PAR will not include clock skew in a period analysis if a clock is routed on the secondary backbone of Virtex. After running TRCE, I receive the following warning:
WARNING:bastw:544 - Clock nets using non-dedicated resources were found in this design.
Clock skew on these resources will not be automatically addressed during path analysis. To
create a timing report that analyzes clock skew for these paths, run TRCE with the '-skew' option.
Follow the suggestion in the warning and run TRCE with the -skew option. This will analyze the design, including the clock skew in the PERIOD constraint. If the addition of the clock skew causes the PERIOD constraint to fail, the design will need to be run through PAR again.
Before this is done, an environment variable needs to be set that directs PAR to include the skew when it tries to meet timing. Do this by setting the following environment variable: XILINX_DOSKEWCHECK=1
If an unused BUFG exists, route this clock using that resource. (PAR will include the skew in a period analysis for clocks that are routed using BUFGs.)
NOTE: This warning does not mean that no global resources were used; rather, it indicates only that some nets that could be classified as "clock" nets are not on global clock resources.