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"rfftN" function of CCES Lib Timing problem

Question asked by john_cheng on Feb 21, 2018
Latest reply on Mar 2, 2018 by Jithul_Janardhanan


When I use the "rfftN" function of CCES Lib on my ASDP-21479 EVM, there is one problem about the calculate timing.

When I do 256~8192 points FFT, it seems like increase linearly.

But when the points are over 8192, it takes too much time.

The test result is as below:

code is as below:


    for(int i=0;i<N;i++)
        input[i] = sin(2*pi*f*i*0.005);

    SRU(LOW, DAI_PB06_I);
    rfft16384 (input, output); //rfft function
    rfft_mag (output, spectrum, N);//rfft magnitude function
    SRU(HIGH, DAI_PB06_I);


after 8192 to 
16384 points,
takes time from 0.815 mSec to 29.5 mSec , and I do't know why

Could anyone tell me why?

Thanks! ^^