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78 Xilinx Development System
Xilinx System Generator v2.1 Reference Guide
Figure 3-52: FFT Timing Characteristics
For 16-point FFTs, the block is always in the "ready for data" state and output frames
are delivered continuously. Thus, there are no stall periods (stall = stall_0 = 0), and the
frame variable of the timing diagram defaults to 16 sample periods. There is,
however, a pipeline delay (i.e., it takes some time for the first output frame to appear)
with frame_0 = 84 sample periods.
Xilinx LogiCORE
The block always uses the Xilinx LogiCORE fft V1.0 (Virtex) or FFT V2.0 (Virtex-II).
The number of points supported are N=16, 64, 256, or 1024. The 64, 256, and 1024
point FFTs contain external memories implemented with the LogiCORE Dual Port
Block Memory V3.2. The number of memory blocks (either 1, 2, or 3) determines the
timing characteristics and size of the implementation. The FFT LogiCOREs support
only 16-bit data, although in simulation, the System Generator FFT block supports
other data sizes.
The Core datasheets can be found on your local disk at:
For Virtex:
%XILINX%\coregen\ip\xilinx\primary\com\xilinx\ip\vfft\doc\c_ff
t1024_v1_0.pdf
%XILINX%\coregen\ip\xilinx\primary\com\xilinx\ip\vfft\doc\c_ff
t16_v1_0.pdf
%XILINX%\coregen\ip\xilinx\primary\com\xilinx\ip\vfft\doc\c_ff
t256_v1_0.pdf
%XILINX%\coregen\ip\xilinx\primary\com\xilinx\ip\vfft\doc\c_ff
t64_v1_0.pdf
For Virtex-II:
%XILINX%\coregen\ip\xilinx\primary\com\xilinx\ip\vfft_v2_0\doc
\vfft1024v2.pdf
%XILINX%\coregen\ip\xilinx\primary\com\xilinx\ip\vfft_v2_0\doc
\vfft16v2.pdf
%XILINX%\coregen\ip\xilinx\primary\com\xilinx\ip\vfft_v2_0\doc
\vfft256v2.pdf
%XILINX%\coregen\ip\xilinx\primary\com\xilinx\ip\vfft_v2_0\doc
\vfft64v2.pdf
Single Memory Double Memory Triple Memory
64-point
stall_0 = 275
stall = 275
frame_0 = 277
frame = 339
stall_0 = 146
stall = 128
frame_0 = 276
frame = 192
stall_0 = 0
stall = 0
frame_0 = 406
frame = 192
256-point
stall_0 = 1074
stall = 1074
frame_0 = 1076
frame = 1330
stall_0 = 789
stall = 768
frame_0 = 1075
frame = 1024
stall_0 = 0
stall = 0
frame_0 = 1589
frame = 768
1024-point
stall_0 = 5170
stall = 5170
frame_0 = 5172
frame = 6194
stall_0 = 4117
stall = 4096
frame_0 = 5171
frame = 5120
stall_0 = 0
stall = 0
frame_0 = 8246
frame = 4096