DenseChip

CDC / RDC · All levels

Multi-bit CDC Hazards: Step-by-Step Walkthrough

Step-by-Step Walkthrough for Multi-bit CDC Hazards.

Step-by-step analysis walkthrough

Use this sequence when owning Multi-bit CDC Hazards in a signoff review.

  1. Inspect warning class and severity.

  2. Trace crossing endpoints and domains.

  3. Check protocol/reset assumptions against spec.

  4. Review synchronizer or FIFO implementation detail.

  5. Run focused assertion or formal check.

  6. Draft fix options with risk and schedule impact.

  7. Execute regression and publish closure memo.

Artifacts to collect

  • bus crossing inventory, reconvergence report, protocol assumptions doc

  • crossing inventory

  • waiver board notes

  • regression report

Decision memo template

diagram
CDC/RDC DECISION MEMO — Multi-bit CDC Hazards
risk class:
mechanism:
evidence:
fix/waiver:
owners: RTL owner, CDC owner, IP integration owner

Reference visuals

Bus incoherency hazard

diagram
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.

CDC/RDC deep dive

Metastability is managed risk, not eliminated risk.

Concept diagram

diagram
METASTABILITY FLOW

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

Metric graph

diagram
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

Practice this lesson