Honored Contributor
Honored ContributorI wouldn't be trying to do this if the Fitter were doing a good job.
I have a large but relatively straightforward design with a wide (512-bit) internal pipeline. The way the logic is structured, it seems like it should be possible to make the pipeline run left to right (put each stage of the pipeline into 128 ALMs in the same column), and use direct LAB to LAB links for interconnection. Instead of doing that, the fitter gets really creative. Each stage ends up in a cloud-shaped formation, with clouds running top to bottom, or diagonally, or in a spiral. Direct links are idle (usage 4%), everything ends up on C and R interconnects, there is a routing overload, and I end up with Fmax of 140 MHz because some signal somewhere incurs a 5 ns interconnect delay just traveling from a source register to a destination register via a circuitous path. I've been trying to encourage the fitter to do the right thing (or, alternately, to understand why it can't be done), but the fitter is proving extremely resistant to my coaxing.
