Congestion

What is congestion???

If the no. of routing tracks available for routing in a one particular area is less than the required routing tracks then the area said to be congested. There will be a limit for no. of nets that can be routed through particular area.

Reasons for congestion

• High standard cell density in small area,
• Placement of standard cells near macros,
• High pin density at the edge of macros
• Bad floorplan
• During IO optimization tool does buffering, so lot of cells placed in the core area. 

How to fix congestion???

• Rerun the fast placement with congestion driven option (congestion drive placement).

• Modify physical constrains such as adjust cell density in congested areas. Because higher cell density cause for congestion.

• Use/modify proper blockage i.e. soft and hard blockages, macro padding(halos) are used proper locations to minimize the congestion near macros.

• Modify floorplanning such as moving macros, change core shape/size, move pins to give enough room for routing.

Congestion Driven Placement

Congestion driven placement is perform to reduce the congestion.

During congestion driven placement, the cells (higher cell density) which caused for congestion are

spread apart.

Q. If the cells along timing critical paths are spread apart to minimize congestion, what happens???

Answer: If the cells along timing critical paths are spread apart, the timing constraints along that particular paths are not met which cause for timing violations. But these violations can be fixed during incremental optimization.