how can i measure and compare voltage

at first i want adjust over and under voltage and timer with pot after this if ac line voltage is exceed to my edge ,how can i start the time(1 to 20 second--its adjustable) and how can i compare this voltages...

pic is could be 16F876
thanx to who help me or not...

hm. I cannot understand your question..

I think what he wants to do is use a voltage divider with a potiometer so that it can be used for different levels of inputs. From there, he wants to moniter it for its maximum (minimum too?) value and trigger something if it goes out of range.

I'd do it with an ADC. Set up a voltage divider with potentiometers and resistors, connect it to an ADC, and then let it run. You'll have to find out what the maximum impedance is for the ADC, and if the AC line goes below ground, come up with some kind of protection for that.... like a 1n4001 diode.

But I'm not familiar with pics so I can't offer anything more.