Hardware architectural metrics

Various dependability metrics are of course safety relevant too. These include failure rate and other hardware architecture metrics. 

For the electronic we often find very tight intersections, particularly because the evaluation of the hardware architectural metrics (ISO 26262, part 5, chapter 8) as well as the evaluation of safety goal violations due to random hardware failures (ISO 26262, part 5, chapter 9) are based on the failure probability of electric components or occurrence probability of random hardware errors. Section 4.4.2.S covers such quantitative safety analyses in detail. Often, chapter 7 part 5 of ISO 26262 is overlooked, which covers the correct dimensioning and verification of... 

The objective of this clause is to evaluate the hardware architecture of the item against the requirements for fault handling as represented by the hardware architectural metrics. 

This clause describes two hardware architectural metrics for the evaluation of the effectiveness of the architecture of the item to cope with random hardware failures. 

The hardware architectural metrics can be applied iteratively during the hardware architectural design and the hardware detailed design... 

The hardware architectural metrics are dependent upon the whole hardware of the item. Compliance with the target figures prescribed for the hardware architectural metrics is achieved for each safety goal in which the item is involved. 

These hardware architectural metrics are defined to achieve the following objectives ... 

NOTE I If an item contains different kinds of hardware elements with significantly different failure rate levels, the risk exists that compliance with the hardware architectural metrics only focus on the kind of hardware elements with the highest magnitude of failure rates. (One example where this can... 

These architecture metrics are based on reliability data of elements and bring them in relation to the implemented control mechanism, which are the implemented safety mechanism. Random hardware failures of the electronic components are used as a basis for the data. 

The note 2 refers already to the problem with random hardware faults and systematic failures. The cause of an error in a system is never only related to random hardware faults, the cause for a random hardware fault is often already a systematic fault such as wrong selection of the part, wrong estimation of environmental impacts, production errors etc. That means all quantitative methods rely on systematic analysis, where the quantification could only consider as an indication or as a metric for comparison or balancing of the architecture or design. In other standards those approaches are considered also as semi-quantitative analysis. Furthermore, the question is if these methods are only the kind of representation of the result of the analysis rather than the indication for the analysis itself. 

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