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Constraint Bug Patterns: Pitfalls & Red Flags

Pitfalls & Red Flags for Constraint Bug Patterns.

Pitfalls and red flags

Pitfalls & Red Flags for Constraint Bug Patterns focuses on false violation count after SDC fix, constraint audit pass rate. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

  • A path looks clean in one view but fails in signoff SPEF.

  • The SDC allows behavior that the implementation team assumed was illegal.

  • A generated_clock definition shifts capture edge silently.

  • Hold fails on short paths after setup-focused ECO.

  • The fix improves one corner but regresses another MMMC scenario.

Layer responsibility check

diagram
STA OWNERSHIP LAYERS — Constraint Bug Patterns

layer              owns                         failure mode
----------------   --------------------------   ---------------------
constraints        clocks, IO, exceptions       false violations
implementation     cells, nets, placement       real path delay
extraction         SPEF/RC corners              view mismatch
signoff policy     derate, MMMC matrix          margin disputes
closure           ECO order, regression        fix breaks other corner

STA deep dive

Debug hygiene separates senior STA leads from report readers.

Concept diagram

diagram
DEBUG ORDER

1. same tags?  2. constraints?  3. view?  4. real path?  5. ECO

Metric graph

diagram
ROOT CAUSE MIX (typical)

constraint bugs  ████████  35%
view mismatch    ██████    25%
real path        ██████    25%
methodology      ████      15%

Reports and artifacts

  • run manifest

  • SDC diff

  • SPEF tag

  • scenario isolation

Mini case study

Mystery regression: SPEF corner tag typo in run script — not a design bug.

Debug branches

  • Manifest every run

  • War room by path group

Senior review question

Ask: what corner/mode/view proves this topic is closed or failing?

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.

Principal STA review addendum

Most 'mystery' STA failures are wrong clocks, missing generated clocks, bad I/O delay, or over-broad exceptions.

Metric: false violation count after SDC fix, constraint audit pass rate

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