Constraint Bug Patterns: Step-by-Step Walkthrough

Step-by-step analysis walkthrough

Use when you own Constraint Bug Patterns in a timing war room.

1. Open signoff scenario list — note worst WNS corner.

2. report_timing -max/-min on failing group.

3. Trace launch and capture clocks on critical path.

4. Check exceptions and generated clocks on path.

5. Break down cell vs net delay.

6. Compare implementation vs signoff view if delta suspicious.

7. Assign owner: RTL, synth, PD, CTS, SI.

8. Propose ECO with hold/SI check plan.

9. Run regression matrix.

10. Update closure dashboard and memo.

Artifacts to collect

• SDC diff, constraint validator log, before/after path report

• SDC file

• SPEF tag

• MMMC scenario list

• ECO ticket

Decision memo template

STA DECISION MEMO — Constraint Bug Patterns
corner/mode:
WNS/TNS:
hypothesis:
ECO:
regression:
owners: STA owner, RTL owner, synthesis owner

Reference visuals

Top constraint bugs

CONSTRAINT BUG TOP 5
1. missing generated_clock
2. wrong clock on latch/path
3. IO delay wrong direction
4. false_path too broad
5. case_analysis stale

STA deep dive

Debug hygiene separates senior STA leads from report readers.

Concept diagram

DEBUG ORDER

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

Metric graph

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