To achieve a system functional safety level of ASIL-B, the following PDN features are available:

• PMIC over voltage and under voltage monitoring on the power resource voltage outputs
• Watchdog monitoring of safety processor
• MCU error monitoring
• MCU reset
• I²C communication
• Error indicator, EN_DRV, for driving external circuitry (optional)
• Read-back of EN_DRV pin

The PDN has an in-line, external power FET, as shown in Figure 3-1, between the input supply and PMICs. The voltage before and after the FET is monitored by the PMIC, and the PMIC controls the FET through the OVPGDRV pin. The FET can quickly isolate the PMICs when an over-voltage event greater than 6 V is detected on the input supply to protect the system from being damaged. This includes all power rails sourced from the FET. Any power connected upstream from the FET is not protected from over voltage events. In Figure 3-1 the load switches that supply power to the MCU and Main I/O domains, the discrete buck supplying the DDR, and the discrete LDO supplying EFUSE are all connected after the FET to extend the over voltage protection to these processor domains and discrete power resources.

The PMIC internal over voltage and under voltage monitoring and their respective monitoring threshold levels are enabled by default and can be updated through I²C after startup. PMIC power rails connected directly to the processor are monitored by default. The unused feedback pin of BUCK3 on TPS65941213-Q1, FB_B3, is assigned to monitor the load switch output voltage that supplies the MCU I/O of the processor, VDD_MCUIO_3V3_LS. The POK monitor built into the VDDSHV0_MCU voltage domain can also be used, but it is still required to connect a 3.3V supply to the feedback pin in order to prevent an error since the PMIC is expecting the 3.3V to be present. For monitoring the load switch voltage that supplies the Main I/O, an unused feedback pin of the TPS65941111- Q1 (FB_B3 or FB_B4) can be configured through I2C and connected to the output of the load switch to enable monitoring. An example of enabling an unused monitor is provided in Section 7.4.

The internal Q&A Watchdog is enabled on the primary TPS6594-Q1 device. Once the device is in ACTIVE state, the trigger or Q&A watchdog settings can be configured through the secondary I2C in the device. The primary and secondary I2C CRC is not enabled by default but must be enabled with the I2C_2 trigger described in section Table-6-1. Once enabled the secondary I2C is disabled for 2ms. It is recommended to enable I2C CRC and wait a minimum of 2ms before starting the Q&A Watchdog. The steps for configuring and starting the watchdog can be found in the TPS6594-Q1 datasheet. Setting the DISABLE_WDOG signal high on primary TPS6594-Q1 GPIO_8 disables the watchdog timer if this feature needs to be suspended during initial development or is not required in the system. An example of re-purposing GPIO_8 is provided in Section 7.4.

GPIO_7 of the primary TPS6594-Q1 PMIC is configured as the MCU error signal monitor, and must be enabled though the ESM_MCU_EN register bit. MCU reset is supported through the connection between the primary PMIC nRSTOUT pin and the MCU_PORz of the processor. Lastly, there are two I2C ports between the TPS6594-Q1 and the processor. The first is used for all non-watchdog communication, such as voltage level control, and the second allows the watchdog monitoring to be on an independent communication channel.

There is an option to use the EN_DRV of the primary TPS6594-Q1 PMIC to indicate an error has been detected and the system is entering SAFE state. This signal can be utilized if the system has external circuitry that needs to be driven by an error event. In this PDN, the EN_DRV is not utilized, but available if needed.