Maziar Navahan wrote:
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i need fft for find harmonic of 50hz
power

That can be done easily without FFT, which is too much computing intensive in a 8051.

Try this: do a *comb filtering* of your sampled signal with a 50Hz frequency filter. This will kill frequencies of 50Hz, 100Hz, 150Hz, ... Then substract this result from your original signal.
You'll get the desired armonics.

A comb filter is no more than a delay line !! Search the web with the keywords 'comb filter' and forget about FFT.

Best regards,
Txip.