and program

For example, if you want to track a square wave and light an LED, a
simple way would be to poll the input. Say you have chosen B0 / INT0
as the input

repeat

btfss portb,0 ;wait here until the input goes high
goto $-1

bsf led ;turn the LED on

btfsc portb,0 ;wait here until the input goes low
goto $-1

bcf led ;turn the LED off

goto repeat ;go back to the start

As you can see, that ties up the micro for 100% of its time and you
can do no other work with it. If you use an interrupt then that frees
up the micro to do other things. You'd set up the interrupt system to
react to an INT0 event and alternate the edge trigger polarity. This
process is quite straight-forward but can get quite involved depending
on what else is happening in the software, so you need to account
for everything and anything that could happen to ensure smooth running
and good time management

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