Breaking The Three Laws


Partitioning Poser #1

There are those who say that one should partition so that only sequential eements are allowed to appear at the FPGA edges, thus simplifying the timing and constraints at the FPGA pins.

This is an excellent goal but one that we can not always reach.

We often need to partition combinatorial paths across FPGA boundaries and then we need to ensure good time constraints for the synthesis and, especially, for the P&R for each end of the path. Don’t forget, when implemented in the individual source and destination FPGAs, each part of the path is considered in isolation. The synthesis and P&R tools have no knowledge of the other end of the path and will assume by default that the signals on the stub that they can see have an entire clock period  to propagate (minus set-up or hold for the FF). In reality this is never the case and so we need to apply good timing constraints for these stubs. This is achieved by time budgeting the path to allocate an appropriate portion of the full path propagation delay to each stub. This is automated in tools like the Certify tool that I know and love. When the path runs across three or more FPGAs with the middle FPGAs possibly acting only as feedthroughs then the task becomes really tricky.

We can talk about time budgeting of combinatorial paths in a future blog but in the meantime, I have a poser for you around the best way to partition an example of a inter-FPGA combinatorial path.

 The simplified example in the diagram below, with a mux feeding multiple sources from two source FPGAs into a separate destination FPGA, is not as uncommon as we’d like to hope. After all, many bus standards used in SoCs are actually complex mux’es, and bus sources and destinations are often partitioned into different FPGAs.

Here, then, is a good example of how partition has happened at a multiplexer. We can see that the sources for the mux are in two separate FPGAs and the destination is in a third. The next task is to assign the multiplexer. The question for you is; where is the best place to put the multiplexer, including how and why?

Please use the comment box below to give us your replies.

I’ll explain my favourite answer in our next blog in any case.

All the best,


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