USB descriptors and their types (Part 3/6)

A descriptor contains information such as device type, vendor ID, supported USB version, number of configurations supported by the device, number of endpoints, etc. The USB peripheral must respond with descriptors when requested by the host. During enumeration, the host uses control transfer to request descriptors from the device.

Types of descriptors

There are mainly five types of descriptors

• Device descriptors

• Configuration descriptors

• Interface Descriptors

• Endpoint descriptors

• String descriptors

During enumeration, the host requests the top-level descriptors. As the enumeration progresses, the higher-level descriptors inform the host about the lower-level descriptors (if any). Descriptors contain several fields and most of them use a prefix to indicate the format or type of content. Common prefixes are: b = byte (8 bits), c = word (16 bits), BCD = binary coded decimal, bm = bitmap (bit array), l = index, id = identifier.

Each descriptor contains a series of fields and follows a common format. The first byte specifies the length of the descriptor while the second byte indicates the descriptor type.

bDescriptionType – This field indicates the type of descriptor.

Device descriptors

The device descriptor provides basic information about the device. It represents the entire USB device. It is the first descriptor read by the host. The host receives this descriptor after sending the Get_Descriptor request.

The descriptor contains fields containing device-specific information such as USB version, device class, product ID, vendor ID, number of configurations supported by the device, etc. Check out the table below to learn more about common fields included in device descriptors.

Device_qualifier descriptors

USB devices that support full and high speed modes must have a device_qualifier descriptor. When the USB device changes speed modes, the values ​​in some fields change in the device descriptor. The device_qualifier descriptor provides information only about the values ​​of fields that change when the device switches to an alternate speed mode. This descriptor contains the following fields, of which the first two are common, while others are changed specifically for the device.

Configuration descriptors

A USB device can have one or more configurations. Most devices only have one setting. The configuration provides information related to power source, power consumption, and number of interfaces.

The bNumConfigurations in the device descriptor informs the host about the number of possible configurations. The host uses Set_Configuration request to select a configuration and Get_Configuration request to read the currently selected configuration.

The configuration descriptor contains the following fields.

Other_speed_configuration descriptors

This descriptor is for devices that support full and high speed modes. Returns the currently inactive speed mode setting. The structure is the same as the configuration descriptor, except for the field value for bDescriptor type, which is 0x07.

Interface descriptors

A configuration can have one or more interfaces that can be considered resources or functions implemented by the device. The interface descriptor provides information related to specific device features, such as class, subclass, protocol, and number of uses of the endpoint interface.

The bNumInterfaces in the configuration descriptor tells the host about the number of possible interfaces. After selecting a configuration, the host uses Set_Interface request to select an interface and Get_Interface request to read the currently selected interface.

The Endpoint Descriptor describes the endpoints associated with an interface. There is no descriptor for the default zero endpoint. Endpoint zero is the control endpoint and is configured even before any descriptor is requested. The information contained in this descriptor is used to determine the bandwidth required by the USB bus.

This descriptor includes the following fields –:

String descriptors

String descriptors contain human-readable text. They are generally used to provide information. Other descriptors contain index values ​​that point to strings containing manufacturer, product, serial number, configuration, interface information. Other descriptors like Class and Vendor Specific can contain indexes to string descriptors

Strings are encoded in Unicode format and multiple languages ​​are supported. The host can get the list of supported languages ​​using Get_Descriptor request with 0 in wIndex field. The device returns the String descriptor containing codes for different languages. The first 2 fields are mandatory and the other fields contain language codes.

After that, all subsequent string descriptors (beyond index 0) will have string content and must conform to the following format.