Why do I see extremely long generation times when I try to target a Fixed Fractional Rate filter for Spartan-II/-IIE/-3/-3E/-3A or Virtex/-E/-II/-II Pro?
This is because Fractional Rate filters are not supported for Spartan-II/-IIE/-3/-3E/-3A and Virtex/-E/-II/-II Pro in the FIR Compiler v3.0.
The GUI has been updated in the FIR Compiler v3.1 to automatically grey out the Fractional Rate option.
This limitation has been removed in the FIR Compiler v3.2 and beyond, and Fractional Rate filters are supported for all families.
Please see (Xilinx Answer 29138) for a detailed list of LogiCORE FIR Compiler Release Notes and Known Issues.