Honored Contributor
Honored ContributorI think I made a mistake in my Cyclone III test case (I'd already deleted some files before I could confirm that), but hopefully my EP1C6 test case in unused_pin_test_case.zip is correct. I still had no difference in .ttf files for the unused pin reserved as output driving ground versus hardwired to zero in the source file.
I used a very simple test case for which the back annotation works without a .vqm to keep the node names matching those in the location settings. I have no combinational nodes in the test case. For a real design that includes combinational node names that can change from one compile to the next, a .vqm will reduce the chance of a name change but not prevent it (the .vqm has to be run back through Analysis & Synthesis for the next compile). Even register names can change between compiles if synthesis is creating instance names inside megafunctions (megafunctions often have a suffix like a serial number on the instance names created during synthesis). You have to back annotate routing in addition to placement to make sure the routing is identical in the two programming files you compare. Node name changes make parts of the routing back annotation not work. It can be difficult to preserve everything exactly with back annotation in a real design. If you really needed to back annotate something, incremental compilation would be the better way to do it. In your case though, back annotation isn't the point of what you want to accomplish. You just need confidence that two ways of handling an unused pin produce exactly the same programming file. To reassure yourself of that, a test like the one I did using a trivial little design for which back annotation with routing will work avoids the complications of trying to check what you care about using the real design. With the simple test case, it's easier to make sure that the two different methods of handling the unused pin are the only difference in the two compilations for which you are comparing programming files.
