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This sounds so weird that I can't believe I am asking about this.

I have two exactly the same Redmi 11S (same in terms of purchase date and software installed). One of them has fallen a couple of times and has about 25% of the screen broken, though still fully usable. The second one has screen intact.

Each time I connect an USB-C cable in data transfer mode, the phone with broken screen gets wako:

  1. I cannot start photo app from the lock screen, I cannot draw an unlock patters, I cannot even unlock it, screen is totally unresponsive.

  2. When I unlock the screen before attaching an USB-C cable and only then I attach it, then screen gets wako; "something" is randomly changing launcher pages and touching icons, the phone lives on it's own.

When I connect the very same cable in charge mode (i.e. attach it to the wall socket charger instead to the PC) or connect the second Redmi 11S (using the same cable), the phone operates normally.

Tested on both Redmi 11S, on two different cables and three different USB ports on the same PC.

What am I missing. How can a broken screen and USB cable influence each other?

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    Thank you for all your time and very impressive research. Though this is a guessing game, I still would suggest that you turn these comments into answer. Thank you again.
    – trejder
    Commented Sep 22, 2023 at 10:10

1 Answer 1

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Note: This answer is an educated guess based on my understanding from Guide to USB-C Pinout and Features and anyone with a better understanding is welcome to improve

From a historical perspective USB standards evolution is as depicted

enter image description here

Since USB Type-C is the de-facto standard, the answer is based on this standard. Figure below shows the configuration of USB-C receptacle (plug is similar with minor changes)

enter image description here

  • OP says that both devices charge fine with a wall socket charger. VBUS(A9) and GND(A12) are responsible for this charging so it's working fine.

Without getting into technical details, what is of relevance here is Alternate Modes

This mode of operation allows us to implement third-party protocols, such as DisplayPort and HDMI, using the USB Type-C standard. All Alternate Modes must at least support a USB 2.0 and USB Power Delivery connection

DisplayPort & HDMI are highlighted because the problems faced by OP are around this. especially with the broken screen. If we analyze this further:

  • The document linked talks more in detail about Alternate Modes. They can be unidirectional (One side deciding the source or host) or bi-directional. But when side is connected to PC, PCs always run in host mode.

  • With the PC in host mode and undamaged device connected , it works fine (implied in the question)

  • With the PC in host mode and damaged screen device connected the behavior is wonky and changing cables, ports doesn't make a difference.

  • Conclusion: The broken screen HDMI(maybe digitizer damaged too) is the culprit and needs to be repaired for it work normally. Unable to open lock screen or if the lock screen is open but exhibiting unpredictable behavior are at best symptoms of the underlying root cause.

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    Electronics SE could have provided a more technically valid answer but I am not sure if you can cross post there, given it's been answered here but I surely would like to hear them experts
    – beeshyams
    Commented Sep 22, 2023 at 17:09
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    After spending more time in figuring out the documentation, I am fairly sure of my conclusions (though not 100%sure) // In any case getting the broken screen fixed is worth it if plan to use it (no idea of cost comparison vs new device)
    – beeshyams
    Commented Sep 23, 2023 at 17:02

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