AN4266 datasheet(25/76 Pages) STMICROELECTRONICS

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AN4266

Functional safety requirements for application software

Doc ID 024283 Rev 2

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3.13.4

Write Digital Outputs

For ASIL D applications, digital outputs used for safety purposes are assumed to be written

either redundantly or with read back as described in the following section.

Note:

Application-dependent option: If a sufficient diagnostic coverage can be reached by a

plausibility check on a single output channel for a specific application, a plausibility check

can replace a redundant write or a direct read back.

The element safety function Write Digital Out is implemented as either:

●

Single Write Digital Out With Read Back

●

Double Write Digital Out

Single Write Digital Outputs With Read Back

The SIUL hardware element is used to perform a single Write Digital Output With Read

Back.

Mandatory: The read back must be implemented in one of the two modes shown in

Figure 4.

Note:

Rationale: To verify if written data compares with the expected data

Mandatory: The SIUL element must be correctly configured to provide the output write and

the pad directions as follows:

●

External read back – SIUL is configured to read back the signal from an additional pad,

and the loopback is performed outside the device. In this configuration, only half of the

available digital outputs are available as safety outputs.

●

Internal read back(e) – SIUL is configured to read back the pad value via an internal

read path. All pads dedicated to digital input/output are capable of reading the pad

digital status using the input logic.

Note:

Rationale: To verify if written data is coherent with the expected data

Mandatory: The application software must implement the software test to check the correct

configuration of the pads, SIUL_SWTEST_REGCRC, and to compare the read back with

the digital output write. GPOERB_SWTEST_CMP is used for external read back and

GPOIRB_SWTEST_CMP is used for internal read back.

Internal read back does not cover package faults (e.g., wire bond, etc.).

AN4266 Datasheet(PDF) 25 Page - STMicroelectronics