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Through research I am aware that using the CC lines one can advertise "Default" (500mA for 2.0 and 900mA For 3.0+), 1.5A and 3A before getting into PD territory, but none of these include provisions for backwards compatibility with the standards of a bus powered USB 2.0 Hub which is limited to 500mA total, meaning a realistic limit of typically 100mA per port for a 4 port hub (where 100mA is reserved for the controller itself).

What is the correct procedure downstream facing USB 2.0 Type-C ports in this situation however, where there indeed is a hard limit to the power available to the hub that is in alignment with USB 2.0 standards?

To restate differently; how can one avoid advertising currents that a bus powered USB 2.0 hub controller (limited to 500mA total and 100mA per port), cannot support such as the all of the current ratings that CC lines can advertise using Type-C Downward Facing Ports?


I found a relevant excerpt from the USB Specification that I believe answers my question, though the phrasing is not crystal clear to me so I could definitely use reassurance.

When a Source is advertising USB Type-C Default current, the Sink behavior is defined as follows:

  • It connects as a USB 2.0 or USB 3.2 device, after which the Sink shall follow the appropriate USB specification.
  • It enters a USB PD contract, after which the Sink shall follow the USB PD specification to determine the current (e.g., Rp will no longer be Default as it is superseded by the USB PD contract).
  • It detects a USB BC 1.2 charging port, after which the Sink shall follow the USB BC 1.2 specification.
  • It attaches as a USB Type-C Power Sinking Device (PSD), after which the Sink may draw up to 500 mA.

This to me indicates that if set to the "Default" advertisement, the connecting device should refer to the standard means of evaluating available current which for this use case would be first with the minimum pre-enumeration current of 100mA, followed by the post-enumeration current of 100mA (As this is for a USB 2.0 hub controller in "bus powered" mode).

The specific document referenced was "Universal Serial Bus Type-C Cable and Connector Specification Release 2.0 August 2019"

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    \$\begingroup\$ Befitting Display Name - You posted the question (obviously) and then posted an "answer". That should mean you have solved your own problem and the question is considered solved. But your "answer" implied that it needed replies / help / advice and was only a possible solution. || In this context, it isn't an answer (as you are not saying that it is the solution) and therefore I have moved it into your question, as a piece of your own research (which we do encourage OPs to show in their questions - nothing wrong with that). That way, site members can respond to it in their answers. \$\endgroup\$
    – SamGibson
    Jan 13 at 13:59

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There is no problem. The procedure is exactly same as having a USB2.0 hub with Type-A DFP ports and connected to UFP Type-A port.

The USB 2.0 specification does not allow a device to draw more than 100mA before it has been enumerated.

The computer knows how many devices are behind each port and hub and can refuse to enumerate a USB device, if it requires more current than what is currently impossible to provide.

So setting the hub CC pins to provide the default current does not mean that the plugged in USB2 device can draw the maximum without asking.

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