I have written code for the fft in C language. I use this SW in an 89C52. I have already sent this code by e-mail to some people who requested a copy (free).
Please, send me an e-mail if you are interested.
By the way, I have sent a copy to the forum , too, about 6 months ago. I don't know whether or not it is available there.

Alfredo del Rio.