Xilinx System Generator V2.1 Reference Manual page 77

Other parameters used by this block are explained in the Common Parameters section
of the previous chapter.
The FFT block cannot be placed in an enabled subsystem in System Generator v2.1.
See the Enabled Subsystems section (within the MATLAB I/O library documentation)
explanation for more details.
Block Timing
The timing diagram below illustrates the behavior of the FFT block. The diagram
indicates the number of sample periods between the taking of input samples and the
production of the output samples for a particular frame. (Note that the timing
characteristics depend on the number of points in the FFT and the memory usage
mode selected. For triple memory configurations, the timing numbers are specified in
terms of the output data sample period.)
Figure 3-51: FFT Timing Diagram
DSP
Memory Usage: number of memory banks used to compute the transform, one of
Single, Double, Triple (not used for 16 point FFTs).
Scale Output By: one of 1/N or 1/(2N).
Overflow characteristic: block behavior when internal overflow occurs;
you may choose to invalidate the output (if checkbox is selected) or to stop the
simulation in the event of an overflow (if checkbox is not selected).
Xilinx Blocks
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