Path Depth Sensitivity: Inputs & Outputs

Inputs and outputs contract

Inputs & Outputs 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.

Treat these as a signed interface contract. Ambiguity here is the single biggest source of wasted integration weeks, because two teams debug against different assumptions.

INPUTS
  - SDC (clocks, IO delays, exceptions)
  - Liberty libraries per corner
  - SPEF/parasitics and RC corner
  - MMMC scenario definition

OUTPUTS
  - WNS/TNS reports per scenario
  - path group summaries
  - constraint audit / waiver log
  - signoff closure memo

Timing path sequence

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

Ownership map

OWNERSHIP MAP — Path Depth Sensitivity

artifact              owner
----------------      -----------------
timing report       STA owner
SDC/constraint      library owner

Every WNS cluster needs a named owner before ECO.

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