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Functional safety requirements for application software

AN4266

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Doc ID 024283 Rev 2

Note:

Rationale: To set the correct threshold for the self-tests

Note:

Implementation hint: Since user can not directly read the test sector an SSCM feature,

called Test Flash Enable, shall be exploited. This action is performed through the following

steps:

If code is executing in flash memory, it jumps to execute from RAM.

Write SSCM_SCTR[TFE] = 1.

Test sector is readable at the offset 0x0 of the flash memory address space (See

“System Status and Configuration Module (SSCM)” of the Reference Manual).

Thresholds are copied from the test sector to the respective register.

Write SSCM_SCTR[TFE] = 0.

Code can continue execution from the flash memory.

BAM implements an access method to read the test sector.

Mandatory: Since the BAM is not developed according to the safety standard, a safety

application is not allowed to read the test sector through the BAM access method.

Additionally, a watchdog timer is implemented to check the sequence of the self-test

algorithms.

Mandatory: The customer must enable the watchdog timer for CPU mode and CTU mode.

The programmable watchdog timeout is the FTTI(h).

Note:

Rationale: To check the sequence of the self-test algorithms

Every hardware BIST is activated via a dedicated command sent to the ADC. Refer to the

“Self-testing” section in the “ADC” chapter of the SPC56xL70xx Reference Manual to have

all detailed instructions for implementing one of these modes.

The supply self-test must be executed without interleaved user conversion.

Software tests

●

ADC_SWTEST_TEST1

This software BIST exploits the presampling feature of the ADC. Presampling allows to

precharge or discharge the ADC internal capacitor before it starts the sampling and

conversion phases of the analog input coming from pads. During presampling phase,

the ADC samples the internally generated voltage while in the sampling phase the ADC

samples analog input coming from pads (See Figure ).

Reference voltage which can be used during presampling phase is either

VDD_HV_ADR0/1 or VSS_HV_ADR0/1.

If there is an open failure in the analog multiplexing circuitry, the signal converted by

the ADC is not the analog input coming from the pad, but the presampling reference

voltage (VDD_HV_ADR0/1 or VSS_HV_ADR0/1).

This BIST must be run for each analog input used by the safety function.

Since the pads dedicated to analog inputs are of type INPUT, a missing enable from

the SIUL results in an open failure.

Note:

Rationale: To detect open failures of the channel multiplexing circuitry (See Figure 9)

This action is not mandatory in case of Double Read Analog Inputs.

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