Large Multiplexer fitting problem

Designs that have lots of multiplexers can have fitting problems because they often have very high interconnect demand (per logic element).

One way you can tell whether this is the case for your design is to look for the Fitter info messages during routing that tell you what the interconnect demand is. They look like this:

Info: Average interconnect usage is X% of the available device resources

Info: Peak interconnect usage is Y% of the available device resources in the region that extends from location X??_Y?? to location X??_Y??

Typically, when X > 60% and/or Y > 90%, designs can have trouble routing (may need extra fit attempts, or may fail to route).

If you are using logiclock regions, it is important to try the design without them to make sure that your floorplan is not constraining the design in an un-natural way and causing higher interconnect demand.

Hope that helps.