Technology

StarRC SPEF versus Redhawk extraction engine

Why are StarRC SPEF files required in Ansys Redhawk for IR Drop Analysis? (Even Though It Has Parasitic Extraction Capabilities)

Ansys Redhawk is a powerful tool for power integrity (IR drop) and electromigration (EM) analysis, but it still relies on Standard Parasitic Exchange Format (SPEF) inputs for several critical reasons:

1. SPEF Provides Accurate Post-Layout Parasitics

Redhawk’s Built-In Extraction vs. SPEF:
Redhawk can estimate parasitics, but SPEF files from signoff-quality extraction tools (e.g., StarRC, Quantus) are more accurate because:
- They account for process variations (corner-specific RC).
- Include detailed coupling capacitances (Cc) and distributed RC networks.
- Capture metal density effects (e.g., chemical-mechanical polishing (CMP) variations).
Example Scenario:
If Redhawk estimates parasitics internally, it may miss localized resistance/capacitance hotspots that SPEF captures from a full extraction flow.

2. Consistency with Signoff Timing Analysis

SPEF Ensures Correlation with STA:
Static Timing Analysis (STA) tools (e.g., PrimeTime) use SPEF for delay calculations.
Using the same SPEF in Redhawk ensures IR drop analysis aligns with STA results.
Risk Without SPEF:
If Redhawk uses its own extraction, IR drop-induced delays may not match STA predictions, leading to mismatches in timing signoff.

3. Handling Hierarchical and Mixed-Signal Designs

SPEF Supports Hierarchical Abstraction:
For large SoCs, SPEF files provide black-boxed or reduced-order models of macros (e.g., SRAMs, analog blocks).
Redhawk’s internal extraction may not handle hierarchical partitioning efficiently.
Mixed-Signal Designs:
SPEF includes analog parasitics (e.g., substrate coupling) that Redhawk’s digital-centric extraction might miss.

4. Redhawk’s Role: Power Network Analysis, Not Extraction

Redhawk’s Core Strength:
Focuses on dynamic IR drop, EM, and power noise rather than parasitic extraction.
SPEF lets Redhawk offload extraction to dedicated tools (StarRC, Quantus) while specializing in power integrity.
Performance Trade-off:
Extracting parasitics on-the-fly in Redhawk would slow down analysis for large designs.

5. SPEF Enables Cross-Tool Validation

Golden Reference for Debugging:
If IR drop issues arise, engineers can compare SPEF-based Redhawk results with other tools (e.g., Voltus, PrimePower) to isolate inaccuracies.
Example Workflow:

  graph LR
    A[Layout (DEF/LEF)] --> B[StarRC/Quantus]
    B --> C[SPEF]
    C --> D[Redhawk IR Drop Analysis]
    D --> E[Signoff]

When Does Redhawk Use Its Own Parasitics?

Redhawk can estimate parasitics in limited scenarios, such as:

Early Exploration (pre-layout, rough estimation).
Missing SPEF (with reduced accuracy).
Partial Updates (small ECOs where full re-extraction isn’t needed).

However, for signoff-quality IR drop analysis, SPEF remains mandatory.

Key Takeaways

Reason	Why SPEF is Better
Accuracy	Signoff extraction tools model RC more precisely.
STA Correlation	Ensures IR drop and timing analysis agree.
Hierarchical Support	Handles macros/black boxes efficiently.
Performance	Lets Redhawk focus on power analysis.
Debugging	Cross-tool consistency checks.