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CDC / RDC · All levels

MTBF & Synchronizer Math: Step-by-Step Walkthrough

Step-by-Step Walkthrough for MTBF & Synchronizer Math.

Step-by-step analysis walkthrough

Use this sequence when owning MTBF & Synchronizer Math in a signoff review.

  1. Inspect warning class and severity.

  2. Trace crossing endpoints and domains.

  3. Check protocol/reset assumptions against spec.

  4. Review synchronizer or FIFO implementation detail.

  5. Run focused assertion or formal check.

  6. Draft fix options with risk and schedule impact.

  7. Execute regression and publish closure memo.

Artifacts to collect

  • MTBF worksheet, synchronizer cell characterization, frequency assumptions

  • crossing inventory

  • waiver board notes

  • regression report

Decision memo template

diagram
CDC/RDC DECISION MEMO — MTBF & Synchronizer Math
risk class:
mechanism:
evidence:
fix/waiver:
owners: CDC owner, library owner, architecture owner

Reference visuals

MTBF intuition

diagram
METASTABILITY MODEL

clock edge samples async input near transition
  -> first flop may metastabilize
  -> settle window before second flop sample

More settle time + better tau => higher MTBF.

CDC/RDC deep dive

Metastability is managed risk, not eliminated risk.

Concept diagram

diagram
METASTABILITY FLOW

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

Metric graph

diagram
MTBF TREND

target MTBF ---------
current design   ____/

Reports and artifacts

  • MTBF assumptions

  • synchronizer inventory

  • crossing class summary

  • critical waivers

Mini case study

Pulse sync chosen for a level signal caused intermittent stuck state under voltage stress.

Debug branches

  • Validate crossing class first

  • Check pulse width assumptions

  • Audit synchronizer template usage

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

Metastability cannot be eliminated, only pushed beyond mission lifetime by adequate settle time, stage count, and frequency-aware design.

Metric: MTBF target vs product FIT budget, synchronizer latency budget

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