C language and 16F877

Good evening, I want to achieve a basis for a pic 16F877 device measures the electrical impedance of the three phase lines, my card contains the voltage sensor and current and a peak 16F877 AND an LCD 16 * 2. we know it need to adapt the input signals with the peak everything is fine until now, our pic 16F877 will receive two tesion U1, U2 freq = 50 Hz (current and tesion): Then I'll determine the phase shift between these two signals (detection at zero crossing)
my problem at the moment I can not determine the maximum of my signals to determine the rms voltage or signal received. unfortunately I'm still newbie in C programming with the 16F877. "
I have a good idea to determine the max value of the signal and I can not use it in C languge the idea and the next
Set _adc channel(0)
a=read_adc()
b=read_adc()
c=read_adc()
Then I'll put a while loop to test if it is true it will be equal tens_max ab for b> a and b> c (I'll take this value as max)
else a = b
b = c
set_adc_channel (0)
c=read_adc( )
ohh excuse me if there is something wrong or that I forget I want you to help me determine the maximum values ​​with a fairly accurate method

Be aware that measuring the peak value you´re not able to calculate the "real" rms value beacuse is needed to measure and integrate point-to-point of the wave.

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