Router Initial Congestion (named Initial Estimated Router Congestion for 7 series FPGAs) is only available when the router has been run. It shows the routing congestion initially faced by the router during the early stages of routing.
When the congestion level is 5 or higher, report_design_analysis generates a congestion table that provides details about the nature of congestion and region(s) associated with the highest congestion in a particular direction and type.
- Global congestion is estimated similar to placer congestion and is based on all types of interconnects.
- Long congestion only considers long interconnect utilization for a given direction.
- Short congestion considers all other interconnect utilization for a given direction.
Any congestion area greater than 32x32 (level 5) is likely to impact QoR and routability. Congestion on long interconnects increases usage of short interconnects and results in longer routed delays. Congestion on short interconnects usually induces longer runtimes and if their window size is big, it is also likely to cause QoR degradation.
When analyzing the Router Initial Congestion table, look for the following:
- If the congestion level is greater than 6, the design is unlikely to meet timing and might fail during routing.
- If the congestion level is 4 or 5, then identify the module(s) located in the congested area(s). You can apply a congestion alleviation technique on these modules or rerun the placer with different directives, such as *SpreadLogic*.
- If the congestion level is 3 or less, the congestion is probably not a cause for concern unless the design has a very tight timing budget.
The previous figure illustrates an example of the Router Initial Congestion,
where regions with congestion level 5 or more are reported. To generate a congestion
report with a lower congestion threshold, use the switch -min_congestion_level. The default minimum congestion level is 5. The
values must be between 3 and 8.
In addition to the region where the design has maximum congestion in a given direction and type, the congestion report also contains additional regions with the maximum congestion level in that given direction and type (if any). These regions can have some amount of overlap or they can be present in different regions of the device.
The following figure illustrates an example of where the design has a congestion level 6 for North (Direction) Long (Type) in more than one region.
