MTBF & Synchronizer Math: Inputs & Outputs

Inputs and outputs contract

Inputs & Outputs for MTBF & Synchronizer Math focuses on MTBF target vs product FIT budget, synchronizer latency budget. The goal is to convert issue observations into mechanism-backed closure decisions.

Treat CDC/RDC signoff as a contract across architecture, RTL, DV, and signoff governance. Most late surprises are contract mismatches, not tooling gaps.

INPUTS
  - crossing inventory with intent classification
  - reset tree and release dependency model
  - protocol assumptions and assertion package
  - CDC/RDC tool configuration + waiver policy

OUTPUTS
  - severity-tagged open issue list
  - protocol/reset proof evidence
  - owner-assigned closure plan
  - signoff or escalation memo

Crossing sequence

CROSSING FLOW — MTBF & Synchronizer Math

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

Key metric: MTBF target vs product FIT budget, synchronizer latency budget

Ownership map

OWNERSHIP MAP — MTBF & Synchronizer Math

artifact                  owner
----------------------    -------------------------
design intent           CDC owner
verification evidence   library owner
signoff decision        architecture owner

Every open CDC/RDC issue needs one accountable owner before waiver or fix.

CDC/RDC deep dive

Metastability is managed risk, not eliminated risk.

Concept diagram

METASTABILITY FLOW

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

Metric graph

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