Hi,

are you trying to detect any keys from the keyboard or specific ones?

If you are trying to detect specific key presses, analyzing the USB data using comparators sounds quite tricky to me...

If it was me, I would make a software application that listens for the specific key presses and then on receiving them outputs a specific message to the PIC, either by USB or RS-232, and then the PIC can take the relevant action.

May be a different way of looking at what you are trying to achieve.

Hope this helps

Rob