Simulink HDL Coder currently does not support this particular block (FFT block in Signal Processing Blockset). To find all the blocks supported by Simulink HDL Coder, type "hdllib" from MATLAB prompt.
There are 2 FFT blocks in HDL demo library that you can use for HDL code generation. "HDL Streaming FFT" block supports the Radix-2 with decimation-in-frequency (DIF) algorithm. "HDL FFT" block implements a minimum resource FFT architecture and supports the Radix-2 with decimation-in-time (DIT) algorithm.
John
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