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Is it possible/sane to connect a USB 3.0 host controller's D+/D- pins via a USB 2.0 hub (to allow several downstream USB 2.0 ports) while connecting the USB 3.0 ss pins directly to a USB 3.0 connector and allow for concurrent operation of all ports?

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  • \$\begingroup\$ I'm pretty sure you can get USB hubs with some 3.0 and some 2.0 ports, particularly ones made around when 3.0 was new and expensive. \$\endgroup\$
    – Hearth
    Commented Jun 23, 2021 at 15:41

2 Answers 2

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It may be possible but I'd question the sanity.

The SS lines in USB 3.x can operate independently of the USB 2.0 backward compatibility pins that offer Vbus, Gnd, and D+/D-. That is how the kernel debug cables work, by leaving Vbus and D+/D- not connected.

A problem I see is that if a device is connected by only the SS pins then it is not getting Vbus power or it is sharing Vbus power with another device. If Vbus is not connected then the device connected by Vbus may not know that there is a connection, a self powered device will use that as an indication of being connected to a host. If Vbus is shared then there may be a greater load on Vbus than it is designed to handle as both devices will assume they have the bus to themselves.

A sane solution, in my opinion, is to get a USB 3.0 hub. The hub will provide a path for power management, a shared USB 2.0 data path for the device on the SS pins to announce it's presence to the host and negotiate the SS data path, and the USB 2.0 device will have the bulk of the bandwidth on the D+/D- pins as once the negotiation is complete the USB 3.x device will communicate on the SS pins.

Without the USB 2.0 data path for negotiating power and the SS data path then the SS path and power would have to be managed manually, and even then it might not work since I assume that devices will not even power up until they can talk on USB 2.0 and know that they are allowed to take more power.

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If you mean, connect the a USB 3.0 host to two different devices (whatever is on the connector and the usb 2.0 hub (which is also a device)) then no. The USB spec needs one port per device and does not support two devices on one host (without a hub/controller).

If you want to plug two devices into the same port, then use a USB 3.0 hub controller IC.

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