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FFT's on Blackfin 533

Question asked by whogoesthere on Dec 2, 2011
Latest reply on Dec 6, 2011 by gpan

I'm experimenting with FFT's on the blackfin 533 EZ-KIT using the Visual DSP 5.0 with the latest updates.


I'm using the cfftf_fr32(X,R,twiddle,1,NFFT); function with the X and R vectors located in


section("L1_data_a") complex_fract32 X[NFFT];
section("L1_data_b") complex_fract32 R[NFFT];
I've compared the execution time with those provided in the documentation with a test harness and achieved the same numbers for
FFT's of length from 8 to 1024 ie
        Size FFT  Cycles
             8      217
            16      447
            32     1011
            64     2203
           128     5315
           256    11747
           512    27643
          1024    60334
The surprising figure is obtained for an 2kpt FFT which has a cycle time of 1909978 or 31 times the figure for 1 kpt.
Whats happenening here, why does it take so much longer to execute ? Am I missing something important ?

Any pointers would be greatly appreciated.