# AURIX™ MCU: ISO 26262, fault classification of a HW element and associated HW architecture metrics definition – KBA236342

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Fault classification (according to ISO26262-5:2018):

Failure modes are classified based on the potential to violate a safety goal.

Figure 1   Example of flow diagram for failure mode classification

Among these five failure rate categories, three have a negative contribution as they correspond to the portion of a failure mode of a hardware element that may violate a safety goal in the absence of a safety mechanism:

Table 1  Fault classification

 Faults Description Single-point faults Faults with potential to violate safety goal (PVSG) and they are not covered by any safety mechanism. Residual fault Portion of a random hardware fault that by itself leads to the violation of a safety goal and is not controlled by the safety mechanism. Multiple-point fault, Latent Multiple-point faults are not detected by any safety mechanism nor perceived by the driver.

The two metrics are as follows:

1.Single-point fault metric (SPFM)

According to ISO 26262-5:2018,

The SPFM requirement applies to ASIL B, C, and D of the safety goal. Use the calculation in Figure 2 to determine the single-point fault metric.

Figure 2   SPFM calculation

In other words,

• (SPF + RF) FIT (Failure In Time) < 1% of the total faiure rate (ASIL D).
• Quality Managed (QM) parts are excluded from the calculation.
• FIT is calculated based on the area [mm^2] of all involved HW elements (see Figure 3).

Figure 3    Failure rate contribution to SPFM

Table 2  Possible source for the derivation of the target SPFM value

 ASIL B ASIL C ASIL D SPFM ≥90% ≥97% ≥99%

2.
Latent-fault metric (LFM)

According to ISO 26262-5:2018,

The LFM requirement applies to ASIL B, C, and D of the safety goal. Use the calculation in Figure 4 to determine the latent-fault metric:

Figure 4  LFM calculation

In other words,

• MPF, L FIT < 10% (MPF, DP + Safe + MPF, L) [ASIL D].
• QM parts are excluded from the calculation (see NOTE 1 in Figure 4).
• FIT is calculated based on the area [mm^2] of all involved HW elements (see Figure 5).

Where,

• MPF, L is Multiple point faults latent

Figure 5   Failure rate contribution to LFM

Table 3  Possible source for the derivation of the LFM target value

 ASIL B ASIL C ASIL D LFM ≥60% ≥80% ≥90%

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