I have a RFmodule & a PIC (16F877A). They both can run upto 3V to 5V. One pin of the RF module gives a constant voltage of 2.8V.

I plan to run the PIC on 4 AA batteries, rechargeable. (appx 1.2v - 1.3v when fully charged).

I want to take this lower voltage (2.8V) as a reference and check when battery pack reaches 3.6. Can someone please guide me how I can take the lower voltage as a ref and check the supply voltage. Thanks