Electronic – Does USB-C Alternate Mode require the Billboard Device to be always connected

displayportusb-c

I'm working on a device design that operates in USB-C alternate mode as a displayport dongle. As such it requires a USB 2.0 billboard device to be present to negotiate with the host and enter alt mode.

This might be a silly question but if after the device enters alt mode, I want to turn the USB 2.0 connection over to another device (using a USB Mux), can I disconnect the billboard and then point to my other USB device without losing Alt Mode function? Or will alt mode cease function when the Billboard is disconnected?

In the USB-C Spec from August 2019, the following is stated

E.4.3 Operational Summary:

Host stays in USB DisplayPort Dock mode until

a. Explicitly exited by and Exit Mode VDM.

b. System physically disconnected from the USB DisplayPort Dock

c. Hard Reset on USB PD

d. VBus is removed

Would (b) count as physical device removal? Or is it referring to the USB-C connection and not the USB 2.0 Billboard Interface?

Thanks for the help!

Best Answer

Found the following in the USB-C R2.0 Spec 2019:

When a device offers multiple modes, especially where multiple Alternate Mode definitions are needed in order to be compatible with multiple host-side implementations, successfully entering an Alternate Mode may be predicated on only one of the available modes being successfully recognized by a host. In this case, the device is not required to expose but may still expose a USB Billboard Device Classinterface to indicate to the host the availability and status of the modes it supports.

The host may send an Enter Mode after tAMETimeout. If the device enters the mode, it shall respond with an ACK and discontinue exposing the USB Billboard Device Classinterface. The device may expose the USB Billboard Device Classinterface again with updated capabilities.

According to this, exposure of the interface after Alternate Mode has been entered is optional, and may be re-exposed later with updated information if need be. I will take this to mean "Yes, the interface can be disconnected" after Alternate Mode entry.

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