Signoff vs Implementation Views: Expanded Case Study

Extended case study

Signoff review: view delta WNS, parasitic corner mismatch, SPEF vs signoff RC gap fails after an apparently minor change affecting Signoff vs Implementation Views.

Background

Team had closure last week on full MMMC matrix. New run shows regression clustered in one path group.

Symptoms observed

• view delta WNS, parasitic corner mismatch, SPEF vs signoff RC gap regression

• Disagreement between synth and STA WNS

• Hold violations after setup ECO

Investigation timeline

1. Hour 0: freeze netlist, SDC, SPEF, lib tags

2. Hour 1: diff constraints and run manifest

3. Hour 2: isolate corner/mode that fails

4. Hour 3: read worst path — cell vs net, clocks

5. Hour 4: classify root cause

6. Hour 5: minimal ECO or SDC fix

7. Hour 6: full regression + signoff memo

Root cause

Root cause tied to Signoff vs Implementation Views: Implementation and signoff use different parasitic corners and analysis views; comparing them without mapping causes false panic or false confidence.

Fix and validation

• Bounded ECO or audited SDC change

• Re-run view comparison report, SPEF tag manifest, delta slack log

• Full MMMC + SI regression

Lessons learned

• Tag every run

• Diff constraints before blaming PD

• Setup and hold together

CASE STUDY — Signoff vs Implementation Views
baseline WNS / regressed WNS / post-fix WNS

Sequence under stress

TIMING PATH — Signoff vs Implementation Views

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: view delta WNS, parasitic corner mismatch, SPEF vs signoff RC gap

STA deep dive

Signoff WNS is the minimum across the full scenario matrix.

Concept diagram

MMMC

corners (ss/tt/ff) x modes (func/test) x views (signoff RC)
= scenario set -> min(WNS) is signoff

Metric graph

WNS HEATMAP BY CORNER

        ss    tt    ff
setup  -80   +10   +5
hold   -15   -2    -8

Reports and artifacts

• per-corner WNS

• mode summary

• lib map audit

• view delta

Mini case study

Passed TT typical but failed SS hold — signoff matrix was incomplete; added low-voltage mode.

Debug branches

• Isolate failing scenario first

• Never compare unlike SPEF tags

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

Implementation and signoff use different parasitic corners and analysis views; comparing them without mapping causes false panic or false confidence.

Metric: view delta WNS, parasitic corner mismatch, SPEF vs signoff RC gap