Because placement typically has the greatest impact on overall design maximum clock frequency, applying different placer directives is one of the first techniques that should be tried to reduce congestion. Consider running the alternate placer directives without any existing Pblock constraints in order to give more freedom to the placer to spread the logic as needed.
Several placer subdirectives exist that can help alleviate congestion by spreading logic throughout the device to avoid congested regions. Each subdirective takes a setting of high, med, or low.
- Floorplan.ForceSpreading
- GPlace.ForceSpreading
- GPlace.ReduceCongestion
- DPlace.ReducePinDensity
In addition, other placer directives or implementation strategies might help with alleviating congestion and can be tried after the placer subdirectives mentioned above.
To compare congestion for different placer subdirectives either run
the Design Analysis Congestion report after place_design, or examine the initial estimated congestion in the
router log file.
Routing has less impact on congestion than placement. However, in some cases it is useful to attempt different routing directives. The following directive ensures that the router works harder to access more routing and relieve congestion in the interconnect tiles:
- AlternateCLBRouting
For more information, see this link in the Vivado Design Suite User Guide: Implementation (UG904).