Useful Skew Tradeoffs: Review Checklist

Review checklist

Review Checklist for Useful Skew Tradeoffs focuses on skew budget used, hold impact, clock tree power. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

• SDC revision and implemented clock/exception policy are named.

• Clock/reset and CDC assumptions are reviewed.

• Path groups, IO delays, and exception rationale are documented.

• MMMC scenario list maps back to signoff requirements.

• WNS/TNS artifact stored with corner/mode/view tags.

• Owners signed: CTS owner, STA lead, PD owner.

Sign-off ownership

OWNERSHIP MAP — Useful Skew Tradeoffs

artifact              owner
----------------      -----------------
timing report       CTS owner
SDC/constraint      STA lead
implementation      PD owner

Every WNS cluster needs a named owner before ECO.

STA deep dive

ECO without regression is a coin flip.

Concept diagram

ECO LOOP

failing path -> fix -> setup check -> hold check -> SI -> MMMC matrix

Metric graph

ECO PROGRESS

WNS
 0 |              o target
-50|        o
-100|   o baseline
    +-----------------> ECO #

Reports and artifacts

• pre/post WNS

• hold delta

• leakage delta

• ECO ticket log

Mini case study

48-hour tapeout crunch: VT swap fixed setup; hold fixed with route on short path — order mattered.

Debug branches

• Never ship setup-only proof

• Logical ECO needs equivalence

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

Intentional clock skew can borrow setup time but increases hold risk and clock tree complexity; useful skew is a negotiated margin.

Metric: skew budget used, hold impact, clock tree power