Two USB ports as gadget/device mode?

[Pavel_Hofman]

Hi, please is it possible to use both USB3 ports of Rock Pi 4A/B as gadget/device ports? IIUC RK3399 has two OTG-capable dwc3 blocks, and two OTG-capable USB2/3 PHYs.

IIUC the board schematics show one of the USB3 A ports is OTG by design (utilizing TYPEC0_U2VBUSDET). But the second USB3 A port is hooked directly to 5V and TYPEC1_U2VBUSDET is not connected.

I would not mind hacking a custom cable with the +5V cable disconnected (USB A -> USB A cables are custom only anyway). Also reconfiguring device tree would not be a problem either.

The purpose would be a dual-gadget USB2/3 bridge to connect between two PCs (network, audio, etc.)

Or please is there a cheaper/smaller Radxa board capable of dual-gadget operation?

Thanks a lot,

Pavel.

[Pavel_Hofman]

Going through the SoCs used by Radxa … IIUC the only two chips with dual dwc3 and two USB phys are RK3399 and RK3588.

[jack]

You are right, RK3399 and RK3588 both has two USB OTG. ROCK 5B OTG0 works and OTG1 can not work. ROCK 5B+ OTG0 and OTG1 both work. ROCK 4A/4B OTG1 is not tested.

[Pavel_Hofman]

Jack, thanks a lot. I am looking at 5B and 5B+ schematics to understand the difference, so that I can compare with Rock 4. Please can you comment on the following?

Both 5B and 5B+ connect complete OTG0 + TYPEC0 to the USB-C connector which by design supports host and device.

Both 5B and 5B+ connect OTG1 + TYPEC1 to one port of the USB3 type A combo. 5B uses one power switch for both A ports of the dual combo, while 5B+ has a separate power switch for the TYPEC1 port of the combo:
5B:

5B+:

Also 5B has TYPEC1_USB20_VBUSDET not connected:

while 5B+ has it connected to 3.3V fixed:

Now, IIUC to use the TYPEC1 as a device, I would need USB A-A cable. Please how will the DWC3-1 controller for TYPEC1 know that it should switch to device when a cable is connected, when no OTG-ID is used?

I would understand the DWC3-1 must be “static device” as described in RK3588 datasheet:

Configured in device tree.

But please, assuming the above is true (which I am not sure of), why OTG1 on 5B cannot work while on 5B+ it can, if both must be switched to the device statically?

TYPEC1 on Pi4 seems to be connected similarly to 5B, the USBVDET is unconnected to H:

But still it would seem to me that if the second DWC3-1 is statically switched to device in device tree and the custom USB A-A cable has its 5V line disconnected (so that the host 5V does not conflict the 5V in the device port type A), both 5B and 4A/B should work OK.

Please can you comment on that? Thanks a lot!