Path Depth Sensitivity: Theory Deep Dive

Foundational theory

Path Depth Sensitivity is central to OCV, AOCV, POCV & Variation. Short paths see different variation pessimism than long paths; depth-aware models prevent both optimism and excess margin. Senior STA engineers always tie slack movement to corner, mode, constraint set, and parasitic view — not just a single report line.

Core concepts explained

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

• Primary metric: short-path hold margin, long-path setup margin, depth bucket WNS

• Primary artifact: path depth vs slack scatter, AOCV table dump, hold/setup split report

• Owners: STA owner, library owner

• Setup: max delay path; Hold: min delay path

• WNS/TNS summarize tail risk across endpoints

Why this matters at signoff

At signoff, Path Depth Sensitivity failures block tapeout or force risky ECO. Variation models translate process spread into timing margin. Wrong analysis setup wastes weeks of PD effort.

Mental model

SLACK vs LOGIC DEPTH

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

Short paths cluster left — hold risk zone.

Worked intuition

1. Name corner, mode, and view for the failing run.

2. Open short-path hold margin, long-path setup margin, depth bucket WNS and identify worst path group.

3. Read critical path: cell vs net, launch/capture clocks.

4. Check constraints on that path (exceptions, generated clocks).

5. Collect path depth vs slack scatter, AOCV table dump, hold/setup split report and tag database versions.

6. Classify: constraint bug, view mismatch, or real path.

7. Propose minimal ECO and list MMMC+SI regression.

Common misconceptions

• One typical corner is enough for signoff.

• False paths can be applied broadly to green-wash violations.

• Implementation WNS equals signoff WNS without view mapping.

• Setup fix automatically preserves hold.

Visual reinforcement

Depth vs slack scatter

SLACK vs LOGIC DEPTH

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

Short paths cluster left — hold risk zone.

Layer responsibilities

STA OWNERSHIP LAYERS — Path Depth Sensitivity

layer              owns                         failure mode
----------------   --------------------------   ---------------------
constraints        clocks, IO, exceptions       false violations
implementation     cells, nets, placement       real path delay
extraction         SPEF/RC corners              view mismatch
signoff policy     derate, MMMC matrix          margin disputes
closure           ECO order, regression        fix breaks other corner

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.

Theory reinforcement

Variation models translate process spread into timing margin.