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Congestion + timing violations. How to approach this?

potat0e
Just like to find out what's the general approach in this situation. You had a floorplan, placement and CTS done. After optDesign -postCTS, there were a handful of huge violation WNS. Looking through the timing report, some are multicycle path which can be resolve by adjusting the constraints. Whereas, some paths had a lots of buffers added during hold fixing. From the floorplan, there are also local area where the congestion are quite high, H:75/40, V:45/20. Generally, what approach would you guys be taking with this situation?
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  • I think there are several things you should look at.

    - Now that you know what your multicycle paths are, rerun from the beginning with the corrected constraints. The optimizer may have spent a lot of time and area trying to fix these paths, which could have contributed to congestion.

    - For the hold fixing, are your margins reasonable? If you have an unrealistically large uncertainty, then lots of buffers will be added. Also take a look at your clocks. Sometimes a small clock adjustment can fix many holds, without buffers being added.

    - For the local congestion, you could try using density screens (partial placement blockages) or cell padding. This may help the hold fixing as well; we typically use cell padding around flops so that there is always room later for buffers to fix hold. If the congestion is more serious, you may need to take another look at your floorplan. Do you have a lot of macros/rams? Give Masterplan a try! See BobD's blog entry "Demo: Automatic Floorplan Synthesis in Encounter".

    - Kari

Reply

  • I think there are several things you should look at.

    - Now that you know what your multicycle paths are, rerun from the beginning with the corrected constraints. The optimizer may have spent a lot of time and area trying to fix these paths, which could have contributed to congestion.

    - For the hold fixing, are your margins reasonable? If you have an unrealistically large uncertainty, then lots of buffers will be added. Also take a look at your clocks. Sometimes a small clock adjustment can fix many holds, without buffers being added.

    - For the local congestion, you could try using density screens (partial placement blockages) or cell padding. This may help the hold fixing as well; we typically use cell padding around flops so that there is always room later for buffers to fix hold. If the congestion is more serious, you may need to take another look at your floorplan. Do you have a lot of macros/rams? Give Masterplan a try! See BobD's blog entry "Demo: Automatic Floorplan Synthesis in Encounter".

    - Kari

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