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STA Mastery · All levels

Scenario: Constraint Ghost Violation

Nightly STA shows new violations. Netlist identical. Suspect SDC merge, generated clock, or view tag regression.

Scenario

Nightly STA shows new violations. Netlist identical. Suspect SDC merge, generated clock, or view tag regression.

diagram
OBSERVED METRIC
1000+ failing paths appear overnight with no netlist change

45-MINUTE INTERVIEW FLOW
0-5: clarify corner/mode/view and path group
5-15: read critical path and constraints
15-25: classify constraint vs view vs real path
25-35: propose ECO order (setup/hold/SI)
35-45: state MMMC regression and signoff criteria

Common pitfalls

  • Start layout ECO immediately

  • Ignore SDC git diff

  • Compare different SPEF tags

Scenario debrief

Score: corner/mode, path group, constraint check, ECO, regression.

diagram
constraints -> graph -> slack
diagram
WNS trend

Debrief prompts

  1. Which scenario failed first?

  2. Setup, hold, or both?

  3. What regression did you mandate?

Key takeaways

  • State corner, mode, view, and database tag with every slack claim.

  • Run setup and hold plus MMMC regression after every ECO.

Common pitfalls

  • Comparing STA runs with different SPEF or SDC tags.

  • Broad false_path to green-wash violations.

  • Setup-only ECO without hold check.

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