PPA Optimization strategies

ASIC Physical Design Engineers play a critical role in PPA (Power, Performance, Area) recovery throughout the RTL-to-GDSII flow.

Below is a structured breakdown of their key tasks and techniques for PPA optimization, categorized by Power, Performance, and Area:

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1. Dynamic Power Reduction
   
   Clock Gating:
   

Insert clock gating cells (ICGs) to disable clocks for idle logic blocks.
Example: Automate gating with TCL scripts in synthesis/PnR tools.

• Multi-Bit Flip-Flop (MBFF) Merging: 
  Combine single-bit FFs into multi-bit FFs to reduce clock network power.
  Tool: Synopsys DC/ICC2 set_clock_gating_options.
  
  
  • Voltage Scaling:
    Implement Multi-Vt libraries (LVT for critical paths, HVT for non-critical).
    Challenge: Balancing timing vs. leakage power
    
    

    2.   Leakage Power Reduction

• Power Gating:
  Use header/footer switchesto shut off power to idle blocks (e.g., sleep transistors). Example: Automated power switch insertion in Cadence Innovus.
  
  Body Biasing:
  Apply reverse body bias (RBB) to HVT cells to further reduce leakage.
  
  

   3.    IR Drop Mitigation

• Decoupling Capacitors:

Add decap cells near high-switching logic (e.g., CPU cores).
Tool: RedHawk/Voltus for decap placement analysis.

• Power Mesh Optimization:
  Increase metal width/pitch in high-current regions.
  AWS Example: Graviton chips use hierarchical power grids.

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2. Performance Optimization

A. Timing Closure

Useful Skew:

• Deliberately imbalance clock tree latency to improve setup/hold margins.
• Tool: PrimeTime + ECO scripts.
  
  Critical Path Optimization:
• Cell Sizing: Upsize drivers on critical paths.
• Buffer Insertion: Break long nets to reduce RC delay.
  
  Example:
  tcl resize_cell -lib_cell BUF_X4 [get_cells critical_path_ff]

B. Clock Tree Synthesis (CTS)

H-Tree Structures:

• Balance clock skew for high-frequency designs (e.g., AWS Graviton).
  
  Clock Mesh:
• Use mesh networks for ultra-low skew in multi-GHz designs.

C. Crosstalk Mitigation

• Shielding:
  Route critical nets (clocks) between VDD/VSS lines.
  Tool: Innovus add_shield
  Spacing Rules:
  Increase spacing for high-aggressor nets.

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3. Area Recovery

A. Congestion Reduction

• Macro Placement:
  Place macros at chip edges to reduce routing congestion.
  Example: Use Cadence Innovus plan_design
  
  
• Cell Padding:
  Add whitespace around high-density cells.

B. Logic Optimization

 Library Cell Selection

• High-Density Libraries:
  Use low-drive cells in non-critical paths.
  Tradeoff: Timing vs. area.

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4. Toolflows & Automation

A. PPA-Aware Scripting

• TCL/Python for ECOs:

  # Example: Auto-resize cells with negative slack

  foreach cell [get_cells -filter "slack < 0"] {
      resize_cell -lib_cell BUF_X4 $cell
  }

• Machine Learning:
• Use ML-based tools (e.g., Synopsys DSO.ai) for PPA exploration.

B. Signoff Correlation

• Cross-Tool Validation:
• Compare power/timing between PrimeTime (signoff) and PnR tools.

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