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TUSB320: Is UFP to UFP USB device connection safe?

Brian Han1
Intellectual 530 points
Part Number: TUSB320


I'm designing a USB device for a data transfer and I want to make the circuit as simple and small footprints as possible.
And to achieve this, I need to know if 2 self-powered USB UFP devices won't die when I connect them to each other.

For more details,

The system is a USB device(Slave only) using USB C connectors, but this is an only USB 2.0 device so SS pins are not used.

So I was planning to use SPI peripherals over SS pins(Tx,Rx) to have devices send and receive data without having all of my devices connect to host.
Currently, I have set a USB C controller in UFP mode and I know if I make the controller to be in a DRP mode, this system won't have any problem.
But If I make the device in DRP mode, I need to add a load switch, which I don't want to use since this system is a self-powered USB slave device.

So I'm wondering if I'm ok to connect 2 self-powered USB UFP Devices without blowing anything up.

  • 0
    TI__Guru 64326 points

    Hi Brian,

    Unfortunately connecting two UFP devices together is not allowable in the USB-C spec. IS this device meant to be USB compliant? Further more TUSB320 will not see a proper voltage on the CC line to register a connection had been made. Does your MCU have I2C to talk to TUSB320? I recommend enabling Debug accessory mode on "Conn 2" and have both TUSB320 in DRP mode so "special" but valid connection can be made. If you are looking to reduce you footprint and size, you could even do this with one connector however I2C must be used in to enable Debug Accessory mode. 

  • 0 in reply to Malik Barton57
    Intellectual 530 points

    Hello Malik,

    Thanks for the answer. This device is USB Compliant, but I won't be using the USB connection when I connect my 2 devices.

    I just need the SS pins on the USB C ports.

    I can use I2C, but I'm not sure what register I need to write to set the device in DRP Debug accessory mode.

    0x08 3:1 bits ACCESSORY_CONNECTED bit only seems to be Read/Update registers.

    Regards

    Brian

  • 0 in reply to Brian Han1
    TI__Guru 64326 points

    Hi Brian,

    Are you using TUSB320 or TUSB320LAI (HAI)?

  • 0 in reply to Malik Barton57
    Intellectual 530 points

    Hi Malik,

    Actually, I was not aware of TUSB320LAI, but it seems the configuration is easier with TUSB320LAI.

    From the information given, I will probably use TUSB320LAI.

  • 0 in reply to Brian Han1
    TI__Guru 64326 points

    Hi Brian,

    I do recommend using the TUSB320LAI as it is one of our most up to date devices for CC controlers. 

  • 0 in reply to Malik Barton57
    Intellectual 530 points

    Hi Malik,

    Is there any other option than the Debug Accessory mode for a safe connection? It seems I need to add 2 Rd resistors at one side of the CC pins and since the circuit of the 2 boards will be the same, I need to add a MUX to switch signals between receiving TUSB320LAI CC pins and 5.1k resistors to ground.
    like the picture below.

    If we set the default mode to DRP, will connecting DRP USB port to a DRP USB port cause any problem?
    I understand the valid connection can't be made, but I can use the I2C to set the slave system as the UFP when the board to board connection is required.

    Is using the MUX the only way to solve this issue?

  • 0 in reply to Brian Han1
    TI__Guru 64326 points

    Hi Brian,

    When TUSB320LAI is in DRP it will toggle between Rp and Rd on both CC lines and connecting two DRP ports together is not a problem as they will resolve themselves to valid Type-C connection. The mux in this case is not needed. 

    One important point to mention is that when a Type-C cable is used only one CC line carried over the cable. In order to realize the debug port properly both CC pins should be exposed as in a standard Type-C plug. there are two options here

    • Order a special Type-Cc cable where both CC pins traverse the entire cable.
    • Implement a capative into you design so one Type-C port is a standard receptacle and the other is a plug.

    Please let me know if you have questions. I can also point you to the relevant sections in the Type-C spec for additional details.

  • 0 in reply to Malik Barton57
    Intellectual 530 points

    Hi Malik,

    Thank you so much for the clear answer. I think I'm all set now with the setup.

    I will look into the cable options you suggested.