Cell vs Net Delay Budgets: Expanded Case Study

Extended case study

Signoff review: cell delay %, net delay %, high fanout net count, transition violations fails after an apparently minor change affecting Cell vs Net Delay Budgets.

Background

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

Symptoms observed

• cell delay %, net delay %, high fanout net count, transition violations 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

Root cause tied to Cell vs Net Delay Budgets: Path delay splits between library cell arcs and interconnect parasitics; implementation fixes target the dominant contributor.

Fix and validation

• Bounded ECO or audited SDC change

• Re-run report_timing -cap -tran -nets, delay breakdown report, RC extraction log

• Full MMMC + SI regression

Lessons learned

• Tag every run

• Diff constraints before blaming PD

• Setup and hold together

CASE STUDY — Cell vs Net Delay Budgets
baseline WNS / regressed WNS / post-fix WNS

Sequence under stress

TIMING PATH — Cell vs Net Delay Budgets

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: cell delay %, net delay %, high fanout net count, transition violations

STA deep dive

Slack is a path story — read the graph, not just the number.

Concept diagram

TIMING GRAPH

FF --cell-- logic --net-- logic --cell-- FF
launch                              capture

Metric graph

SLACK TAIL

paths |    ***
      |  *******
      |*********  <- fix WNS first

Reports and artifacts

• WNS/TNS

• path slack histogram

• cell vs net %

• path group summary

Mini case study

WNS -90ps but 80% net delay on one congested route — PD route fix beat upsizing three cells.

Debug branches

• Cell-heavy -> sizing/VT

• Net-heavy -> route/layer

• Check launch/capture clocks

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

Path delay splits between library cell arcs and interconnect parasitics; implementation fixes target the dominant contributor.

Metric: cell delay %, net delay %, high fanout net count, transition violations