Synchronizer Selection Guide: Expanded Case Study

Extended case study

Milestone review flags crossing class coverage, incorrect pattern usage rate, waiver count around Synchronizer Selection Guide.

Background

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

Symptoms observed

• crossing class coverage, incorrect pattern usage rate, waiver count 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 Synchronizer Selection Guide: Crossing intent (level, pulse, bus, control/data coupling) determines legal synchronization topology and verification evidence.

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 — Synchronizer Selection Guide
critical issues before / after / signoff

Crossing sequence under stress

CROSSING FLOW — Synchronizer Selection Guide

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

Key metric: crossing class coverage, incorrect pattern usage rate, waiver count

CDC/RDC deep dive

Metastability is managed risk, not eliminated risk.

Concept diagram

METASTABILITY FLOW

async event -> first sample may metastabilize
 -> settle window
 -> downstream sample confidence

Metric graph

MTBF TREND

target MTBF ---------
current design   ____/

Reports and artifacts

• MTBF assumptions

• synchronizer inventory

• crossing class summary

• critical waivers

Mini case study

Pulse sync chosen for a level signal caused intermittent stuck state under voltage stress.

Debug branches

• Validate crossing class first

• Check pulse width assumptions

• Audit synchronizer template usage

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

Crossing intent (level, pulse, bus, control/data coupling) determines legal synchronization topology and verification evidence.

Metric: crossing class coverage, incorrect pattern usage rate, waiver count