Sundance FC100 v.2.3 - Manual (Page 6)

FC100 - Floating Point Fast Fourier Transform
v2.3
Fast Fourier Transform product manual
October 2005 www.sundance.com - 6 -
Transform length
The FFT transform length is a parameter fed to the core. This parameter can be either
constant or can be changed on the fly in order to perform an FFT or Inverse FFT with a
different transform length.
The FFT length parameter as well as the FFT direction (FFT_nIFFT
) is registered when a
start pulse is sent to the core. In the case the FFT transform length is a constant parameter
passed to the core, it is recommended to match the address bit width (addr_width) with
the length N of the transform: addr_width=log2(N). This will yield the best synthesis
results and guarantee an optimal clock frequency for this implementation. In any other
case 2
addr_width must be bigger or equal to the longest transform length N.
The following table shows the FFTlength code for a given transform length:
Transform length FFTlength
code
Number of passes
through the core
256 00010 2
512 00011 2
1024 00100 2
2048 00101 3
4096 00110 3
8192 00111 3
16384 01000 3
32768 01001 3
65536 01010 4
131072 01011 4
262144 01100 4
524288 01101 4
1048576 01110 4
Table 3 : FFTlength codes