Variation Signoff Dashboard: Mechanism

Mechanism to understand

Mechanism for Variation Signoff Dashboard focuses on WNS/TNS across derate corners, hold/setup closure rate, margin trend. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

Variation signoff tracks closure across the full derate matrix and flags corners where margin is consumed by policy not implementation. Think of STA as a signed contract between constraints, parasitics, and implementation — slack is the measurable outcome.

• Identify launch and capture clocks on the failing path.

• Identify path group: reg2reg, in2reg, reg2out, or async.

• Identify whether setup, hold, or both fail in the scenario.

Layered view

STA CLOSURE FLOW — Variation Signoff Dashboard

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.

Weekly WNS trend

WNS TREND (all signoff scenarios)

 0ps |           ___----
-50ps |      ____/
-100ps| ____/
      +-----------------> week
      w1   w2   w3   w4

Layer responsibilities

STA OWNERSHIP LAYERS — Variation Signoff Dashboard

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

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.

Mechanism deep dive

Variation signoff tracks closure across the full derate matrix and flags corners where margin is consumed by policy not implementation.