Useful Skew Tradeoffs: Pitfalls & Red Flags

Pitfalls and red flags

Pitfalls & Red Flags 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.

• A path looks clean in one view but fails in signoff SPEF.

• The SDC allows behavior that the implementation team assumed was illegal.

• A generated_clock definition shifts capture edge silently.

• Hold fails on short paths after setup-focused ECO.

• The fix improves one corner but regresses another MMMC scenario.

Layer responsibility check

STA OWNERSHIP LAYERS — Useful Skew Tradeoffs

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

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