What is Freedom From Interference (FFI)?

Why?

Functional safety revolves around ensuring that a system can detect, react to, and manage potential failures without compromising safety. Without FFI, a system’s vulnerability increases significantly, especially as the system grows more complex.
Preventing Catastrophic Failures: Without FFI, errors in one system could interfere with critical operations in another. For example, a malfunction in a vehicle’s GPS module should never cause a failure in the steering or braking system. Ensuring FFI minimizes the risk of catastrophic system-wide failures.
Protecting Safety-Critical Functions: In environments where lives are at stake—such as in autonomous driving or medical devices—ensuring the continuous, reliable operation of safety-critical systems is essential. FFI ensures these critical functions operate independently, without disruption.
Compliance with Safety Standards: Functional safety standards like ISO 26262 (for automotive) and IEC 61508 (general functional safety) emphasize the need for FFI. Achieving FFI helps organizations comply with these stringent regulations, ensuring their systems meet safety benchmarks.

What?

Freedom From Interference (FFI) refers to the complete separation of critical safety functions from non-critical ones. This means that any failure or malfunction in a non-essential system—whether it’s hardware or software—should not affect the functioning of safety-critical function.
For example, in a car, your entertainment system and your braking system serve different purposes. While the entertainment system is important for user experience, it should not, under any circumstance, interfere with the braking mechanism.
According to ISO26262 FFI is “absence of cascading failures between two or more elements that could lead to the violation of a safety requirement”

Cascading Failures: 

Cascading Failures occur when the failure of one element within a system leads to the failure of another element, or even multiple elements, within the same system. These failures are interdependent but are not caused by a single common event or root cause. A failure in one part triggers a chain reaction, causing subsequent elements to fail, creating a cascading effect.

The figure below illustrates that failure in element 2 does not cause element 1 to fail, hence achieving Freedom From Interference between element 1 and element 2

The figure below illustrates that failure in element 3 does cause element 4 to fail, there is no Freedom From Interference between element 3 and element 4

 
In simple words, a failure in less safety critical elements shall not cause any failure in more safety critical element

How to Achieve FFI in Functional Safety Systems?

There are multiple methods to do this, one of which is a popular tool Dependent Failure Analysis (DFA)
Dependent Failure Analysis is the systematic examination of how failures can propagate across different systems, subsystems, or components that are intended to function independently. In theory, these elements should operate without influencing each other, but in practice, external factors, environmental conditions, or design flaws can create unwanted dependencies.
In simpler terms, DFA assesses situations where a failure in one part of a system may unintentionally cause another supposedly independent part to fail. Such failures can be devastating in safety-critical systems, making DFA a vital tool in functional safety engineering.
DFA also reveals where systems may interfere with each other, even if they share resources. This allows for better design and mitigation strategies.

Partitioning in Hardware and Software: One of the most direct ways to achieve FFI is by partitioning critical and non-critical components. This partitioning can occur at both the hardware and software levels

Hardware Partitioning: You can dedicate separate processing units, memory, or communication interfaces to safety-critical functions, thereby physically isolating them from non-critical tasks.
Software Partitioning: Critical software functions can run in isolated memory spaces, utilizing robust mechanisms like memory protection or containerization to prevent unintended interactions with non-critical applications.

Conclusion

Achieving Freedom From Interference (FFI) is crucial in ensuring that safety-critical systems remain protected, reliable, and functional, even in the event of failures or malfunctions elsewhere in the system.
By implementing FFI through methods like partitioning, time separation, and redundancy, engineers can build robust, safety-focused systems that continue to perform under pressure.