RTL to GDSii: Congestion Reduction strategies

In ASIC physical design, congestion occurs when there are more routing demands than available resources (tracks) in a region, leading to routing failures, increased delays, and DRC violations. Managing congestion is crucial for achieving a routable design with optimal performance.

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Methods to Reduce Congestion

1. Floorplanning Techniques

• Macro Placement Optimization
• Place large macros (RAMs, CPUs) at the periphery to free up routing channels.
• Avoid placing macros in high-density standard cell regions.
• Use channel spacing between macros for smoother routing.
• Hierarchical Floorplanning (Block-Level)
• Divide the design into sub-blocks with controlled interconnect density.
• Use fence regions to restrict cell placement near congested areas.
• Power Grid Planning
• Avoid overly dense power stripes that block routing tracks.
• Use staggered power stripes instead of continuous ones.

2. Placement Optimization

• Cell Spreading
• Increase cell spacing in congested regions (e.g., setPlaceMode -place_global_cell_spacing in Cadence Innovus).
• Use density screens (-uniformDensity) to prevent hot spots.
• Utilization Control
• Keep standard cell utilization ≤ 70-80% in congested areas.
• Use partial placement blockage to restrict cell density.
• Congestion-Driven Placement
• Run global placement with congestion awareness (e.g., -timingDriven -congEffort high).
• Use ECO placement to redistribute cells post-optimization.

3. Routing Strategies

• Layer Assignment
• Route local nets on lower metals (M1-M3) and global nets on higher metals (M4+).
• Avoid excessive via stacking in dense regions.
• Track Utilization Balancing
• Use alternate routing directions (e.g., M1: Horizontal, M2: Vertical).
• Avoid routing bottlenecks by spreading signals across layers.
• Non-Default Rules (NDR)
• Use double-width spacing for critical nets to reduce conflicts.
• Apply shielding for noise-sensitive nets to prevent detours.

4. Logic & Netlist Optimizations

• Buffer & Inserter (Buf/Inv) Tree Balancing
• Reduce fanout congestion by inserting buffers in high-fanout nets.
• Use clock tree synthesis (CTS) optimizations to minimize skew-induced congestion.
• Logic Restructuring
• Replicate high-fanout cells to distribute load.
• Split long wires with repeaters to reduce routing pressure.
• Net Weighting
• Assign higher priority to timing-critical nets (setNetWeight).

5. Physical Synthesis & ECO Fixes

• Congestion-Aware Synthesis
• Use physical synthesis tools (e.g., Fusion Compiler, Genus iSpatial) to optimize for routability.
• Apply gate sizing to reduce cell density.
• Post-Route ECOs
• Perform minor placement legalization to relieve congestion.
• Reroute problematic nets with manual guidance.

6. Advanced Techniques

• Pin Access Optimization
• Use offset pins or middle-of-line (MOL) routing to ease pin access.
• Avoid pin congestion by spreading pins in macros.
• DFM (Design for Manufacturability) Adjustments
• Follow metal density rules to avoid CMP (Chemical Mechanical Polishing) issues.
• Use fill cells to maintain uniform density.

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Congestion Analysis & Debugging

Early-Stage Checks:

• Use global routing congestion maps (e.g., in Innovus, ICC2).
• Identify hotspots (red/yellow zones) in congestion reports.

Metrics to Monitor:

• Overflow (%): Percentage of unroutable nets.
• Cell Density: Should be < 80% in most regions.
• Track Utilization: Should not exceed 90%.

Common Fixes:

• If congestion is localized: Adjust placement, add routing blockages.
• If global: Reduce utilization, optimize netlist.
  
  

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Example Flow for Congestion Reduction

1. Initial Floorplan → Identify congestion hotspots.
2. Adjust Macro Placement → Free up routing channels.
3. Run Congestion-Aware Placement → Spread cells uniformly.
4. Optimize Power Grid → Avoid blocking routing tracks.
5. Rerun Routing & Verify → Check overflow reduction.
   

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Summary

Method	When to Apply	Impact
Macro Placement	Early floorplanning	High (reduces routing blocks)
Cell Spreading	Post-placement	Medium (improves track usage)
Buffer Insertion	High-fanout nets	Medium (reduces net crowding)
Layer Optimization	During routing	High (better resource usage)
Net Weighting	Critical nets	Low (prioritizes key routes)

Final Tip: Use iterative refinement—congestion fixes often require multiple placement/routing trials.