Gray Code Crossing: Design Space

Design space exploration

For Gray Code Crossing, signoff options trade reliability, latency, and schedule.

Option A — conservative

• Conservative synchronization: helps robustness

• Risk: latency increase

• Validate with: stress regressions

Option B — balanced

• Balanced protocol design: helps throughput + safety

• Risk: higher design effort

• Validate with: formal + simulation

Option C — aggressive

• Aggressive waiver posture: helps schedule relief

• Risk: escape risk

• Validate with: periodic waiver audit

Option D — refactor

• Architectural refactor: helps long-term safety

• Risk: schedule hit

• Validate with: system bring-up

DESIGN SPACE — Gray Code Crossing
robustness <-> latency <-> complexity <-> schedule

Design pitfalls

• Waive-first behavior

• No owner for residual risk

Tradeoff curve

BEFORE / AFTER — Gray Code Crossing

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

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