Path Depth Sensitivity: Worked Example

Worked example

Worked Example for Path Depth Sensitivity focuses on short-path hold margin, long-path setup margin, depth bucket WNS. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

A signoff run shows short-path hold margin, long-path setup margin, depth bucket WNS. 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 Path Depth Sensitivity.

Sequence under inspection

TIMING PATH — Path Depth Sensitivity

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: short-path hold margin, long-path setup margin, depth bucket WNS

Depth vs slack scatter

SLACK vs LOGIC DEPTH

slack
  + |     *  *  *
    |   *   *  *
  0 |--*--*-----*---  WNS
    | *
  - |*
    +----------------> depth (stages)

Short paths cluster left — hold risk zone.

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 path depth vs slack scatter, AOCV table dump, hold/setup split report.

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

Did the fix work?

BEFORE / AFTER ECO — Path Depth Sensitivity

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

STA deep dive

Variation policy is negotiated margin, not a tool checkbox.

Concept diagram

VARIATION STACK

nominal delay + OCV/AOCV/POCV margin = signoff delay

Metric graph

DERATE IMPACT ON WNS

OCV only     WNS -120ps
AOCV         WNS -45ps
POCV         WNS -60ps (policy dependent)

Reports and artifacts

• derate table

• AOCV depth report

• POCV sigma

• hold/setup split

Mini case study

Switched to AOCV without re-baselining hold — short paths failed min analysis.

Debug branches

• Short paths for hold

• Policy approval for derate changes

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

Short paths see different variation pessimism than long paths; depth-aware models prevent both optimism and excess margin.

Metric: short-path hold margin, long-path setup margin, depth bucket WNS