Setup / Hold Closure Order: Worked Example

Worked example

Worked Example 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.

A signoff run shows setup WNS, hold WNS, simultaneous violation count. The first review mistake is to blame implementation without checking constraints. A better review starts by pinning one critical path, proving where slack was lost, and checking whether the SDC rule is correct for Setup / Hold Closure Order.

Sequence under inspection

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

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.

1. Capture report_timing and the relevant SDC excerpt.

2. Tag corner, mode, view, and path group.

3. Find the first cell or net that dominates critical delay.

4. Compare against setup/hold trend chart, ECO sequence log, path replay.

5. Choose one reversible ECO and write the regression matrix before editing netlist or constraints.

Did the fix work?

BEFORE / AFTER ECO — Setup / Hold Closure Order

WNS (ps)
  0 |                    --- target
-50 |    ● regressed
-80 |         ● before ECO
-20 |              ● after ECO
    +-------------------------------> ECO iteration
Prove setup AND hold moved safely.

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

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

Metric: setup WNS, hold WNS, simultaneous violation count