TIMING AND CONGESTION FIXES IN PLACEMENT

 

Placement Optimization Techniques for Reducing Timing and Congestion in Physical Design

1. Introduction

Placement plays a crucial role in timing, congestion, and overall performance of the design. Poor placement can cause setup and hold violations, routing congestion, and increased power consumption.

Goal: Optimize placement using floorplan-aware techniques and tool commands to improve timing, congestion, and power.

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2. Key Placement Optimization Techniques

1️⃣ Congestion-Aware Placement

🔹 Why? Congested areas lead to routing detours, increasing wire length and resistance.
🔹 Fix:
✔ Spread cells evenly to avoid hotspots.
✔ Use placement blockages in high-density regions.

 Command Example (Cadence Innovus):

setPlaceMode -congEffort high
place_design
analyze_congestion

(Sets high congestion effort during placement and analyzes congestion after placement.)

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2️⃣ Timing-Driven Placement

🔹 Why? Poor placement of setup-critical paths increases delay, causing setup violations.
🔹 Fix:
✔ Place high-fanout cells close to their loads.
✔ Prioritize critical paths using tool settings.

 Command Example (Synopsys ICC2):

set_place_opt -timing_driven true
place_opt
report_timing

(Enables timing-driven placement, optimizes, and checks violations.)

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3️⃣ High-Fanout Net Optimization

🔹 Why? High fanout causes high capacitance, leading to longer delays.
🔹 Fix:
✔ Insert buffering to reduce fanout load.
✔ Clone high-fanout drivers for better timing.

 Command Example (Synopsys ICC2):

setOptMode -fixHighFanout true
optDesign -preCTS

(Fixes high-fanout nets before clock tree synthesis (CTS).)

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4️⃣ Cell Spreading to Reduce Density

🔹 Why? High cell density areas cause congestion and IR drop issues.
🔹 Fix:
✔ Spread cells apart to improve routing and power integrity.

 Command Example (Cadence Innovus):

setPlaceMode -density 70
place_design

(Limits placement density to 70% to reduce congestion.)

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5️⃣ Macro Placement Optimization

🔹 Why? Poor macro placement causes routing congestion and timing issues.
🔹 Fix:
✔ Align macros along the edges to allow better routing channels.
✔ Avoid macro clustering to prevent routing bottlenecks.

 Command Example (Cadence Innovus - Macro Guidelines):

place_instance U1 -coordinate {100 200}
place_instance U2 -coordinate {300 400}

(Places macros at specific locations to avoid congestion.)

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6️⃣ Wirelength Reduction for Better Timing

🔹 Why? Long wires increase RC delay, affecting setup timing.
🔹 Fix:
✔ Optimize cell locations to minimize net wirelength.
✔ Use global and detailed placement refinements.

 Command Example (Synopsys ICC2):

setOptMode -wirelengthEffort high
optDesign -postPlace

(Reduces wirelength after placement to improve timing.)

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3. Placement Optimization Flow

Step 1: Pre-placement analysis (floorplan, congestion map, timing paths).
Step 2: Enable congestion-driven and timing-aware placement.
Step 3: Adjust macro locations and cell density.
Step 4: Optimize high-fanout nets and long wire paths.
Step 5: Perform post-placement optimization (cell spreading, buffering, wirelength reduction).
Step 6: Validate timing and congestion reports.