Problem Description: On Virtex design Worst case paths are not always covered by the PERIOD constraint.
MAP is dropping some FFS from the timing groups. Hence under timing analysis worst case path is not always shown.
A work around is to use a FFS to FFS constraint instead.
This problem is fixed for the 2.1i release scheduled to be released in June, 1999.
Do custom timing analysis in the Timing Analyser to cover the FFs to FFs path.
The missing paths may not just be FF to FF but can also be from FFs to BlockRams. So the period timespec has to be replaced by 4: From:To's - FF->FF, FF->Ram, Ram->FF, Ram->Ram. Although this can be reduced to 1 timespec with some use of time groups.
Also the period spec always has lower priority than from:to so when you replace it you need to start using explicit PRIORITY values to preserve multicycle from:to definitions.
In 1.5i the priority scheme is: tig, from:to, and period. In 2.1i the priority scheme is: tig, from:thru:to, from:to, offset, and then period.
Add the missing entries to the PCF file time groups so that these paths are considered by PAR.