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I want to implement multiple USB ports using one USB3.0 and one USB2.0 hub with four ports each. The microprocessor I want to implement these ports for has just a single USB3.0 port left (of which the USB3.0 PHYs port is muxed and routed separately through a SerDes. The USB2.0 PHY is exported directly). Now the obvious implementation approach is cascading both ports. However I want to use all four USB3.0 ports.

My question is now whether it is possible to put the USB3.0 hub behind the USB2.0 hub (connections: MPU->2.0hub and MPU->3.0hub and 2.0hub->3.0hub). So the USBx lanes would separately be connected to the corresponding hubs and then the 2.0 hub supplies the 3.0 hub.

This way I would have all four 3.0 ports (with backward compatibility) available (as opposed to the hierarchical correct solution MPU->3.0->2.0), however I am not sure whether this kind of "port splitting" is possible.

Thx in advance

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Is your end goal to have downstream 4 USB3 ports and 4 USB2 ports total?

As I understand you have 1 downstream path from your MCU (which is full USB3 w USB2 signals as part of it). In this case you cannot split the USB3 and USB2 downstream paths because they are the same channel. The way USB3 works is it will try to work over the super speed pairs and if that fails default back to USB2. At no time are the USB2 and USB3 SS paths both working and I would be surprised if the libraries/hardware would support a logical split of the channels from a single physical channel.

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  • \$\begingroup\$ My goal is to have 4 USB3 ports, yes, but only 3 USB2 downstream ports. The 4th USB2 port would supply the USB3 hub with USB2. So I'm not really splitting USB2 from USB3 as I would reunite these two paths with the USB3 hub. But I want to use a USB2 hub to create additional 2.0 ports first. \$\endgroup\$
    – 0x4859
    Commented Aug 27, 2019 at 20:31
  • \$\begingroup\$ As this answer explains, you cannot do that \$\endgroup\$
    – Chris Stratton
    Commented Aug 27, 2019 at 21:07
  • \$\begingroup\$ @0x4859. You cannot split the USB2 signal from a downstream port and then try to rebind them after with a USB2.0 hub in between. The USB 2.0 protocol and USB 3.0 stacks are independent which makes it seem like in theory you could use both in parallel but USB protocol requires that one be selected and the other turned off for a given port. What you are talking about would be a science experiment and not USB complaint. \$\endgroup\$
    – EasyOhm
    Commented Aug 27, 2019 at 22:04

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