STA Mastery · All levels

OCV / AOCV / POCV Mechanisms: Interview Drills

Interview Drills for OCV / AOCV / POCV Mechanisms.

Interview drills

Interview Drills for OCV / AOCV / POCV Mechanisms focuses on OCV derate %, depth-adjusted pessimism, POCV sigma contribution. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

diagram

PROMPT
You see OCV derate %, depth-adjusted pessimism, POCV sigma contribution on OCV / AOCV / POCV Mechanisms. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Variation models add margin for on-chip delay spread; AOCV/POCV reduce over-pessimism by accounting for path depth and statistical correlation.
3. Requests variation report, derate table, depth histogram, POCV policy doc.
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

OCV vs AOCV depth effect

diagram

DERATE PESSIMISM vs PATH DEPTH

OCV:   fixed % on every stage  -> over-pessimistic on long paths
AOCV:  derate reduces with depth -> less pessimism on long paths
POCV:  statistical sigma model  -> signoff-grade distribution

Short hold paths still need careful min-delay analysis.

Root-cause tree to narrate

diagram

ROOT-CAUSE TREE — OCV / AOCV / POCV Mechanisms

OCV derate %, depth-adjusted pessimism, POCV sigma contribution 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

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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

Variation models add margin for on-chip delay spread; AOCV/POCV reduce over-pessimism by accounting for path depth and statistical correlation.

Metric: OCV derate %, depth-adjusted pessimism, POCV sigma contribution

Practice this lesson