Problem Description: On Virtex design Worst case paths are not always covered by the PERIOD constraint.
ソリューション
1
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.
2
Do custom timing analysis in the Timing Analyser to cover the FFs to FFs path.
3
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.
4
Add the missing entries to the PCF file time groups so that these paths are considered by PAR.