The overflow flag does not assert when an overflow occurs in my 64- or 128-point FFT. Why is this the case?
This is a known problem that is fixed in the FFT v5.0.
The overflow issue occurs only when the following core conditions occur:
Device Family: Spartan-3/E/A/AN, Virtex-II/II Pro/4
FFT Architecture: Pipelined Streaming I/O
Scaling Options: Scaled
Output Ordering: Natural order output
Transform Length: 64- or 128-point transform
Number Of Stages Using Block RAM: 0
You can work around the problem by using the Digit Reversed output ordering and using a RAM buffer to externally reorder the FFT outputs.
Please see (Xilinx Answer 29209) for a detailed list of LogiCORE Fast Fourier Transform (FFT) Release Notes and Known Issues.