Setup / Hold Closure Order: Expanded Case Study

Extended case study

Signoff review: setup WNS, hold WNS, simultaneous violation count fails after an apparently minor change affecting Setup / Hold Closure Order.

Background

Team had closure last week on full MMMC matrix. New run shows regression clustered in one path group.

Symptoms observed

• setup WNS, hold WNS, simultaneous violation count regression

• Disagreement between synth and STA WNS

• Hold violations after setup ECO

Investigation timeline

1. Hour 0: freeze netlist, SDC, SPEF, lib tags

2. Hour 1: diff constraints and run manifest

3. Hour 2: isolate corner/mode that fails

4. Hour 3: read worst path — cell vs net, clocks

5. Hour 4: classify root cause

6. Hour 5: minimal ECO or SDC fix

7. Hour 6: full regression + signoff memo

Root cause

Setup sizing ECO fixed WNS but created hold violations on short paths — regression caught on min analysis.

Fix and validation

• Bounded ECO or audited SDC change

• Re-run setup/hold trend chart, ECO sequence log, path replay

• Full MMMC + SI regression

Lessons learned

• Tag every run

• Diff constraints before blaming PD

• Setup and hold together

CASE STUDY — Setup / Hold Closure Order
baseline WNS / regressed WNS / post-fix WNS

Sequence under stress

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

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