Functional Safety for Integrated Circuits

July 18, 2018 // By Tom Meany, Analog Devices, Inc.
Integrated circuits supply the logic and either control the sensors or, to a growing extent, are the sensors. Integrated circuits drive the final elements to achieve a safe state and they are the platform on which the software runs. The level of integration possible within semiconductors can simplify the system-level implementation at the cost of the added complexity within the IC itself. Surprisingly, while there are functional safety standards that address process control, machinery, elevators, variable speed drives, and toxic gas sensors, there is no functional safety standard dedicated to integrated circuits. Instead, bits and pieces of the requirements and knowledge are spread around IEC 61508 and other Level B and C standards. This article gives guidance on interpreting the existing functional safety standards for semiconductors.

Introduction

Typically, integrated circuits are developed to either IEC 61508 or ISO 26262. In addition, there are sometimes additional requirements in the level two and level three standards. Developing and assessment to the functional safety standards are what give the confidence that these sometimes complex integrated circuits are sufficiently safe. When IEC 61508 was written it was targeted at bespoke systems, as opposed to open market mass produced integrated circuits. This article will review and comment on the known functional safety requirements for integrated circuits. While the article concentrates on IEC 61508 and its application in industrial sectors, much of the material is relevant to applications such as automotive, avionics, and medical.

Functional Safety

Functional safety is the part of safety that deals with confidence that a system will carry out its safety related task when required to do so. Functional safety is different from other passive forms of safety such as electrical safety, mechanical safety, or intrinsic safety.

Functional safety is an active form of safety; for example, it gives confidence that a motor will shut down quickly enough to prevent harm to an operator who opens a guard door or that a robot will operate at a reduced speed and force when a human is nearby.

Standards

The key functional safety standard is IEC 61508.1 The first revision of this standard was published in 1998 with revision two published in 2010 and work beginning in 2017 to update to revision three with a probable completion date of 2022. Since the first edition of IEC 61508 was published in 1998, the basic IEC 61508 standard has been adapted to suit fields such as automotive (ISO 26262), process control (IEC 61511), PLC (IEC 61131-6), IEC 62061(machinery), variable speed drives (IEC 61800-5-2), and many other areas. These other standards help interpret the very broad scope of IEC 61508 for these more limited fields.

Some functional safety standards such as ISO 13849 and D0-178/D0-254 have not been derived from IEC 61508. Nevertheless, anybody familiar with IEC 61508 and reading these standards would not be too surprised by the contents.

Within a safety system, it is the safety functions that perform the key functional safety activities when the system is running. A safety function defines an operation that must be carried out to achieve or maintain safety. A typical safety function contains an input subsystem, a logic subsystem, and an output subsystem. Typically, this means that a potentially unsafe state is sensed, and something makes a decision on the sensed values and, if deemed potentially hazardous, instructs an output subsystem to take the system to a defined safe state.

 


Figure 1. A sample of functional safety standards.

The time between the unsafe state existing to achieving a safe state is critical. A safety function might, for instance, consist of a sensor to detect that a guard on a machine is open, a PLC to process the data, and a variable speed drive with a safe torque off input that kills a motor before a hand inserted in a machine can reach the moving parts.

Safety Integrity Levels

SIL stands for safety integrity level and is a means to express the required risk reduction needed to reduce the risk to an acceptable level. According to IEC 61508, the safety levels are 1, 2, 3, and 4, with an order of magnitude increase in safety as you go from one level to the next. SIL 4 is not seen in machinery and factory automation where generally no more than one person is typically exposed to a hazard.

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