CDC / RDC · All levels

Metastability & Synchronizers: Tricky Q&A

Senior interview and review questions for Metastability & Synchronizers.

Section Q&A bank

Use these drills after completing all topics in Metastability & Synchronizers. Answer with crossing class, mechanism, risk, evidence, owner, and signoff decision.

Why can metastability never be eliminated?

diagram

[INT][CDC-RDC][METASTABILITY-SYNCHRONIZERS]

Q: Why can metastability never be eliminated?

A:
Because asynchronous sampling can always violate setup/hold; design only lowers propagation probability via settle time and topology.

FOLLOW-UP TRAP: Saying two flops fully eliminate metastability.

When is two-flop sync unsafe?

diagram

[INT][CDC-RDC][METASTABILITY-SYNCHRONIZERS]

Q: When is two-flop sync unsafe?

A:
For narrow pulses, multi-bit buses, or coupled control/data transactions without protocol guarantees.

FOLLOW-UP TRAP: Applying two flops blindly to any crossing.

What is MTBF tied to?

diagram

[INT][CDC-RDC][METASTABILITY-SYNCHRONIZERS]

Q: What is MTBF tied to?

A:
Clock rates, synchronizer resolve window, metastability constants, and target FIT budget.

FOLLOW-UP TRAP: Treating MTBF as static across product modes.

Q&A drill guide

diagram

INTENT -> FAILURE MODE -> EVIDENCE -> FIX/WAIVER -> REGRESSION

Sketch while answering

diagram

METASTABILITY FLOW

async event -> first sample may metastabilize
 -> settle window
 -> downstream sample confidence

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.

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