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I am working on a product that is made of 2 parts: a host with a male USB-C connector and a device with a female USB-C connector that plugs onto it.

The plan is to use USB-C because it's a nice reversible connector, but we'll only use the USB 2.0 specification and pins. We want to use the unused "Alternate mode" pins (in blue) to transmit our own signals.

If I do, what happens if the consumers plugs the device to his computer using a USB 3.0 USB-C cable? Will the host detect that the device is simply USB 2.0 and will not use the Alternate lanes? Or could this cause hardware damage somehow? What resistors should I put on CC1/CC2 pins?

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    \$\begingroup\$ USB C is already quite a mess because it's impossible to tell without detailed technical manuals what a particular USB port can and cannot do and because there are so many non-compliant implementations. Please don't make it worse. If custom signals are really needed, implement it such that the two devices first communicate via the USB 2.0 protocol to ensure that they both support the custom signals before they are activated. \$\endgroup\$
    – Codo
    Commented May 6, 2020 at 11:52
  • \$\begingroup\$ But how to I make sure that the two devices will communicate via USB 2.0 first? If the devices has 5.1k PD resistors on the CC lines, will that be enough? I want to make sure a DFP like a computer will not use the custom signal pins. \$\endgroup\$
    – coriv
    Commented May 7, 2020 at 16:06
  • \$\begingroup\$ electronics.stackexchange.com/a/291427/142001 answers how to wire a USB-C connector such that it appears as a USB 2.0 device. \$\endgroup\$
    – Codo
    Commented May 8, 2020 at 5:36

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