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.

Power Grid Planning

• • Avoid overly dense power stripes that block routing tracks.

• • Use staggered power stripes instead of continuous ones.

Hierarchical Floorplanning (Block-Level)

• • Divide the design into sub-blocks with controlled interconnect density.

• • Use fence regions to restrict cell placement near congested areas.

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

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• • 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

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• • Use alternate routing directions (e.g., M1: Horizontal, M2: Vertical).

• • Avoid routing bottlenecks by spreading signals across layers.

Non-Default Rules (NDR)

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• • 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. 1. Initial Floorplan → Identify congestion hotspots.

1. 1. Adjust Macro Placement → Free up routing channels.

1. 1. Run Congestion-Aware Placement → Spread cells uniformly.

1. 1. Optimize Power Grid → Avoid blocking routing tracks.

1. 1. 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.