CDC Verification for Safety-Critical Designs

 

CDC Verification for Safety-Critical Designs: What You Need to Know

Clock Domain Crossing (CDC) verification has become one of the most critical elements in modern chip design—especially when the chip is destined for safety-critical applications like avionics. A single CDC-related failure in silicon can lead to unpredictable behavior, costly re-spins, and in the worst cases, catastrophic system failures.

In this article, we break down the key concepts from a recent Siemens EDA white paper on CDC verification for airborne hardware and highlight what engineers must know to meet the strict design assurance requirements of DO-254.


Why Verification Matters More Than Ever

In every chip project:

  • Re-spins increase cost and delay time-to-market

  • Bugs in silicon damage brand trust and revenue

  • In safety-critical fields (like aerospace), failure isn’t an option

For avionics hardware, the consequences of intermittent or undetected bugs are severe. This is why design teams must follow stringent guidelines—especially those defined in RTCA/DO-254, the global standard for airborne electronic hardware.

The Challenge: CDC Issues & Metastability

CDC failures are among the most dangerous bugs in digital systems because:

  • They often create intermittent, hard-to-reproduce failures

  • They don’t appear during normal simulation

  • They pass static timing checks, since they occur between asynchronous clock domains

What is Metastability?

Metastability occurs when:

  • Data and clock signals hit a flip-flop at nearly the same time

  • The flop output becomes unstable, oscillates, and eventually settles to a random value

This can lead to:

  • Data loss

  • Data corruption

  • Unpredictable behavior in cross-clock paths

As the number of asynchronous clock domains increases (common in today’s SoCs), the metastability risk grows dramatically.

Why CDC Bugs Are Dangerous in Safety-Critical Designs

In safety-critical systems (like flight control units):

  • Intermittent CDC failures may never appear in pre-silicon testing

  • Failures often show up only in deployed hardware

  • A single event can contribute to hazardous or catastrophic outcomes

This is why explicit CDC verification is mandatory in DO-254 compliant projects.

Understanding DO-254 Design Assurance

The DO-254 standard ensures that airborne hardware performs reliably and safely.
Projects are classified under Design Assurance Levels (DAL A–E):

  • DAL A – Catastrophic failure (plane crash)

  • DAL B – Hazardous/major failure

  • DAL C – Significant reduction in safety margin

  • DAL D – Minor failure

  • DAL E – No impact on safety

DAL A/B designs require the highest level of rigor, documentation, traceability, and verification—including comprehensive CDC verification.



Building a DO-254-Compliant CDC Verification Methodology

According to Siemens, a complete CDC verification solution must include four core components:

1. Structural Analysis

Identify all clock domains, crossings, synchronizers, and violations.

2. Transfer Protocol Verification

Ensure that handshake signals follow safe communication rules across domains.

3. Global Reconvergence Checks

Address issues where signals diverge and reconverge unsafely after synchronization.

4. Netlist Glitch Analysis

Detect potential glitch sources introduced during synthesis or optimization.

These capabilities are crucial for uncovering CDC bugs that traditional verification flows miss.

Why Many Companies Use Siemens Questa CDC

The Siemens white paper highlights several organizations using Questa CDC, including:

  • Major storage & networking companies

  • Global PC & consumer electronics makers

  • Wireless communication vendors

  • Aerospace & defense technology providers

  • Military/space system developers

These companies rely on structured CDC verification not only for bug detection but also as part of their design assurance strategy under DO-254.

Tool Assessment: A Mandatory DO-254 Requirement

DO-254 doesn't stop at design processes—it also requires verifying that the tools used in design and verification are operating correctly.

Key points:

  • Tool vendors cannot self-certify their tools

  • FAA/EASA do not approve tools outright

  • Every project must perform its own tool assessment

  • Tool qualification depends on the tool’s impact on hardware safety

The white paper provides practical guidance on meeting assessment requirements using Questa CDC.

Final Thoughts

CDC verification is no longer optional—especially for airborne or mission-critical electronics.
As clock domains multiply and system complexity grows, the risk of metastability-driven failures increases exponentially.

A robust CDC flow, aligned with DO-254, ensures:

  • Predictable behavior

  • Verified safety

  • Reduced re-spins

  • Compliance with global aviation standards

For engineers building safety-critical chips, mastering CDC verification is essential.

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