Combinational loop

Bujosa,

You did the right thing by asking the owner to modify the IP. This warning is serious and demonstrates bad design practices. Latches are ok and won't generate combinational nodes with more than two or three nodes. For TimeQuest to go into exponential time computation, you are dealing with a lot more nodes all connected into what is known as a strongly connected component in computer science. Basically when any node in the loop can get to any other node in the loop through a combinational path. TimeQuest is suppose to be smart enough to detect when the node is too large to try to analyze. This is important as a large enough strongly connected component can take as much time as stars in the universe :-)

Seems like the TimeQuest algorithm is estimating what it will tak incorrectly, and therefore trying to analyze it, instead of switching to the alternative algorithm where it does not try to analyze it exhaustively.

Anyway, I do believe there is a hidden QSF variable to change the threshold, but I don't have access to Quartus now and I don't remember it. But you can definetely use set_disable_timing to fix this problem. But because of the characteristics of the loop, you may need to cut several nodes ( or all of them ). TimeQuest should have givien you a message with the nodes in the loop. You should be able to copy and paste this names into a [get_nodes] collection. Set_false_path will only work to fix a report timing path ( a path between two registers) but won't prevent TimeQuest from analyzing it from some of the global reports that need to analyze the whole net list.

As I don't have access to Quartus to confirm my statements, I hope they are right, but if you post the full name of one of the nodes in the loop, we can help you with the right collection. Also note that you can use the Name Finder in TimeQuest to confirm the collections are correct.

Hope this helps.