Derate Policy & Approval

What this topic teaches

Derate Policy & Approval is about converting constraints and path rules into measurable slack. Derating policy is a contractual margin between implementation and signoff; changes need documented approval and regression. The hard part is never the textbook inequality; it is proving, under the right corner/mode/view, which layer — constraint, view, or implementation — broke the contract.

The senior-engineer question

When approved derate set, waiver count, signoff pessimism budget moves, can you identify the failing path group, the STA layer, the responsible owner, and the smallest experiment that proves the root cause?

STA CLOSURE FLOW — Derate Policy & Approval

constraints (SDC)
        |
        v
timing graph (cell + net delays)
        |
        v
MMMC scenarios (corner x mode x view)
        |
        v
variation + SI adjustments
        |
        v
slack (WNS/TNS) -> ECO -> regression

Debug rule: always state corner, mode, view, and database tag with slack.

Picture the timing closure flow

Start every study session by drawing the behavior before reading signals. The diagrams below are the mental models to reproduce on a whiteboard.

Derate approval flow

DERATE CHANGE REQUEST
  -> STA lead review
  -> PD director sign
  -> foundry policy check
  -> re-run full MMMC matrix
  -> archive before/after WNS

Timing path sequence

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

Who owns which layer

STA OWNERSHIP LAYERS — Derate Policy & Approval

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

Evidence to collect

• Primary metric: approved derate set, waiver count, signoff pessimism budget.

• Primary artifact: derate approval memo, signoff policy PDF, before/after WNS comparison.

• Owners to bring into review: STA lead, PD director, foundry signoff.

• Spec clause or requirement ID for every constraint claim.

• One failing report_timing excerpt and one reduced path that isolates the rule.

Ownership map

OWNERSHIP MAP — Derate Policy & Approval

artifact              owner
----------------      -----------------
timing report       STA lead
SDC/constraint      PD director
implementation      foundry signoff

Every WNS cluster needs a named owner before ECO.

Subpages in this topic

Each topic is taught across mechanism, inputs/outputs, reports, debug, worked example, pitfalls, interview, checklist, theory, design space, expanded case study, walkthrough, comparison matrix, software view, and silicon PPA impact.

Key takeaways

• State corner, mode, and view with every slack number.

• Separate constraint bugs, view mismatches, and real path delays before ECO.

• Run setup and hold plus full MMMC after every change.

Common pitfalls

• Comparing runs with different SPEF or SDC tags.

• Broad false_path to hide violations.

• Setup-only ECO without hold regression.

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.