Tco Constraints

You didn't mention what type of constraint it was(or I missed it, or it's implied to be a Tco). Anyway, one thing about how the assignment editor works. There are multi-point assignments, which have a node in the from and to columns, and single-point assignments, which always are listed in the to column, regardless of if it's from or to anything. Sometimes this doesn't make sense though. For example, if doing a Tsu constraint, you could do:

From -> pin_name

To -> internal_registers

So that all paths from the pin_name the internal_registers it feeds have this Tsu requirement. Or you could do a single point assignment of:

To -> pin_name

In this case it's a single point assignment, so the concept of from/to is ignored, and Quartus just knows your putting a Tsu constraint on the pin pin_name.

Just pointing out that you have a single point assignment.

(And to get more confusing, different timing constraints work differently for single points assignments. For example, Cuts propogate forward, i.e. you're cutting the timing on all paths that that node fans out to, while if it were a Multicycle single-point assignment, you're applying the multicycle to all paths leading into that node. This is all another reason why Classic Timing Analyzer was abandoned for TimeQuest...)