USB System Architecture

•Isochronous Endpoints — Isochronous endpoints are used for transmitting and receiving data to ensure a constant bit rate that is required for real time communications requiring low latency.

•Bulk Endpoints — These endpoints are used when the data consists of bursts that are not as time sensitive, such as data to and from a conventional modem.

Communications Class-Specific Descriptors

Two types of class-specific descriptors are defined for communications devices. These descriptors include:

•Class-specific Configuration Descriptors

•Class-specific String Descriptors (also called Protocol Descriptors since they define aspects of a device based on its protocol code)

Please refer to the specification for details regarding the format and definition of these descriptors.

Communications Class-Specific Requests

The following class-specific requests are defined by the communications device class specification. These are:

•Send Encapsulated Command

•Get Encapsulated Response

•Report Format (encapsulated protocol message)

•Notification of Interface Availability

•Select Interface Protocol Command

•Get Interface Command

Please refer to the specification for details regarding the format and definition of these requests.

Display Device Class

This device class defines the mechanism used to control display settings, such as brightness, contrast, and color. Traditionally, these controls have been implemented via manual controls on a hardware control panel. The USB display function permits these adjustments to be made under software control.

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Chapter 21: Device Classes

The display class and subclasses are defined within the Device Descriptor as illustrated in Table 21-3.

Table 21-3: Display Class Standard Device Descriptor Definition

The Standard Display Device Class Interface

USB display devices require only the default control endpoint for passing control information to the display. Only one configuration and one interface is defined by this class. Note that since no endpoint other than the default endpoint is used, the interface descriptor specifies no endpoints.

Display Device-Specific Descriptors

A USB display device uses three device-specific descriptors:

•Display Descriptor — The display descriptor defines which controls are supported by this device, and specifies the displays characteristics. This information is based on the VESA Extended Display Identification (EDID) specification. This descriptor is read via the Get Display ID request.

•Display Status Descriptor — This descriptor provides status information on a variety of display settings, along with the horizontal and vertical frequency used by the display. This descriptor is read via the Get Display Status request.

•Display Control Descriptor — This descriptor is used to determine the possible values that can be set for each controls supported and its current setting. A separate Display Control descriptor exists for each control, defined by control codes in the specification. The control code to be referenced is specified in the Get Max and Get Current requests, which return the contents of the descriptor.

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