When I run my design through the implementation tools, I get the an error referencing a constraint that previously existed in my sources files but no longer does. For example:
ERROR:Place:382 - The placer was unable to find a feasible solution for the IOBs in your design. This is possibly due to SelectIO banking constraints.
Each Group of a specific Standard is listed.
Standard LVCMOS25 (Vref=NR Vcco=2.50 Terminate=none) 10 IOs, 1 locked.
Why is this constraint still active? What can I do to clear this constraint from my design?
This is a known issue with the ISE 10.1 implementation tools, where a constraint database is sometimes not correctly updated to reflect the removal of a constraint. The phantom constraint is contained in a directory structure in your project directory named project_xdb, where "project" is the name of your project. This problem can be worked around by removing the project_xdb directory before rerunning the implementation tools.
This problem is scheduled to be fixed in ISE version 11.1.