3.7 USB Configuration

Before the USB function (or functions, in a compound device) can be operated, the device must be configured. The host does the configuring by acquiring the configuration information from the USB device. USB devices report their attributes by descriptors. A descriptor is the defined structure and format in which the data is transferred. A complete description of the USB descriptors can be found in Chapter 9 of the USB Specification (see http://www.usb.org for the full specification).

It is best to view the USB descriptors as a hierarchical structure with four levels:

• The Device level
  
• The Configuration level
  
• The Interface level (this level may include an optional
  sub-level called Alternate Setting)
  
• The Endpoint level
  

There is only one device descriptor for each USB device. Each device has one or more configurations, each configuration has one or more interfaces, and each interface has zero or more endpoints, as demonstrated in Figure 3.3 below.

Figure 3.3: Device Descriptors

Device Level:

The device descriptor includes general information about the USB device, i.e. global information for all of the device configurations. The device descriptor identifies, among other things, the device class (HID device, hub, locator device, etc.), subclass, protocol code, vendor ID, device ID and more. Each USB device has one device descriptor.

Configuration Level:

A USB device has one or more configuration descriptors. Each descriptor identifies the number of interfaces grouped in the configuration and the power attributes of the configuration (such as self-powered, remote wakeup, maximum power consumption and more). Only one configuration can be loaded at a given time. For example, an ISDN adapter might have two different configurations, one that presents it with a single interface of 128 Kb/s and a second that presents it with two interfaces of 64 Kb/s each.

Interface Level:

The interface is a related set of endpoints that present a specific functionality or feature of the device. Each interface may operate independently. The interface descriptor describes the number of the interface, the number of endpoints used by this interface and the interface-specific class, subclass and protocol values when the interface operates independently.

In addition, an interface may have alternate settings. The alternate settings allow the endpoints or their characteristics to be varied after the device is configured.

Endpoint Level:

The lowest level is the endpoint descriptor, which provides the host with information regarding the endpoint's data transfer type and maximum packet size. For isochronous endpoints, the maximum packet size is used to reserve the required bus time for the data transfer - i.e. the bandwidth. Other endpoint attributes are its bus access frequency, endpoint number, error handling mechanism and direction.

The same endpoint can have different properties (and consequently different uses) in different alternate settings.

Seems complicated? Not at all! WinDriver automates the USB configuration process. The included DriverWizard utility [5] and USB diagnostics application scan the USB bus, detect all USB devices and their configurations, interfaces, alternate settings and endpoints, and enable you to pick the desired configuration before starting driver development.

WinDriver identifies the endpoint transfer type as determined in the endpoint descriptor. The driver created with WinDriver contains all configuration information acquired at this early stage.