Hey there, fellow automotive engineers! If you’ve been wrestling with ISO 26262 compliance lately (and who hasn’t?), you’ve probably encountered the somewhat intimidating concepts of Dependent Failure Analysis (DFA) and Safety Measures. Today, I want to break these down into practical, applicable insights that you can actually use in your day-to-day work.
The Safety Challenge We’re All Facing
Let’s be honest – modern automotive systems are incredibly complex. With advanced driver assistance systems (ADAS) and autonomous features becoming standard, ensuring safety isn’t just a regulatory checkbox anymore – it’s absolutely critical. And that’s exactly why ISO 26262 puts so much emphasis on DFA and Safety Measures.
What’s DFA All About, Really?
Think of DFA as your safety detective work. It’s all about finding those sneaky failures that can cascade through your system or take down multiple components at once. Unlike simple random failures that we can calculate with traditional probability, dependent failures are trickier beasts.
DFA helps you identify two main types of troublemakers:
- Cascading Failures (CF): When one component fails and triggers a domino effect through the system. It’s like when your phone overheats, causing the battery management system to fail, which then corrupts your data storage.
- Common Cause Failures (CCF): When a single event knocks out multiple components simultaneously. Imagine a power surge taking down both your primary and backup systems at once.
The real value of DFA is that it forces you to think about these connections early in your design process, not as an afterthought when changes become exponentially more expensive.
Safety Measures: Your Practical Defense Toolkit
Once DFA identifies potential risks, Safety Measures are your actual tools to address them. Here are the four major strategies that have saved my bacon more times than I can count:
1. Freedom from Interference (FFI)
This is all about keeping your safety-critical functions isolated from less critical ones. When you’ve got ASIL D functions sharing hardware with QM (non-safety) functions, you absolutely need to prevent interference.
In practice, I’ve implemented FFI through:
- Memory partitioning: Using hardware memory protection units in processors like the Infineon AURIX to create strict boundaries between safety and non-safety code
- Time slicing: Guaranteeing execution time for safety-critical tasks through AUTOSAR OS scheduling
- Communication filters: Creating strict rules about what data can pass between different safety levels
A real-world example: On a recent project using a TI TDA2x SoC, we used hardware memory partitions to ensure our ASIL D lane-keeping function couldn’t be corrupted by our QM infotainment system, even though they shared the same chip.
2. Independence
When you need redundancy (especially after ASIL decomposition), independence ensures your backup plans aren’t compromised by the same factors that might take down your primary systems.
Some practical approaches I’ve used:
- Physical separation: For a steering system, we used completely separate power supplies and PCBs for each redundant channel
- Different chip vendors: For a braking system, we used both an STM32 and an NXP S32K MCU to implement the same function with different hardware
Yes, this adds cost and complexity – but it’s worth it for true safety redundancy.
3. Common Cause Failure Prevention
This is my favorite area to really think creatively. How do we prevent a single root cause from taking down multiple safety channels?
Some techniques that have worked well in my experience:
- Diverse implementation: Using different algorithms to achieve the same function (one channel using a Kalman filter, another using a simpler moving average)
- Supply diversity: Sourcing critical components from different vendors or manufacturing batches
- Environmental hardening: Implementing different EMI/EMC protection strategies for each channel
On my last project, we used both MEMS and Hall-effect sensors for position detection – completely different technologies measuring the same parameter, making them highly resistant to common failures.
4. Diagnostics & Monitoring
This is your early warning system. Good diagnostics can detect problems before they become dangerous.
The most effective approaches I’ve implemented include:
- Self-tests: Running memory and CPU tests during startup and periodically during runtime
- Lockstep processing: Using dual-core lockstep in processors like the Infineon TC3xx series to catch computational errors
- Plausibility checks: Cross-verifying sensor inputs against expected physical behaviors
- Window watchdogs: Monitoring not just that software is running, but that it’s running on schedule
A quick example: For a throttle control system, we implemented signal monitoring that checked not just the pedal position sensor value, but also its rate of change and correlation with other vehicle parameters.
Bringing It All Together
The key insight I’ve gained after years of implementing these concepts is that DFA and Safety Measures aren’t just compliance exercises – they’re fundamental design principles that make your systems more robust.
Here’s my practical advice for making this work in your own projects:
- Start early: Run your first DFA during architecture definition, not as a verification step
- Get specific: Identify actual physical failure mechanisms in your system, not just abstract concepts
- Test your assumptions: Verify your safety measures with fault injection testing
- Document clearly: Make the connections between identified risks and implemented measures explicit
The Road Ahead
As our vehicles get smarter and more connected, the complexity of dependent failures will only increase. Systematic approaches to DFA and Safety Measures aren’t optional extras – they’re the foundation of building systems that people can trust with their lives.
What’s your experience with implementing ISO 26262 safety measures? I’d love to hear your practical insights in the comments below!
If you are interested in other articles about ISO 2626 Series, please refer to the links below!
[ISO 26262] #1. Part4-6 Technical Safety Concept (TSC)
[ISO 26262] #2. Safety Mechanisms for Electrical and Electronic
[ISO 26262] #3. Safety Mechanism for Processing Unit
[ISO 26262] #4. Safety Mechanisms for IO units and Interfaces
[ISO 26262] #5. Safety Mechanisms for Communication Bus
[ISO 26262] #6. Safety Mechanisms for Power Supply
[ISO 26262] #7. Safety Mechanisms for Temporal monitoring and logical programme sequence monitoring
[ISO 26262] #8. Safety Mechanism for Sensors and Actuator
[ISO 26262] #10. Fault Tree Analysis (FTA)
[ISO 26262] #11. DFA, Dependent Failure Analysis
[ISO26262] #12. Understanding ASIL A Assignment Through Freedom From Interference (FFI) in ISO 26262
[ISO26262] #13. ISO 26262 ASIL in Practice: Key Challenges and Innovative Strategies
[ISO26262] #14. DFA Core: Mastering Dependent Failure Initiator Analysis
[ISO26262] #15. The Hidden Key to ISO 26262 DFA: Everything About Coupling Factors
From this post onwards, I will be adding podcasts created via Google Notebook lm.