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I would like to understand the consequences of pulling the TX+ pin of a USB-C 3 connector to 3V.

For context, a community of retro-gamers has taken to building controllers & similar items that work both on PC over USB, and on a console via a custom USB-C-to-console-connector cable. USB provides 5V, but the console provides 3.3V. Consequently, either must be able to power the board. Custom cables connect the 3.3V pin from the console connector to the TX+ pin (standardized accross the community), and the data line of the console to the RX+ pin (pulled up by the device).

Typically the MCU used accepts 3.3V, and a buck converter is used to convert the incoming 5V to 3.3V. The 3.3V won't cause 5V to flow back because the buck converter is one-way; but in most designs nothing prevents the 3.3V from flowing back through the TX+ pin in case that custom cable is used on PC with a USB 3 port.

I'm no EE but I've been wondering, what are the consequences of this flow back happening in this particular case ?

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  • \$\begingroup\$ Depends. What's it ever plugged into? Is the question, what happens when plugging such cables/devices into devices/cables that don't expect it? \$\endgroup\$ Nov 20, 2023 at 2:43
  • \$\begingroup\$ Which TX and RX pins are you talking about? USB-C has pins for USB2 pins and USB3 pins. Be sure to mention the designator of the pin, i.e. A1 or B12 or whatever. Using a standard connector for custom usage (and in case of Type-C; without first negotiating the said custom usage) is dangerous. \$\endgroup\$
    – Justme
    Nov 20, 2023 at 6:22
  • \$\begingroup\$ @TimWilliams The question is what happens when it's plugged in a USB3 port, on a PC or a mobile, considering we use USB 3 pins for custom things. TX1+, TX2+, RX1+, RX2+ (A2/B2/A11/B11), not D-/D+. \$\endgroup\$ Nov 20, 2023 at 9:59

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Here is the schematics for USB ports internal connections, per USB3.2 specifications: enter image description here The Tx drivers are de-coupled with "AC capacitors", so nothing will happen to port electricals, be it USB-A or USB-C, and these caps were there from the very beginning.

But if you connect 3.3V to Rx, the effect might vary, depending on whether the manufacturer opted to use the optional AC capacitors on Rx side. To my limited knowledge, manufacturers do use the optional caps today, so the 3.3V will unlikely to damage the receivers. However, older ports might have a problem.

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  • \$\begingroup\$ Does this depend on if the port is Type-C or Type-A? I have looked inside multiple USB3 and not a single one of them have had caps on RX pins, while the mandatory TX caps are of course there. \$\endgroup\$
    – Justme
    Nov 21, 2023 at 5:46
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I would say connecting 3V to any of the USB3 SSTX or SSRX is dangerous and extremely bad idea. Just like any case of using a standard connector for custom purposes that is incompatible electrically.

People are expecting that if you see a USB connector, you can plug it safely to any USB device, and even if it does not work, it does no damage like how USB is required to work. With the standard and allowed cables, not with non-standard non-allowed cables that is.

The 3V is dangerous especially for the hub or PC SSRX side, which may be a direct connection to an USB chip.

The USB3 data wires are low voltage signals, the pins may use much lower voltages for data transmission than 3V and so the on-chip structures cannot handle a DC of 3V.

The SSTX pins are capacitively decoupled from IC, so slightly less damage expected, but still it will receive a pulse of 3V when the capacitor charges.

Using a standard connector for custom purpose by having 3V supply directly on a data pin means you may blow up your expensive laptop just by plugging in the gizmo.

This is exactly what happens when people do something for themselves at first. They might make a prototype, and they may use an USB2-only Type-C cable which does not connect the SS pins at all, or they may splice an USB3 Type-C cable for accessing the custom pins so it will be safe to plug it into standard USB ports.

But as the hobby project goes out of your hands, people come in and try it with their cables, and then they don't realize the danger, and blow up their laptop or whatever.

It is better to use custom connector for custom purposes, or standard connectors for standard purposes.

Maybe a better solution; have the device accept a range of 3V to 5V input with a properly designed power supply section.

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