New USB specifications have been released by the USB 3.0 Promoter Group, and are now available to download from http://www.usb.org/developers/docs. They are USB Power Delivery v2.0 and USB Billboard Device Class v1.0 – said to be ‘essential’ for the development and implementation of new Alternative Modes (Display Monitor Vol 21 No 38) for USB-C.
The new Power Delivery (PD) specification updates USB PD v1.0 to meet the requirements of USB-C, and incorporate additional changes. If a USB-C host or device supports Alt Modes, they will use USB PD ‘Structured Vendor Defined Messages’ (Structured VDMs) to discover, configure and enter/exit Alt Modes. The USB PD Structured VDMs are defined to extend the functionality a device exposes. They provide a standard method to identify the Alt Modes a device supports, and to command it to enter/exit Alt Modes.
The USB Billboard Device Class specification outlines the methods used to communicate if an Alt Mode is supported by a host or device that is USB-C-capable. ‘In particular, if a device fails to successfully enter an Alternate Mode then the device will minimally expose a USB 2.0 Billboard interface powered by VBUS’, says the Promoter Group.