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CDC / RDC · All levels

Multi-bit Crossing Strategies: Interview Drills

Interview Drills for Multi-bit Crossing Strategies.

Interview drills

Interview Drills 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.

diagram
PROMPT
You see coherency guarantee coverage, data corruption incidents, protocol overhead in Multi-bit Crossing Strategies. Explain root cause and signoff decision.

STRONG ANSWER
1. Names crossing/reset class and context.
2. Explains Bundled-data handshakes, FIFOs, source-synchronous strobes, and encoded protocols each trade latency, area, and proof complexity.
3. Requests strategy decision matrix, interface timing contract, verification plan.
4. Chooses fix/waiver/escalation with regression plan.

WEAK ANSWER
Lists tool commands or generic advice without mechanism and risk framing.

Diagram to draw on whiteboard

Strategy matrix

diagram
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

Debug tree to narrate

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ROOT-CAUSE TREE — Multi-bit Crossing Strategies

crossing failure observed
        |
   reproducible?
     /        \
   no          yes
   |            |
stress mode   classify issue
expansion       /      |      \
            synchronizer protocol reset/reconvergence
                 |         |           |
            MTBF fit    liveness    release ordering

CDC/RDC deep dive

Reconvergence failures are timing + logic coupling hazards.

Concept diagram

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RECONVERGENCE

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

Metric graph

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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

Practice this lesson