How to find FFT of the values generated from signal generator using Arduino?

I have a data of values recorded from signal generator using aduino.I would like to find the fft of the signals.I gave a 15Hz sine wave with amplitude 2Vpk-pk but I am getting the wrong fft.Please help me.

X=load('data3.txt');
fs=256;
T=1/fs;
L=10000;
t = (0:L-1)*T;
Y=fft(X);
P2 = abs(Y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);       
f = fs*(0:(L/2))/L; 
plot(f,P1); 
title('FFT Plot of a signal')

[Mark Sherstan]

Your code appears correct. What was your sampling rate and did you confirm your arduino was actually sampling at that rate? Please post your data so we can have a closer look.

[Chandrika Avvaru]

That is where I got stuck.I don't know the sampling frequency.How can I find?

[Mark Sherstan]

Theres lots of different ways but you could rerun your data acquistion and print a timer to record how long it takes to execute one loop (e.g. using micros(); on the Arduino). Then use MATLAB to calculate the difference between time steps using the diff function and average the vector to get an approximation of the sampling rate.