Reconvergence Debug: Expanded Case Study

Extended case study

Milestone review flags hazard reproduction success, debug cycle time, recurrence rate around Reconvergence Debug.

Background

Team believed crossings were stable until stress mode exposed intermittent anomalies.

Symptoms observed

• hazard reproduction success, debug cycle time, recurrence rate regression

• Mismatch between structural report and dynamic behavior

• Waiver debate under schedule pressure

Investigation timeline

1. Freeze design, reset, and tool configuration tags.

2. Reproduce failing scenario with minimized stimulus.

3. Map path/protocol/reset dependencies.

4. Classify root cause and containment options.

5. Execute minimal safe fix.

6. Run stress regression and review board.

Root cause

Root cause links to Reconvergence Debug: Reconvergence failures are intermittent and mode-dependent; debugging requires path correlation, event alignment, and protocol state reconstruction.

Fix and validation

• Targeted RTL/protocol/reset correction

• Evidence refresh

• Signoff board decision

Lessons learned

• Treat waivers as temporary risk contracts

• Track owner and expiry

• Regression before closure

CASE STUDY — Reconvergence Debug
critical issues before / after / signoff

Crossing sequence under stress

CROSSING FLOW — Reconvergence Debug

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

Key metric: hazard reproduction success, debug cycle time, recurrence rate

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

Reconvergence failures are intermittent and mode-dependent; debugging requires path correlation, event alignment, and protocol state reconstruction.

Metric: hazard reproduction success, debug cycle time, recurrence rate