The FMEDA and FTA, recommended by Part 5 of ISO 26262, are the most widely used quantitative safety analysis techniques in the automotive industry. In any quantitative analysis, “Diagnostic Coverage (DC)” of the safety mechanisms is a crucial parameter that affects the final safety metrics. So, what is the meaning of “Diagnostic Coverage,” and how do we use it in practice?

Diagnostic coverage is a measure of effectiveness of the diagnostics implemented in the system. Mathematically, it is the ratio of the failures detected and/or controlled by a Safety mechanism to the total failures) in the element. For example, if a given Cyclic Redundancy Check (CRC) can detect 1,022 out of a possible 1,024 potential errors in a transmitted message, then the diagnostic coverage would be 1,022 divided by 1,024, or 99.8%.

Determining diagnostic coverage in practice is not trivial. To simplify the process, ISO 26262 provides a “starting point” for estimating the DC values of a safety mechanism based on their applicability to a system. They are classified as Low (60%), Medium (90%) and High (99%) diagnostic coverage. The safety mechanisms are classified to these corresponding levels (low, medium, high) depending on factors varying from:

  • Variations in the source of the fault type detected by the diagnostic
  • Specific implementation of a safety mechanism
  • Technologies implemented in the system
  • The execution timing of the safety mechanism, etc.

The table shown above is a short summary for power supplies. Many additional elements, including semiconductor elements such as ALUs, digital and analog I/O, memory, and bus communications, are summarized in the same table in ISO 26262 Part 5 Annex D.

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  1. shashikanth comments on Determining Diagnostic Coverage

    I would like to know, From which document we refer DC coverage values While developing the FMEDA?

    • kVA by UL comments on Determining Diagnostic Coverage
      kVA by UL

      Diagnostic coverage can be estimated using ISO 26262 part 5 annex D. This Annex not only provides estimates of Diagnostic Coverage, but it also provides some practical guidance on how to implement the safety mechanisms.

      For semiconductors and digital designs, ISO 26262 part 11 also provides useful information.

  2. Ramu comments on Determining Diagnostic Coverage

    how are the levels of power spikes and oscillations amplitude and frequency defined?