Why is the "Number of Stages using Block RAM" option on page 3 of the GUI grayed out, when the Implementation Option on page 1 is set to Automatic?
If you attempt to implement a fully pipelined FFT, this will result in the following error:
ERROR:sim - Number of Stages : 0 is invalid.
This is due to a know issue with the Fast Fourier Transform (FFT) v5.0, that only affects the Auto Implementation option when the resulting Implementation Option is Pipelined, Streaming I/O. This will be fixed in the next release of the FFT.
To work around this issue, use the Pipelined, Streaming I/O Implementation Option, instead of the Auto option.
See (Xilinx Answer 29209) for a detailed list of LogiCORE Fast Fourier Transform (FFT) Release Notes and Known Issues.