Shutdown on MX8MP

  • Hi,

    I have another question regarding the MX8MP device.
    After testing shutdown/poweroff commands with the supplied image with all possible additional parameters it was found that our device always reboots.
    In the hardware documentation I was not able to locate any kind of GPIO that could be set to a different level to prevent this behaviour, apparently this is required.
    Could you please point me in the correct direction to implement a shutdown procedure that leaves the device in poweroff state until reboot is requested by hardware reset button or powercycle?

    Thank you once again

  • Thanks for the information. We have also seen this strange behavior ourselves not long ago. We then analyzed this problem and found a solution.

    This is actually a regression that NXP introduced in a recent modification of the ARM Trusted Firmware (ATF). In the shutdown function, which is part of ATF, NXP unconditionally enables the alarm clock before shutting down the power. This is meant for cases, where the system should start up at a specific time, i.e. the alarm time.

    Activating the alarm clock in such a way obviously assumes that the chip-internal SNVS Real Time Clock (RTC) is used and the alarm is configured correctly. However on F&S boards and modules, there is an external RTC chip and the internal SNVS RTC is not used at all. Therefore the alarm is also not initialized. And right from the start, there always seems to be a pending alarm latched in a register. We assume that this happens when the system is powered on and the internal RTC time value as well as the alarm time value are both reset to zero. In this one second, before the RTC begins to count, there is a match between time value and alarm value, causing the alarm pending bit to be set.

    When the regular SNVS RTC driver initialized the internal RTC including the alarm value, it will also clear this pending bit. But as this driver is not started on F&S boards, this does not happen. So when the system is switched off, the alarm pending bit from the beginning is still active and immediately wakes up the system again. This is the behavior that you experience. Even after a reboot, this pending flag remains set, because it is part of the SNVS-powered domain that keeps its contents also across reboots. So the alarm is triggerd each and every time the system is shut down.

    In the meantime, we have improved the shutdown procedure in ATF, so that the power-off sequence should work again. On fsimx8mp, ATF is actually part of our NBoot, so you basically need a newer NBoot. There will be a new fsimx8mp release in a few days. At that time, we will also upload a new NBoot version that should fix this issue.

    Please note that the armStoneMX8MP is not designed for a real standby mode because the ONOFF-pin of the CPU is not connected to a button. This signal would be needed to switch on the device again after a poweroff command. Also pressing Reset will not work. So you really need a full power cycle of the external +5V supply to restart the board.

    Your F&S Support Team

    F&S Elektronik Systeme GmbH
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