Reconvergence Analysis: Worked Example

Worked example

Worked Example for Reconvergence Analysis focuses on reconvergence hazard count, false alarm ratio, silicon-escape risk index. The goal is to convert issue observations into mechanism-backed closure decisions.

A milestone review shows reconvergence hazard count, false alarm ratio, silicon-escape risk index. Teams disagree on severity. The right move is to isolate one representative issue, prove mechanism class, and decide fix or waiver with explicit residual risk.

Crossing under inspection

CROSSING FLOW — Reconvergence Analysis

source clock domain -> launch signal -> crossing structure -> destination sample
      |                    |                 |                    |
   source FF           protocol           sync / fifo         destination FF

Key metric: reconvergence hazard count, false alarm ratio, silicon-escape risk index

Reconvergence skew

signal A --sync path 1--                             logic merge -> hazard
signal B --sync path 2--/

Individually synchronized signals can still combine illegally.

1. Capture warning, waveform, and owning module context.

2. Tag mode/reset/traffic state for the failure.

3. Validate assumptions against spec and assertions.

4. Compare outcome with reconvergence report, fanout cone map, functional hazard classification.

5. Choose one reversible action and define regression upfront.

Did the action work?

BEFORE / AFTER — Reconvergence Analysis

open critical issues
  ^
  |  o baseline
  |     o after fix batch
  |         o after protocol proof
  |             o signoff-ready
  +---------------------------------> closure iteration

Track issue burn-down with evidence quality, not only count.

CDC/RDC deep dive

Reconvergence failures are timing + logic coupling hazards.

Concept diagram

RECONVERGENCE

branch A sync --                 -> merge logic -> illegal combination window
branch B sync --/

Metric graph

HAZARD REPRODUCTION RATE

before fix: 1/2000 runs
after fix: 0/100000 runs

Reports and artifacts

• reconvergence warnings

• gray transition checks

• hazard replay traces

• multi-bit strategy map

Mini case study

Gray pointer was legal, but downstream merge decoded mixed-era values during burst stress.

Debug branches

• Trace fanin cones

• align event windows

• verify decode assumptions

Senior review question

Ask: what evidence proves this risk is closed for silicon, not just tool-clean?

Key takeaways

• State crossing class, assumptions, and owner with every issue.

• Run structural and dynamic regressions after each fix.

Common pitfalls

• Treating all warnings as equivalent risk.

• Waiving issues without containment evidence.

• Skipping reset and reconvergence stress after CDC fixes.

Principal CDC/RDC review addendum

Signals synchronized along different paths may reconverge with phase skew, creating functionally illegal combinations despite structurally legal crossings.

Metric: reconvergence hazard count, false alarm ratio, silicon-escape risk index