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How the mobile phone (that is off, but had been just plugged into USB) informs the Windows PC (with installed drivers "MediaTek PreLoader USB VCOM port" and "MediaTek USB Port") how to identify itself: as device - "MediaTek PreLoader USB VCOM port" or as - "MediaTek USB Port"?

I don’t see any pattern - when I plug the phone to USB Windows PC handle it sometime through "preloader USB", sometimes through "just USB" ports, sometimes both in sequence (I see it from WMI events).

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  • A mobile phone that is off won't do anything at all. Please edit and clarify your question.
    – DavidPostill ♦
    Commented Dec 7, 2018 at 21:27

1 Answer 1

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When you plug in a MediaTek phone, it will turn on, but not fully on. The boot ROM will run, which will run the preloader, and depending on implementation, either show the charging animation or starts the Linux kernel and enters into a mode called kernel power-off charging (KPOC) that shows the charging animation. The devices you're seeing are associated with the first two stages. Both devices are used for downloading firmware to the phone, but they represent different stages and support different command sets. MediaTek USB Port is created by the boot ROM, and MediaTek PreLoader USB VCOM port is created by the preloader (it runs after the boot ROM and does further hardware initialization).

Usually, only the preloader device would show up upon plugging the phone in, but it seems in your case the phone might be configured to enter the boot ROM's download mode, and if it doesn't see a connection, to continue into the preloader, which will offer another download mode. It's also possible that there is some sort of issue with the preloader that causes the phone to reset back to boot ROM to start an emergency download mode.

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  • brilliant, thank you. btw how you distinct those two "off" states - when phone is compleatly off and when it is off but ready to run the preloader? DavidPostill have asked me to improve the question. Do we have exact naming for those two states?
    – Roman Pokrovskij
    Commented Jun 17, 2019 at 19:42
  • I'd say unplugged is completely off and plugged in is charging. When charging the phone is on to display the charging animation, but otherwise does not function fully.
    – cyanic
    Commented Jun 18, 2019 at 6:33

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