thanks bert...slight oversight in my previous (noe edited)...the toggle frequency on my scope is HALF the interrupt frequency!

Re the pseudo cap - a peak detect cicuit (the end goal here)....it's basically a diode followed by a cap...when the incoming ac signal is higher in magnitude in voltage than that across the cap the diode allows it through...and the cap's voltage increases......when the incoming signal is lower the diode switches off....the cap holds the voltage, but for this circuit to be useful, we actually add in a resistor to discharge it at the designer's choice.....hence wanting to decrement the ADCsample variable when the incoming signal is lower.

http://hades.mech.northwestern.edu/i...#Peak_Detector