Graph-based analysis (GBA) and path-based analysis (PBA) give significantly different slack on the same critical path. What's going on, and which one do you trust?
From PDVerse MMMC Interview Masterclass, part of the pdVerse Mentor Series
Direct answer
GBA (Graph-Based Analysis) is conservative and pessimistic: at multi-input gates, it assumes worst-case input slew across all inputs propagates to the output, compounding pessimism down the tree. PBA (Path-Based Analysis) re-evaluates the path using the exact propagating slew. PBA is trusted for signoff because it removes artificial mathematical pessimism.
Mentor explanation
PBA removes artificial GBA pessimism by calculating exact path-specific transition times.
Key terms
- GBA (Graph-Based Analysis) — fast, pessimistic timing analysis used during PnR optimization; propagates worst-case slew across all merged arcs.
- PBA (Path-Based Analysis) — accurate, path-specific analysis used for signoff; re-evaluates exact slews along the critical path.
- PBA-path vs PBA-exhaustive — PBA recalculates top failing paths to recover 20–100+ ps of artificial GBA pessimism.
- Silicon correlation — PBA matches real silicon behavior, whereas GBA is an intentionally conservative bound.
Always run PBA on top failing paths before designing physical ECOs to avoid fixing artificial GBA violations.
Practical example
GBA vs PBA Slew Propagation:
Mux Gate U1:
• Pin A0 Input Slew = 20 ps (Actual active path arc)
• Pin A1 Input Slew = 85 ps (Unrelated slow control net)
GBA Approach: Propagates 85 ps slew to Output Y -> High downstream delay (Pessimistic)
PBA Approach: Propagates exact 20 ps slew along Pin A0 -> Realistic fast delay (+45ps slack reclaimed!)Interview trap
Spending engineering effort fixing a GBA timing violation that is positive and clean under PBA. Always run PBA on critical paths before committing ECO silicon edits.
Key takeaways
- GBA merges worst-case input slews at multi-input gates, compounding pessimism.
- PBA propagates exact path-specific transition times, reclaiming 30ps–150ps of false negative slack.
- PBA is the authoritative golden standard for final tapeout timing signoff.
Self-check: can you answer this aloud?
Try a 45-second answer using this structure:
- State the direct answer.
- Explain the timing or physical reason.
- Name one caveat.
- Say how you would verify it in a real flow.
MMMC Signoff Guide
Master Multi-Mode Multi-Corner Timing Signoff
Explore the full 10-chapter MMMC guide on modes, PVT corners, RC parasitics, analysis views, and correlation between implementation and signoff tools.
Offline PDF Bundle
Want all 1109 questions offline?
Get the complete 4-book PDF bundle (PnR, STA, MMMC, Low Power) with a clickable table of contents - no ads, no internet needed.
Keep connecting concepts