Gray Code Crossing: Inputs & Outputs

Inputs and outputs contract

Inputs & Outputs for Gray Code Crossing focuses on single-bit transition compliance, pointer decode correctness, CDC flag integrity. The goal is to convert issue observations into mechanism-backed closure decisions.

Treat CDC/RDC signoff as a contract across architecture, RTL, DV, and signoff governance. Most late surprises are contract mismatches, not tooling gaps.

INPUTS
  - crossing inventory with intent classification
  - reset tree and release dependency model
  - protocol assumptions and assertion package
  - CDC/RDC tool configuration + waiver policy

OUTPUTS
  - severity-tagged open issue list
  - protocol/reset proof evidence
  - owner-assigned closure plan
  - signoff or escalation memo

Crossing sequence

CROSSING FLOW — Gray Code Crossing

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

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

Ownership map

OWNERSHIP MAP — Gray Code Crossing

artifact                  owner
----------------------    -------------------------
design intent           RTL owner
verification evidence   CDC owner
signoff decision        formal owner

Every open CDC/RDC issue needs one accountable owner before waiver or fix.

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

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