If you are using the Virtex-6 MMCM simulation model, it has been seen in ISim and third-party simulators that the output is not correct when using fractional dividers.
This answer record describes a patch to repair this issue.
This patch will overwrite existing files, please make a backup copy of your original files prior to installing this patch. This patch applies only to the ISE 14.7 tool set. Installation and use in prior versions of ISE are not supported.
Place the patch files in the correct location:
$<install_path> : the Xilinx ISE tool suite installation path.
This patch requires that files be moved to multiple locations. Copy the following files to the listed destination.
In order for the patch files to be used during simulation, the simulation tool libraries must be recompiled. Please refer to the Synthesis and Simulation Guide for more information on how to compile for your specific tool.
ISE Simulator (ISim) Tool Instructions:
This patch will overwrite existing files, please make a backup copy of your original files prior to installing this patch. This patch only applies to the ISE 14.7 tool set. Installation and use in prior versions of ISE are not supported.
The patch contains pre-compiled libraries for all of the supported Operating Systems.
Instructions for the different OS are as follows:
<ise_install_path>/vhdl/hdp/ : the Xilinx ISE tool suite installation/library path.
/<patch_path>/isim/lin/ to : /<ise_install_path>/vhdl/hdp/lin/
/<patch_path>/isim/lin64/ to : /<ise_install_path>/vhdl/hdp/lin64/
/<patch_path>/isim/nt/ to : /<ise_install_path>/vhdl/hdp/nt/
/<patch_path>/isim/nt64/ to : /<ise_install_path>/vhdl/hdp/nt64/