Path Depth Sensitivity: Design Space

Design space exploration

For Path Depth Sensitivity, closure options trade margin, schedule, and regression risk.

Option A — conservative

• Conservative margin: helps safe signoff

• Risk: over-pessimism, area/power

• Validate with: first silicon

Option B — balanced

• Balanced derate: helps reasonable margin

• Risk: needs approval

• Validate with: mature node

Option C — aggressive ECO

• Aggressive ECO: helps fast WNS gain

• Risk: hold/SI regressions

• Validate with: late schedule

Option D — RTL restructure

• RTL restructure: helps structural fix

• Risk: schedule hit

• Validate with: chronic paths

DESIGN SPACE — Path Depth Sensitivity
margin <-> performance <-> schedule
pick based on short-path hold margin, long-path setup margin, depth bucket WNS on product scenarios

Design pitfalls

• ECO without regression matrix

• Constraint change without audit

Tradeoff curve

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