Each Universal Serial Bus (USB) device has a set of descriptors. The descriptors are read by the Host during enumeration. Descriptors inform the Host of the following information about a device:
- The version of USB supported by the device
- Who made the device
- How many ways the device can be configured by the Host
- The power consumed by each device configuration
- The number and length of endpoints on the device
- What type of transfer method is to be used to communicate with endpoints
- How often the endpoints are to be serviced
- What text to display if the Host operating systems accept text descriptions
Descriptor Structure
There are several types of descriptors for USB devices arranged in a logical hierarchy. The following diagram illustrates the hierarchy of a descriptor for a single device with two possible configurations for the Host to activate. Each of these configurations has a single interface with two endpoints.
Descriptor Types
The most commonly used descriptors include:
- Device Descriptor
- Configuration Descriptor
- Interface Descriptor
- Endpoint Descriptor
- String Descriptor
Every USB device must have one Device Descriptor and at least one each of the Configuration, Interface, and Endpoint Descriptors.
A full description of the Byte by Byte contents of the descriptor tables can be found in the USB specifications. This page explains some of the important features of each of the descriptor types and how they are used.
Device Descriptor
The Device Descriptor is the first descriptor read by the Host during enumeration. The purpose of the Device Descriptor is to let the Host know what specification of USB the device complies with and how many possible configurations are available on the device. Upon successful processing of the Device Descriptor, the Host will read all the Configuration Descriptors.
Structure of a Device Descriptor
Key Elements of a Device Descriptor
| bcdUSB | Informs the Host of what version of USB the device supports |
| bDeviceClass | 00 - Thedevice class is defined in the Interface Descriptor FF - the device class is Vendor class any other number is the specification for the class of this device |
| idVendor | 16-bit number assigned by USB.org to the product's manufacturer |
| idProduct | 16-bit product model ID assigned by the Vendor to this product |
| bNumConfigurations | How many different configurations are available for this device |
All certified USB products must have a valid Vendor ID and Product ID. See the"Developing USB Applications with Microchip" page for details on obtaining these numbers
Configuration Descriptor
A device may have more than one configuration. Each device configuration is assigned a number. The Configuration Descriptor serves two purposes:
- Informs the Host as to how many interfaces (i.e., virtual devices) are in the configuration. While it is common for a configuration to offer only one interface, Devices that appear like two or more products have more than one interface.
- How much power the device will consume if this configuration is activated by the Host. If the device is capable of controlling its power consumption, it may offer more than one configuration. Each configuration will advertise how much power would be consumed if the configuration were to be activated.
Structure of a Configuration Descriptor
Key Elements of a Configuration Descriptor
| bNuminterfaces | Number of Interface Descriptor tables available |
| MaxPower | Power load of this device if the Host activates this configuration |
Interface Descriptor
An Interface Descriptor describes the details of the function of the product. Key elements include the number of endpoints on the device and which USB device class is implemented by the endpoints. For example, if the device were a keyboard, the specified device class would be Human Interface Device (HID) and the number of endpoints would be two. See the"USB Device Classes" page for details on how device classes are implemented in USB.
Structure of an Interface Descriptor
Key Elements of an Interface Descriptor
| bNumEndpoints | Number of endpoints in the interface |
| bInterfaceClass | USB device class used to set transfer types for the endpoints |
Only one configuration can be active at any time. When a configuration is active, all of its interfaces and endpoints are available to the Host. Devices that have multiple interfaces are referred to as Composite Devices. One physical product with one available USB connector would appear to the Host as two separate devices. A keyboard with an integrated mouse (or trackball) is an example of a composite device.
Endpoint Descriptor
Each endpoint on a device has its own descriptor. The descriptor provides the endpoint address (i.e., endpoint number), the size of the endpoint, and thedata transfer type used to access the endpoint.
Key Elements of Endpoint Descriptors
| bEndpointAddress | The address of the endpoint (i.e. endpoint number) |
| wMaxPacketSize | Length of the endpoint |
| bInterval | How often in frames is this endpoint to be serviced by the Host |
Even though the device class will usually indicate the endpoint numbers and transfer types to be used, these items must be included in the Endpoint Descriptor
String Descriptor
Strings Descriptors are optional human readable strings which the Host OS may display.
String Descriptor Structues
| Field | Size (in bytes) | Description |
|---|---|---|
| bLength | n + 2 | Size of the String Descriptor |
| bDescriptorTypes | 1 | 0x03 used for String Descriptors |
| bString | n | Unicode string of length n |
