Adding units to the value of the CLKIN_PERIOD constraint, as in the
Constraints Guide example, will result in an ngdbuild error:
ERROR:NgdBuild:686 - A parsing error has occurred while reading design.ucf.
Additionally, using a value over 32 for the CLKIN_PERIOD results in a bitgen error:
WARNING:Bitgen:198 - CLKIN period -xxxxx ps is less than minimum of 2000 ps.
This issue will be resolved in ISE 4.2i and addressed in the next major release's