Multi-bit Crossing Strategies: Worked Example

Worked example

Worked Example for Multi-bit Crossing Strategies focuses on coherency guarantee coverage, data corruption incidents, protocol overhead. The goal is to convert issue observations into mechanism-backed closure decisions.

A milestone review shows coherency guarantee coverage, data corruption incidents, protocol overhead. 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 Crossing Strategies

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

Key metric: coherency guarantee coverage, data corruption incidents, protocol overhead

Strategy matrix

bundle+valid/ready  : moderate latency, strong coherency
async FIFO         : high robustness, area cost
source strobe      : throughput friendly, timing contract heavy
token protocol     : explicit ordering, more control logic

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 strategy decision matrix, interface timing contract, verification plan.

5. Choose one reversible action and define regression upfront.

Did the action work?

BEFORE / AFTER — Multi-bit Crossing Strategies

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

Bundled-data handshakes, FIFOs, source-synchronous strobes, and encoded protocols each trade latency, area, and proof complexity.

Metric: coherency guarantee coverage, data corruption incidents, protocol overhead