This Answer Record discusses input sample period restrictions for System Generator 64-point, 256-point, and 1024-point FFTs, using TMS (Triple Memory Space).
In version 1.0, the D_IP1 FFT core has issues related to clock enable, so it must run at the system clock rate. (The system clock equates to a sample period of "1" in System Generator 1.0, and it only supports integer sample periods in general.) This means that the sample rate out of a realizable FFT block must always be 1; therefore, the input sample periods are required to be either 1, 3, or 4.
For the 64- and 256-point FFT triple memory space (TMS), the input sample period is 3x the output sample period; for the 1024-point FFT TMS, the input sample period is 4x the output sample period.
As System Generator 1.0 supports only positive integer sample periods, this implies that the input sample rate must be 3 for the 64- and 256- point FFT, and an integer of 4 for the 1024-point FFT.
For example, if the input sample period for a 64-point FFT TMS is 3, the output sample period will be 1. This restriction applies only to the TMS FFTs. (For all other configurations, the input rate is the same as the output rate.)