This is a readable FFT code with simple comments. Everyone with C knowledge should be able to read it and modify it.
The code is run on Arduino Due.
myFFT(Xr, Xi, re, im, N, 0, 1)
or
myFFT2(Xr, Xi, 0, re, im, N, 0, 1)
where Xr and Xi store the real and imagine parts of the frequency, and re and im are the time signal. Usually im are set to be zero. N is the number of samples.
More information can be found in the comments.