I am going to try using a voltage divider to detect a voltage drop below 4.0v. I think my AT89c55 will run on 3v. so with the right cap 470uf(it's what I had handy). According to my atmel spec sheet .2v is the cut off point from digital high to low. my resistor are from V+ is 2.7k connected to 150ohm to ground. I connected my AT89c55 pin to the junction.