Setup / Hold Closure Order

What this topic teaches

Setup / Hold Closure Order is about converting constraints and path rules into measurable slack. Setup and hold compete through delay changes; closure order and ECO type must avoid fixing one at the expense of the other. The hard part is never the textbook inequality; it is proving, under the right corner/mode/view, which layer — constraint, view, or implementation — broke the contract.

The senior-engineer question

When setup WNS, hold WNS, simultaneous violation count moves, can you identify the failing path group, the STA layer, the responsible owner, and the smallest experiment that proves the root cause?

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.

Picture the timing closure flow

Start every study session by drawing the behavior before reading signals. The diagrams below are the mental models to reproduce on a whiteboard.

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.

Timing path sequence

TIMING PATH — Setup / Hold Closure Order

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: setup WNS, hold WNS, simultaneous violation count

Who owns which layer

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

Evidence to collect

• Primary metric: setup WNS, hold WNS, simultaneous violation count.

• Primary artifact: setup/hold trend chart, ECO sequence log, path replay.

• Owners to bring into review: timing closure lead, PD owner, STA owner.

• Spec clause or requirement ID for every constraint claim.

• One failing report_timing excerpt and one reduced path that isolates the rule.

Ownership map

OWNERSHIP MAP — Setup / Hold Closure Order

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

Every WNS cluster needs a named owner before ECO.

Subpages in this topic

Each topic is taught across mechanism, inputs/outputs, reports, debug, worked example, pitfalls, interview, checklist, theory, design space, expanded case study, walkthrough, comparison matrix, software view, and silicon PPA impact.

Key takeaways

• State corner, mode, and view with every slack number.

• Separate constraint bugs, view mismatches, and real path delays before ECO.

• Run setup and hold plus full MMMC after every change.

Common pitfalls

• Comparing runs with different SPEF or SDC tags.

• Broad false_path to hide violations.

• Setup-only ECO without hold regression.

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.