How voltage references and supervisors help achieve ASIL functional safety goals


Many safety related automotive systems are required to meet Automotive Safety Integrity Level (ASIL) as defined by International Organization for Standardization (ISO) 26262.

It is a common misconception that integrated circuits (ICs) not developed following the ISO 26262 standards cannot be used to achieve functional safety goals. Many automotive OEMs have been able to use the features and reliability of non-ASIL compliant semiconductor devices to develop systems that target ASIL requirements. In this post, it will be demonstrated how both voltage references and supervisors can help you achieve ASIL compliance for your automotive systems.

Voltage references and supervisors

Devices such as voltage references and supervisors (reset ICs) are common semiconductor devices that can help automotive system integrators develop functionally safe systems. When used in automotive applications, these devices provide diagnostic coverage or redundant monitoring capability.

Figure 1 is taken from ISO26262-10:2018, 9.2.3.4 and is an example of how safety elements out of context (SEooC) can implement voltage supervisors and watchdogs as safety mechanisms.

Figure 1: System-level design assumptions for SEooC based on ISO 26262

 

Features and mechanisms of voltage reference and supervisors

A voltage supervisor can help achieve system-level functional safety targets by providing power supply fault detection. A voltage supervisor implements a safety mechanism to the microcontroller (MCU) when an overvoltage or undervoltage failure mode is detected on the power supply. Some voltage supervisors can also provide digital diagnostics with watchdog timers that can detect clocking failures of an MCU. Clocking failures include late pulses or early pulses sent from the MCU. The window watchdog timer can monitor these pulses and alert the system that a fault has occurred. Another method of under and overvoltage monitoring is to use an analog-to-digital converter (ADC) with a precision voltage reference to monitor multiple voltage rails. Figure 2 shows how a window watchdog timer operates. In some cases, systems with very high diagnostic coverage goals may require redundant safety mechanisms in order to achieve system-level functional safety goals. This means that in addition to an ADC and voltage reference to monitor potential voltage supply failures, a supervisor is also required to monitor the same voltage rails to ensure safety and diagnostic coverage.

Figure 2: Window watchdog timing diagram

Device functional safety collateral

Risk assessments of automotive systems show that faults can occur due to IC failures; therefore evaluations at the device level are required in some functionally safe systems. TI can provide device information needed for evaluating the IC versus the requirements of the functional safety system concept. TI can provide device collateral such as qualification reports, failure in time (FS-FIT), failure mode distributions (FMD), and design failure mode and effect analysis (DFMEA) for voltage references and supervisors.

Automotive reference designs with functional safety considerations

The “ADAS power reference design with improved voltage supervision” shows how voltage references and supervisors can help in implementing functionally safe systems. The voltage reference and supervisors used in this reference design can help enable the designers achieve the system-level functional safety goals when combing the devices’ functionality, features and device collateral.

The reference design provides an automotive power solution with additional voltage supervision and a window watchdog for safety MCUs in advanced driver assistance systems (ADAS). The design helps achieve accurate voltage monitoring with precision supervision of 1% maximum across temperature and includes features such as flexible reset delay and manual reset. The TPS3703-Q1 provides overvoltage and undervoltage monitoring in a small footprint, with minimal needs for external components to help solve space constrained problems.

Figure 3 describes how the TPS3703-Q1 detects overvoltage and undervoltage. For potential clocking failures, the TPS3850-Q1 doubles as an overvoltage/under-voltage monitor and window watchdog timer which is illustrated in Figure 2 and Figure 3. It also has the flexibility of changing the watchdog timeout and window ratio and disabling the watchdog timer. In cases where only undervoltage monitoring is necessary, the TPS3890-Q1 can provide accurate voltage monitoring at a very low quiescent current to save system power consumption. Last but not least, the LM4132-Q1 provides precision voltage to reference the ADC for voltage monitoring. With 0.05% initial accuracy and low temperature drifts of 10 ppm/°C, the LM4132-Q1 solves accurate voltage monitoring at a low supply current cost of 60 µA.

Figure 3: Under-voltage and over-voltage window detector timing diagram

Accommodating the ISO 26262 standard in the ADAS power reference design

The reference design takes ISO 26262 and its guidance on power-supply voltage monitoring and watchdog diagnostics into consideration. Figures 4 explain the need for detecting failures in the power supply and failures in a defective program sequence. Figure 4 is taken from ISO26262-5:2018, Annex D. This annex is intended to evaluate diagnostic coverage and is used as a guideline to choose appropriate safety mechanisms to detect possible system failures. The reference design can help in implementing system-level safety mechanisms shown in Figure 4.

Figure 4: Safety mechanism examples for power-supply and watchdog failures based on ISO 26262

The voltage supervisors and references used in this reference design can provide an additional layer of safety by providing extra diagnostic coverage, safety mechanisms or redundant safety monitoring. The product’s performance and functionality of detecting faults can help achieve functional safety goals in automotive systems. Additionally, TI can provide collateral to improve time-to-market for system integrators.

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