Multi-bit CDC Hazards: Worked Example

Worked example

Worked Example for Multi-bit CDC Hazards focuses on data coherence violations, reconvergence warning count, escaped protocol bugs. The goal is to convert issue observations into mechanism-backed closure decisions.

A milestone review shows data coherence violations, reconvergence warning count, escaped protocol bugs. 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 — Multi-bit CDC Hazards

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

Key metric: data coherence violations, reconvergence warning count, escaped protocol bugs

Bus incoherency hazard

bus[3:0] bits cross independently

sample N : 0 1 1 0
sample N+1: 1 0 0 1   (illegal mixed state)

Need protocol or encoded transfer, not per-bit sync.

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 bus crossing inventory, reconvergence report, protocol assumptions doc.

5. Choose one reversible action and define regression upfront.

Did the action work?

BEFORE / AFTER — Multi-bit CDC Hazards

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

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

Independent bit synchronization causes skew and data incoherency; buses need protocolized transfer (FIFO, handshake, token, or encoded sequencing).

Metric: data coherence violations, reconvergence warning count, escaped protocol bugs