Path Depth Sensitivity: Reports & Metrics

Reports and metrics

Reports & Metrics 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.

The job of a report is to turn short-path hold margin, long-path setup margin, depth bucket WNS into a closure decision. A single WNS number is almost never enough; you need path group breakdown, cell vs net split, and a clear gap between current slack and signoff target.

Metric movement

METRIC GRAPH — short-path hold margin, long-path setup margin, depth bucket WNS

WNS (ps)
  ^
  |                         target (0)
  |                       - - - - - - -
  |                  o after ECO + hold check
  |              o
  |         o baseline run
  |    o failing regression
  +--------------------------------------> closure iteration
    view audit     path group fix     MMMC signoff

Readout:
  - explain why WNS/TNS moved and which path group drove the change

Slack distribution

SLACK HISTOGRAM — Path Depth Sensitivity

path count
  |      ***
  |    *******
  |  ***********
  |*************  <- WNS (worst path)
  +--------------------> slack (ns)
  -0.15  0.0  0.10

Fix the left tail first; average slack misleads.

• Track short-path hold margin, long-path setup margin, depth bucket WNS by path group, corner, and analysis view.

• Report WNS/TNS and failing endpoint count together.

• Include setup and hold splits; they often diverge after ECO.

• Always store the metric next to the artifact that produced it.

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.

Reading reports