How to detect low battery in battery powered F683 circuit

[tekart]

I do a lot of battery powered designs with li-po (3.7v) batteries - mostly using 8 pin chips like the F683. I am trying to figure out if there is a way to use an AD input - or comparator configuration to determine if the battery that is powering the chip is running low. A threshold of 3.2 V would be good. A resistor divider connected to the A/D input will not work because it will always read the resistor ratio voltage as a constant value as the battery voltage changes. I have been fooling around with a zener diode/resistor combination with no luck. I don't have any experience using the comparators inside PICs and wonder if there is some way to configure a comparator to detect a low battery threshold. Does anyone have an elegant solution for detecting a low battery condition? In my current design I only have AN0 available for battery sensing on the F683.

I really need a very low part count/low cost solution! No external comparators or circuits, is this practical?