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Path Depth Sensitivity: Interview Drills

Interview Drills for Path Depth Sensitivity.

Interview drills

Interview Drills 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.

diagram

PROMPT
You see short-path hold margin, long-path setup margin, depth bucket WNS on Path Depth Sensitivity. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Short paths see different variation pessimism than long paths; depth-aware models prevent both optimism and excess margin.
3. Requests path depth vs slack scatter, AOCV table dump, hold/setup split report.
4. Proposes one reduced sequence and one system regression.

WEAK ANSWER
Jumps to upsizing cells, widening routes, or blaming tools without reading constraints and path delay.

Diagram to draw on the whiteboard

Depth vs slack scatter

diagram

SLACK vs LOGIC DEPTH

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

Short paths cluster left — hold risk zone.

Root-cause tree to narrate

diagram

ROOT-CAUSE TREE — Path Depth Sensitivity

short-path hold margin, long-path setup margin, depth bucket WNS looks wrong
        |
   same netlist tag?
     /        \
   no          yes
   |            |
 view/SDC     real path or
 mismatch     constraint bug
 /    \           |
lib   SPEF    delay breakdown
diff  diff    cell vs net

STA deep dive

Variation policy is negotiated margin, not a tool checkbox.

Concept diagram

diagram

VARIATION STACK

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

Metric graph

diagram

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

Practice this lesson