1. Can you do a 'peak-detector' in software? Continuously 
read the ADC, keep comparing all the readings and the 
maximum value would be what you are after.

2. If you know the frequency (F), then detect zero-crossing 
  and begin to take a set of ADC readings a few tens of 
milliseconds before (zero-cross + 1/4F seconds) and carry on 
till a few seconds after (zero-cross + 1/4F seconds). 
Compare these readings and get the maximum value. This would 
be less heavy on the processing resources than 1 above as 
you will be reading the ADCs lesser times per cycle, but 
will need hardware for zero-detection (an extra PIC pin).

HTH,
Mohit Mahajan.
www.BioZen.co.in

Dario Greggio wrote:
> I am to use a 24FJxxGA106 to (along other things) measure 220V mains 
> voltage with some coarse accuracy, say 1-2%
> 
> I have a small 220->6V transformer, whose inputs goes via a partitor 
> (still have to decide, but let's assume it 4K7 - 2K2) to AN2 of the PIC. 
> So, more or less 2Vrms or 3Vpp circa when mains is nominal 220V.
> 
> I'm planning on sampling/reading the input voltage some ... 100? times / 
> second, and perform a ... FFT or something simpler upon it.
> 
> Can you suggest the simplest way to achieve that result?
> 
> TIA
> 
> 
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