CDC / RDC · All levels

Synchronizer Selection Guide: Comparison Matrix

Comparison Matrix for Synchronizer Selection Guide.

Comparison matrix

Synchronizer choices trade latency, reliability, and proof burden.

diagram

+------------------+----------------+----------------+----------------+
| Approach         | Strength       | Weakness       | Best when      |
+------------------+----------------+----------------+----------------+
| Conservative     | safe           | extra latency  | high-risk crossings |
| Balanced         | practical      | more setup     | most production paths |
| Aggressive       | fast schedule  | escape risk    | late-stage crunch |
| Refactor         | structural gain | long cycle     | recurring failures |
+------------------+----------------+----------------+----------------+

When to choose each approach

Select approach by silicon risk, not only schedule urgency

Interview traps

Global waiver spray
Unproven protocol assumptions

Evidence comparison

diagram

CDC/RDC EVIDENCE MATRIX — Synchronizer Selection Guide

+-----------------------+----------------------------+----------------------------+---------------------------+
| Evidence              | Tells you                  | Does not prove             | Next action               |
+-----------------------+----------------------------+----------------------------+---------------------------+
| structural report     | crossing topology class    | protocol liveness          | add assertions/formal     |
| reset map             | release dependencies       | dynamic startup behavior   | run reset stress tests    |
| formal proof          | bounded safety/liveness    | real firmware sequence     | correlate with simulation |
| simulation stress     | observed mode behavior     | full state-space closure   | target unhit corners      |
| waiver log            | accepted residual risk     | technical correctness      | periodic revalidation     |
+-----------------------+----------------------------+----------------------------+---------------------------+

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

Crossing intent (level, pulse, bus, control/data coupling) determines legal synchronization topology and verification evidence.

Metric: crossing class coverage, incorrect pattern usage rate, waiver count

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