I need to implement a low battery detection circuit using already available components. My 8051 polls a digital I/O line in which the level should change when the voltage gets too low.

The circuit is powered with a standard 9 volt battery. The 8051 and remaining circuit is pretty low power and operates off of a 3.3 volt regulator. The board also has a regulated 5 volt supply available.

I have two general purpose, low-power, single supply opamps available from an LM324 IC.

Any circuits already available or information on the best way to implement this would be great.

Thanks
Javier