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
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
- 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
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  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.