Question: What is a Billboard Device? When is it needed? Where do we implement it?
The USB Billboard Device Class definition describes the methods used to communicate the Alternate Modes supported by a device container to a host system. More details on Billboard Devices are available in the USB Billboard Device Class specification at the following link: http://www.usb.org/developers/docs/devclass_docs/.
The USB Billboard Device indicates the failure of Alternate Mode entry. According to USB Type-C specification, if a device fails to successfully enter an Alternate Mode within tAMETimeout (maximum of 1000 ms), then the device will minimally expose a USB 2.0 interface (USB Billboard Device Class) that is powered by VBUS to indicate the failure of Alternate Mode entry. The host then reports that an unsupported device has been connected, identifying the type of device from the USB Billboard Device Class information.
The USB Billboard Device is implemented on the device container, which supports one or more Alternate Modes. The USB Type-C specification mandates the implementation of the Billboard Device for better user experience. For example, in the reference design for Type-C to DisplayPort cable, Billboard functionality is implemented using a CY7C65210 device. CY7C65210 belongs to Cypress’s USB-Serial bridge controller family and is a full-speed USB 2.0 device that supports USB Billboard Device Class. It is available in a 24-QFN package.