Apalis iMX8

Customer Impact: When powering up/booting the SoM at ambient temperatures of -30C and below, the Wi-Fi/Bluetooth module may fail to start and function properly.

Description: Some Apalis iMX8 product configurations feature a Wi-Fi/Bluetooth module. The module featured on the System-on-Module (SoM) is limiting the effective operating temperature range of the product to -30C...85C. When powering up/booting the SoM at ambient temperatures of -30C and below, the Wi-Fi/Bluetooth module may fail to start and function properly. As the Wi-Fi/Bluetooth functionality is not considered boot-critical, the SoM may still be used throughout the full specified temperature range (-40C...85C). When powering up/booting the SoM at ambient temperatures of -30C and below, the warming up the Wi-Fi/Bluetooth module (by the heat dissipated by the other components) may eventually result in the Wi-Fi/Bluetooth module starting and functioning properly.

Workaround: Power up/boot the SoM at ambient temperatures of -30C or above. If this is not feasible, monitor the status of the Wi-Fi/Bluetooth module in software after powering up/booting the SoM, and reset the SoM in case the Wi-Fi/Bluetooth module failed to start/initialize properly. The reset could be initiated both in software and hardware. Repeat this procedure until you have a functional Wi-Fi/Bluetooth module.

Customer Impact: After a software-initiated reset cycle (software reboot), the SGTL5000 audio codec may not work anymore.

Description: The headphone output signals of the SGTL5000 audio codec (SODIMM pin 316: AAP1_HP_L and SODIMM pin 318: AAP1_HP_R) have a DC offset of around 1.65V. Series capacitors must be placed between the SoM edge connector pins and the headphones to block this offset voltage. The Toradex reference design uses values between 47µF and 100µF for this purpose. With headphones plugged in, these series capacitors get charged with 1.65V. In a software-initiated reset cycle (software reboot), all power rails of the SoM are turned off. After a delay of 5ms, the rails get re-enabled, and the SoM starts booting. The SGTL5000 can only be reset by power cycling it, as the codec does not have a dedicated reset input. If headphones are plugged in during a software reboot, the voltage at the series capacitors is backfeeding to the power rails of the audio codec. The series capacitors are not fully discharged since the power rails are turned off only for around 5ms. The voltage at the 3.3V analog rail of the audio codec remains at about 1.5V. The residual voltage can cause the audio codec to not reset properly. This reset issue can prevent the audio codec from functioning properly. Power cycling the SoM resolves the problem. The issue only occurs during software reboots. Regular power cycles are not affected. The issue mainly appears when low-impedance headphones are used. Occurrences are less frequent with high-impedance headphones (devices with smaller drivers). When using the headphone output as a high-impedance line output signal, the resulting backfeeding current is low enough to not cause an issue. Without headphones plugged in, the circuit is open, and therefore no backfeeding occurs. This means, that there is no residual voltage, and consequently, the software reset works properly.

Workaround: A locked-up audio codec can be recovered by power cycling the SoM. Using high-impedance headphones instead of low-impedance ones reduces the backfeeding current (and therefore the risk of having this issue manifest itself). Using the headphone output as a line-out signal with an external amplifier further helps to reduce the backfeeding. In case only high-impedance headphones are used (or the interface is used as a line out signal), the series capacitors' values can be reduced without sacrificing the signal quality at low frequencies. Series capacitors with lower resistances discharge faster and reduce the residual voltage during a software reboot. If the audio codec needs to be able to reliably reset when used in combination with low-impedance headphones, an additional discharge circuit can be added to the carrier board. The idea behind this circuit is to discharge the series capacitors while the RESET_MOCI# signal is low. A resistor value of 47R is a good choice for series capacitors with values between 47µF and 100µF. The example circuit can be found in the related errata document.

Customer Impact: In case of some of the GPIO-capable pins, the reset states are different from the ones documented in the previous versions of the module datasheet. The USBO1_EN and the USBH_EN signals are high during the reset state. Therefore, the USB power rails can get unintentionally enabled. Depending on the carrier board, other affected pins may have an impact on the behavior of the carrier board. It is advised to carefully check the updated reset state documentation in the module datasheet.

Description: The pin configuration registers of the i.MX8 SoC have reset states. These reset states have been stated in the Toradex Apalis iMX8 datasheet (document revision 1.1 and earlier) as reset states in section 4.4. However, the i.MX 8 SoC features a System Controller Unit (SCU) that handles all of the pin configurations. The SCU comes with a firmware (SCFW) that has its own default pin configurations. By the time of the release of the RESET_MOCI# signal, the SCU already overrides the pin configurations. Therefore, the SCU default settings are the relevant reset states for the pins. In case of some of the GPIO-capable pins, the SCFW reset states are different from the register reset states. A comparison list can be found in this article: https://developer.toradex.com/knowledge-base/reset-state-of-the-imx8-soc-pins. The Apalis iMX8 datasheet (document revision 1.2 and newer) has been updated with the reset values dictated by the SCFW. Despite the original intention, the USBO1_EN and the USBH_EN have pull-up resistors enabled instead of being pulled down during reset. This means that the USB power rails can get unintentionally enabled in the reset state of the module. Depending on the carrier board, the different reset states of the SoC pins can result in unintended behavior. For example, on the Ixora carrier board V1.2A, the MMC1_D5 (pin 152) and MMC1_D6 (pin 156) are used for driving LEDs. By default, these two pins have pull-up resistors enabled instead of the pull-down. Therefore, the LEDs are lit during the reset cycle.

Workaround: The values of the internal pull-up and pull-down resistors of the SoC are between 15kΩ and 50kΩ. These pull resistors can be disabled in the bootloader or during kernel boot. The issue caused by the unintentional pull-resistor states persists in time between the release of the reset signal and the bootloader configuring the IO pins in the intended state. In that case, an external pull-resistor on the carrier board can resolve the issue. According to measurements, adding a 10k pull-down resistor to a GPIO-capable pin with an internal pull-up resistor enabled results in a voltage level of around 0.6V. With an external 1k pull-down, the level goes below 100mV. However, these are just typical values at room temperature. For the USBO1_EN and the USBH_EN signals, it is recommended to add a 10k pull-down resistor (or replacing the existing weak pull-down resistor). This keeps the power enable signal level below the maximum input low threshold of the USB power switch IC during the reset cycle.

Description: The HiFi 4 DSP feature will be no longer supported on the future versions of this product. Please don't use this feature on any current product version which theoretically would support it.

Workaround: None

Description: If the RTC battery is present, it is not possible to turn on the module after shutting it down the PMIC in software.

Workaround: Option 1: Do not use the software initiated PMIC shut down. It is possible to shut down the software for making sure the system is in a safe state for removing the power. But the command to the PMIC for shutting down should not be send. Leave the PMIC running and just remove the main input voltage from the module (remove VCC). Option 2: Assemble C267 for making the power button signal pulse long enough. The capacitor C267 is not assembled on Apalis iMX8 modules with the version 1.1A or older. By assembling C267 with a 10µF 6.3V capacitor with the 0603 size. The power button signal pulse gets its required duration. This means the module can be turned on either by power cycling VCC or by pressing the RESET_MICO# button.