fft on nios

Most people use DMAs for this. So they put a block of data into memory and feed it into the FFT block by performing memory to streaming transfers. The output of the FFT block is then streamed back to memory using a DMA.

The reason for using DMAs is that if you are using dedicated hardware for the FFT calculation then you don't want a relatively slow processor in the data path. The DMA can handle moving a word of data every clock cycle which is a closer match to the FFT hardware throughput capabilities.