A FROM:THRU:TO TIG constraint is used to ignore a specific path, but a number of blocked paths are less than expected and/or the instantiated paths.
GroupA = 300 elements/items
GroupB = 200 elements/items
If GroupA and GroupB cover the same elements/items, then GroupA is a superset of GroupB, and GroupB is a subset of GroupA. If we try to remove GroupA from GroupB, the tools are not able to remove these elements/items. If we try to remove GroupB from GroupA, the tools are able to remove these elements/items.
The FROM:THRU:TO TIG constraint is a superset of a smaller FFS time group. The issue is that the FFS time group and the TIG time group intersect; the TIG time group is not a subset of the FFS time group, so the tools cannot remove the correct items.
Unfortunately, the on-line documentation for the TIG constraint (Chapter 6 of the Development System Reference Guide at
http://toolbox.xilinx.com/docsan/4_1i/data/common/dev/chap06/dev06008.htm) does not explain the limitations or the general exceptions. For an exception to be applied to a timing specification by the timing software, the exception (TIG) must be a subset of another constraint. The TIG is actually a superset of FFS time group constraint; therefore, no TIG/exception can be applied.
A simple work-around is to do a global ignore (TIG) on the pins defined in the TIG time group, which will tell the tools to ignore these pins for all time groups.
Another way to work around the problem would be to create a timespec-specific ignore (TIG) of these pins. This will make a TIG group that is a subset of all the other time groups.