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I am developing a device which should be charging and communicating via USB. Therefore, I took USB-C socket and connected CC1 and CC2 to GND via 5.1 k resistors.

The device gets detected correctly as long as I take USB-A to USB-C cable and plug it via a USB-C to USB-A Adapter to the USB-C port of my laptop.

But as soon as I use a USB-C-to-USB-C cable for it, the laptop starts to negotiate something. Charging becomes enabled and disabled periodically and communication is not possible.

Is there something I missed?

enter image description here

Here are some scope screenshots:

  1. Voltage measured at CC line when connected to USB-C at laptop

enter image description here

  1. VBUS

enter image description here

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  • \$\begingroup\$ Have you measured with multimeter if the resistors have correct value and have no bad solder joints? Does it matter which way the USB cable is flipped? How is the other circuitry connected to the USB connector? \$\endgroup\$
    – Justme
    Commented Jul 12, 2022 at 10:11
  • \$\begingroup\$ Yes at least inside the circuit and it has the correct value. Sure a bad soldering can happen at the USB-C pin but I've tested it on several devices with the same circuit. And it does not matter how the cables is connected. The other circuitry is a MacBook which I do not want to open and I expect that it complies to the standard. \$\endgroup\$
    – abeat
    Commented Jul 12, 2022 at 15:26
  • \$\begingroup\$ I don't mean the PC side, I mean what other circuitry is connected to the connector on your board, regulators, MCUs, how it handles USB, etc. \$\endgroup\$
    – Justme
    Commented Jul 12, 2022 at 16:10
  • \$\begingroup\$ I added a bigger picture. As you can see there is a Lipo charger connected to VBUS with a buffer capacitor at the input. \$\endgroup\$
    – abeat
    Commented Jul 13, 2022 at 9:30
  • \$\begingroup\$ Solved ........ \$\endgroup\$
    – abeat
    Commented Jul 14, 2022 at 8:41

1 Answer 1

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Okay, the hint was good to look at the rest of the circuit. The issue was the lipo charger. This chip has no reverse voltage protection. Hence if no USB cable is plugged in, but the battery is in, VBUS is at VBAT - 1.3 V. This is too high for the host to allow VBUS to be switched on and the host starts a negotiation. The solution is a diode in the VBUS path avoiding a reverse current. Thanks, for the support.

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