OCV / AOCV / POCV Mechanisms: Worked Example

Worked example

Worked Example 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.

A signoff run shows OCV derate %, depth-adjusted pessimism, POCV sigma contribution. The first review mistake is to blame implementation without checking constraints. A better review starts by pinning one critical path, proving where slack was lost, and checking whether the SDC rule is correct for OCV / AOCV / POCV Mechanisms.

Sequence under inspection

TIMING PATH — OCV / AOCV / POCV Mechanisms

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: OCV derate %, depth-adjusted pessimism, POCV sigma contribution

OCV vs AOCV depth effect

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.

1. Capture report_timing and the relevant SDC excerpt.

2. Tag corner, mode, view, and path group.

3. Find the first cell or net that dominates critical delay.

4. Compare against variation report, derate table, depth histogram, POCV policy doc.

5. Choose one reversible ECO and write the regression matrix before editing netlist or constraints.

Did the fix work?

BEFORE / AFTER ECO — OCV / AOCV / POCV Mechanisms

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

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