Timing Closure

Section goal

Close setup and hold across all required corners/modes with evidence-backed fixes and zero unconstrained surprises.

Mechanism to narrate

• Treat SDC, SPEF, and corner decks as one signed contract

• Prioritize path groups by tapeout risk and endpoint ownership

• Use a signoff dashboard to prevent last-week blind spots

Senior course bar for this section

• Every topic should end with a signoff decision, not only concept recall.

• Every fix should state expected metric movement and likely regression surface.

• Every open assumption should have an owner, tag, and review date.

• Every recurring issue should become a methodology guardrail or checklist item.

1. sdc-for-implementation/ — SDC for Implementation

2. setup-hold-closure-playbook/ — Setup/Hold Closure Playbook

3. multi-corner-multi-mode-signoff/ — MCMM Signoff Strategy

4. eco-timing-fixes/ — ECO Timing Fixes

5. path-group-prioritization/ — Path Group Prioritization

6. generated-clocks-and-clock-groups/ — Generated Clocks and Clock Groups

7. io-timing-and-block-boundaries/ — IO Timing and Block Boundaries

8. useful-skew-timing-closure/ — Useful Skew in Timing Closure

9. post-route-extraction-and-spef/ — Post-Route Extraction and SPEF

10. timing-signoff-dashboard/ — Timing Signoff Dashboard

Related topics

• RTL STA intro

• RTL timing architecture

• Clock tree synthesis

• Routing closure

Key takeaways

• Name corner, mode, and report source before proposing a fix.

• Avoid hero ECOs; prefer repeatable cluster-level closure loops.

Detailed section notes

Timing closure is a signoff matrix problem, not a single worst-path problem.

Reports and artifacts to inspect

• report_timing -max and -min with corner, mode, SI, and OCV enabled

• path group WNS/TNS: which clock domain is actually blocking signoff

• net delay percentage vs cell delay percentage

• exception audit: false paths, multicycle paths, generated clocks

Mini case study

A -90 ps setup path has 75% net delay and crosses a macro channel. Sizing the launch flop is weak. The stronger response is to layer-promote or shorten the route, then re-run hold at the fast corner.

Debug branches

• If net delay dominates, look for physical fixes before cell sizing.

• If cell delay dominates, consider VT swap, sizing, or RTL micro-architecture.

• If only one mode fails, inspect mode-specific SDC overlays and case analysis.

Senior review question

Ask yourself: what single report line would prove this page's concept is either passing or failing?

What changes at 10+ years

• You are expected to predict what your fix can break before running it.

• You should recognize when the issue is methodology, not one block's implementation.

• You should communicate risk in tapeout language: owner, evidence, impact, mitigation, and decision date.

Principal-level review bar

Hub pages in this course should be read like real closure review material. For a 10+ year PD engineer, the bar is not remembering terminology; it is making a release-quality decision under ambiguity.

What excellent looks like

• Names the failing metric, corner/mode, database tag, and analysis switches before proposing a fix.

• Separates data, constraint, physical, tool, and methodology root causes instead of treating all failures as optimization problems.

• Chooses experiments by information gain and reversibility, not by habit.

• States regression blast radius across timing, route, power, PV, DFT, package, and tapeout manifest.

• Turns recurring failures into methodology guardrails, dashboards, or checklist items.

Closure note template

STAFF / PRINCIPAL CLOSURE NOTE

Context:
  stage: <pre-CTS | post-CTS | post-route | post-fill | signoff>
  tag: <database / netlist / SDC / library stack>
  failing metric: <exact report line>
  affected scope: <block / hierarchy / path group / power domain / region>

Hypotheses:
  H1: <most likely physical or constraint mechanism>
  H2: <competing explanation>
  H3: <methodology or input-data issue>

Decision:
  next experiment: <cheap check that can falsify H1>
  fix candidate: <minimal reversible change>
  rollback trigger: <metric that says the fix is wrong>
  regression set: <timing / route / power / PV / DFT / package>
  escalation owner: <team or reviewer>

Tradeoffs a senior engineer must discuss

Technical tradeoff

Timing closure is a signoff matrix problem, not a single worst-path problem. Explain not only the preferred fix, but what margin or schedule you are spending to get it.

Cross-team tradeoff

• What must RTL, synthesis, CAD, STA, DFT, package, IP, or foundry agree to before this decision is final?

• Which artifact becomes the source of truth after the decision: report, waiver, manifest, ECO script, or methodology deck?

• What is the cost of being wrong: one rerun, ECO churn, mask risk, performance loss, or silicon escape?

Leadership communication

"The current blocker is <metric> in <corner/mode/stage>. The leading cause is <mechanism>. I recommend <fix> because it is bounded and reversible. The regression surface is <domains>. If it fails, we escalate to <owner> with <evidence>."

Key takeaways

• Always connect the concept back to a measurable signoff artifact.

• A fix is not complete until you can name the regression checks.

Common pitfalls

• Optimizing by habit instead of reading the current report.

• Forgetting that a local fix can regress timing, routing, power, or PV elsewhere.