Enumeration and Device Descriptors

Whenever a USB device is attached to the bus it will be enumerated by the USB subsystem - i.e an unique device number (1-127) is assigned and then the device descriptor is read. The desciptor is a data structure which contains information about the device and its properties. The USB standard defines a hierarchy of descriptors (see Figure 2).

Figure 2. USB Descriptor

Standard Descriptors

Device Classes

The standard device and interface descriptors contain fields that are related to classification: class, sub-class and protocol. These fields may be used by a host system to associate a device or interface to a driver, depending on how they are specified by the class specification. Valid values for the class fields of the device and interface descriptors are defined by the USB Device Working Group.

Grouping devices or interfaces together in classes and then specifying the characteristics in a Class Specification allows the development of host software which can manage multiple implementations based on that class. Such host software adapts its operation to a specific device or interface using descriptive information presented by the device. A class specification serves as a framework defining the minimum operation of all devices or interfaces which identify themselves as members of the class.

Human Interface Devices (HID)

The HID class consists primarily of devices that are used by humans to control the operation of computer systems. Typical examples of HID class devices include:

Keyboards and pointing devices for example, standard mouse devices, trackballs, and joysticks.
Front-panel controls for example: knobs, switches, buttons, and sliders.
Controls that might be found on devices such as telephones, VCR remote controls, games or simulation devices for example: data gloves, throttles, steering wheels, and rudder pedals.