I have created OFFSET IN constraints on several negative-edge flip-flops. When I perform timing analysis through PrimeTime, the slack numbers are different than those reported in the TRCE/Timing Analyzer analysis. Which slack equation is correct?
In Timing Analyzer, it might appear that the software is adding the PERIOD into the positive-edge register's OFFSET calculation if the OFFSET constraint and PERIOD constraint have the same requirement. To avoid this ambiguity, ensure that your OFFSET constraint value does not match your PERIOD constraint value.
Another possible conflict in the slack numbers can occur because occasionally PrimeTime does not take the half-PERIOD lost on negative-edge flip-flops into account. This results in the SLACK numbers differing by half the PERIOD.