AR# 5456: 1.5i Map - Correlated back-annotation of Virtex carry chain losing some delays.
1.5i Map - Correlated back-annotation of Virtex carry chain losing some delays.
Due to a bug in the NGM file written by the mapper, ngdanno run with the NGM file will underreport some delays in the carry chain. Problem is a missing map signal connection (aka mapper pinref) when two CYMUXs and two XORCYs are packed into a slice. Test case originated from customer CR 110323.
The net result is that for back annotated simulation, using the .ngm file (default) will under report carry chain delays in the configuration mentioned above.
Workaround is to not use NGM file, but there is no indication to the customer that there is a problem; the carry chain just simulates faster than it should.
The Timing Analyzer can be used to verify the carry chain timing as an alternative to simulation.
This issue is already corrected by the mapper rewrite for the 2.1 release. There are no plans to make a fix in the 1.5i time frame.