Showing results for 
Search instead for 
Did you mean: 


Valued Contributor

Quick question regarding Alt mode for USB-DP dongles.

From what I understand, the Billboard device must present in order to negotiate the PD settings and enter Alt mode for USB-C to Displayport functionality.

Does this connection need to be persistent after alt mode has been entered?  I.E. if I wanted to use the USB 2.0 connection for another device after entering alt. mode via a USB mux, is this something that can be done?  Or will the device fall out of alt mode if the billboard device is disconnected?

Help is most appreciated!


1 Solution
Valued Contributor

According to the USB-C Spec:

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.

So if I'm reading this right, exposure of the USB Billboard Device Classinterface is optional after Alternate Mode has been entered.  Unless I hear differently, this is what I'll assume is the case.

View solution in original post

2 Replies