Integration Problem with microcontroller-uC

I want to make a THD monitor with this PIC, but I have a doubt with the
calculation of the module of each harmonic in the range chosen.

Can I calculate DTFS (Discrete Time Fourier Series) with some routines at
microcontroller?

 

use you can , some where around here i have some codes to do FFT in a
micro controller , it may be strange on it so you mite consider using
a high clock speed and have a good chuck of memory with a good size
flash rom to hold the Tables.
use google and search for FFT
you will find some source code for micro chip examples.
the FFT will monitor frequency spectrums when inserting 2 tones to
monitor the HD..
as far as that is concerned you can make your self an analog
meter that will generate the 2 tones so that they can be in sync
to measure the results back into the meter with phasing..