Two-Flop Synchronizers in Practice: Step-by-Step Walkthrough

Step-by-step analysis walkthrough

Use this sequence when owning Two-Flop Synchronizers in Practice 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

• CDC structural report, RTL synchronizer pattern list, assertion checks

• crossing inventory

• waiver board notes

• regression report

Decision memo template

CDC/RDC DECISION MEMO — Two-Flop Synchronizers in Practice
risk class:
mechanism:
evidence:
fix/waiver:
owners: RTL owner, CDC owner, verification owner

Reference visuals

Two-flop chain limits

SRC -----> [FF1] -----> [FF2] -----> DEST logic
            async sample    filtered sample

Valid for stable level signals.
Not sufficient for narrow pulses or multi-bit buses.

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

A two-flop chain lowers metastability propagation risk for level signals, but only when source assumptions, pulse width, and destination sampling are respected.

Metric: single-bit CDC clean rate, pulse capture reliability, latency cycles