Derate Policy & Approval: Expanded Case Study

Extended case study

Signoff review: approved derate set, waiver count, signoff pessimism budget fails after an apparently minor change affecting Derate Policy & Approval.

Background

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

Symptoms observed

• approved derate set, waiver count, signoff pessimism budget 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 Derate Policy & Approval: Derating policy is a contractual margin between implementation and signoff; changes need documented approval and regression.

Fix and validation

• Bounded ECO or audited SDC change

• Re-run derate approval memo, signoff policy PDF, before/after WNS comparison

• Full MMMC + SI regression

Lessons learned

• Tag every run

• Diff constraints before blaming PD

• Setup and hold together

CASE STUDY — Derate Policy & Approval
baseline WNS / regressed WNS / post-fix WNS

Sequence under stress

TIMING PATH — Derate Policy & Approval

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: approved derate set, waiver count, signoff pessimism budget

STA deep dive

Variation policy is negotiated margin, not a tool checkbox.

Concept diagram

VARIATION STACK

nominal delay + OCV/AOCV/POCV margin = signoff delay

Metric graph

DERATE IMPACT ON WNS

OCV only     WNS -120ps
AOCV         WNS -45ps
POCV         WNS -60ps (policy dependent)

Reports and artifacts

• derate table

• AOCV depth report

• POCV sigma

• hold/setup split

Mini case study

Switched to AOCV without re-baselining hold — short paths failed min analysis.

Debug branches

• Short paths for hold

• Policy approval for derate changes

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

Derating policy is a contractual margin between implementation and signoff; changes need documented approval and regression.

Metric: approved derate set, waiver count, signoff pessimism budget