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CDC / RDC · All levels

Gray Code Crossing: Step-by-Step Walkthrough

Step-by-Step Walkthrough for Gray Code Crossing.

Step-by-step analysis walkthrough

Use this sequence when owning Gray Code Crossing 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

  • gray encode/decode checks, pointer transition proof, FIFO status tests

  • crossing inventory

  • waiver board notes

  • regression report

Decision memo template

diagram
CDC/RDC DECISION MEMO — Gray Code Crossing
risk class:
mechanism:
evidence:
fix/waiver:
owners: RTL owner, CDC owner, formal owner

Reference visuals

Gray transition property

diagram
binary:  0111 -> 1000 (4 bit changes)
gray:    0100 -> 1100 (1 bit change)

One-bit transition reduces sampled ambiguity in crossing.

CDC/RDC deep dive

Reconvergence failures are timing + logic coupling hazards.

Concept diagram

diagram
RECONVERGENCE

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

Metric graph

diagram
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

Gray encoding limits transitions to one bit per step, reducing sampled ambiguity when pointers cross asynchronous boundaries.

Metric: single-bit transition compliance, pointer decode correctness, CDC flag integrity

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