Signoff Regression Hygiene: Mechanism

Mechanism to understand

Mechanism for Signoff Regression Hygiene focuses on run-to-run WNS drift, database tag consistency, closure memo completeness. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

Signoff quality depends on reproducible runs: same lib, SPEF, SDC, scenarios, and tool versions — tagged and archived. 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 — Signoff Regression Hygiene

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.

Run manifest

STA RUN MANIFEST
design: chip_top tag v2.4.1
sdc:    constraints_v18.git
spef:   cworst_v3
libs:   ss_125 / ff_m40 / ...
tool:   PrimeTime Q-2020.03
scenarios: 36 (full matrix)

Layer responsibilities

STA OWNERSHIP LAYERS — Signoff Regression Hygiene

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

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.

Mechanism deep dive

Signoff quality depends on reproducible runs: same lib, SPEF, SDC, scenarios, and tool versions — tagged and archived.