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On an upstream-facing USB3 port, can one put a USB2 hub on the USB2 DP/DM pair?

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karpouzi9:
See title.

I know that something like this is a feature on Cypress' USB3 hubs - they call it Shared Link, you can split a USB3 port into just USB3 and just USB2, TI has something similar - but I'm wondering what the consequences might be of adding a discrete USB2 hub to just the USB2 pair.

Breaking it down:
Downstream facing USB3 port -> Upstream facing USB3 port
Upstream facing USB3 port contains USB2 pair (DP/DM) and USB3 diffpairs (RXP/RXM and TXP/TXM)
Plug USB3 diffpairs into device A's USB3 diffpairs
Plug USB2 pair into the upstream port of a USB2 hub
Plug one downstream port of the USB2 hub into Device A's USB2 diffpairs
Plug the remaining USB2 downstream ports into some other devices.

Anyone know how this would enumerate?

Aiy:
Hi there,
Interesting. If I understand you correctly you want to do something similar to this:
https://numato.com/kb/usb-3-0-a-cost-effective-high-bandwidth-solution-for-fpga-host-interface/

They combine the USB3.0 FTDI FT601 for high bandwidth, with a USB2.0 FT2232H for FPGA JTAG in a sidechannel scheme.

Best regards

SiliconWizard:
I wouldn't expect this to work with most USB hosts. I haven't checked in the USB 3 standard to confirm, I suspect it would not be supported at least as standard, but something to check.

From an OS POV, I don't think you could have one USB port acting as several USB ports (so here a USB 3 port acting both as a USB 3 SS/SS+ and USB HS/FS/LS on the D+/D- pair.)
I'm not aware of this port sharing capability at least, it would somehow imply that each USB 3 port would have its own mini-hub, if you see what I mean.

Also, I think a host with an USB 3 port will first try to enumerate the device on the D+/D- pair before switching to the other pairs for SS/SS+? So if two different devices are actually connected to these pairs, that would wreak havoc on the enumeration process completely.

So don't have a definitive answer, but I wouldn't expect this to be possible. Curious to see if it's actually doable though.

karpouzi9:
Maybe I will throw together an FX3 board with this and see how it pans out.

The basic idea is, how do you add another USB2 peripheral to a USB3 gadget without needing a full blown USB3 hub.

karpouzi9:

--- Quote from: Aiy on March 28, 2023, 06:48:02 pm ---Hi there,
Interesting. If I understand you correctly you want to do something similar to this:
https://numato.com/kb/usb-3-0-a-cost-effective-high-bandwidth-solution-for-fpga-host-interface/

They combine the USB3.0 FTDI FT601 for high bandwidth, with a USB2.0 FT2232H for FPGA JTAG in a sidechannel scheme.

Best regards

--- End quote ---

Nice find, this is precisely the topology I was thinking of. It doesn't look like they built it, though. I will have to report back. Odds are good that some changes would be needed on the USB3 device (ie FX3) side to make it enumerate to the host properly without its own dedicated USB2, though maybe no change would be needed...

Edit: it does look like they used this architecture in the Callisto K7 modules: https://numato.com/help/wp-content/uploads/2018/06/CallistoK7Sch.pdf

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