Setup / Hold Closure Order: Mechanism

Mechanism to understand

Mechanism for Setup / Hold Closure Order focuses on setup WNS, hold WNS, simultaneous violation count. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

Setup and hold compete through delay changes; closure order and ECO type must avoid fixing one at the expense of the other. 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 — Setup / Hold Closure Order

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.

Setup vs hold tradeoff

DELAY CHANGE EFFECT

increase cell delay:
  setup slack  improves  (more time before capture)
  hold slack   worsens    (data arrives later vs hold edge)

ECO must check BOTH sides.

Layer responsibilities

STA OWNERSHIP LAYERS — Setup / Hold Closure Order

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.

Mechanism deep dive

Setup and hold compete through delay changes; closure order and ECO type must avoid fixing one at the expense of the other.