Location constraints and timing analysis

Your pin's locked and the register is locked. There are one of two things changing:

1) Your input delay chain. Look in the fitter report and see what this is and if it's changing. You can lock this down with assignments in the Assignment Editor.

2) The routing. I'm guessing this is it. Run report_timing -show_routing on the paths in TimeQuest to compare two different place and routes. Note that this may change, and the fitter may then change the delay chains to react to that, so they may both be changing.

First, is it passing/failing timing with the changes? What your asking is almost never needed unless the timing is so absolutely tight that it passes/fails compile to compile. If this is something you just want to do without a pass/fail test, it's probably unnecessary.

To lock down routing, you can put the logic in a partition and set to post-fit. That is the easiest and what I would recommend.

Another option is to create a routing constraint file(.rcf) which is not well documented. Basically you need to back-annotate to the routing level, which will create an .rcf. It back-annotates the placement and routing of everything, and then you need to pull out the info you need. I believe the command is "quartus_cdb --back_annotate=routing <projectname>". This is really a method of last resort.