Specifications

Example Device Descriptor Spec (1 of 2)

 A look at a descriptor and its fields

Subclass code (assigned by the USB-IF)

These codes are qualified by the value of the bDeviceClass field

If the bDeviceClass field is reset to zero, this field must also be reset to zero

If the bDeviceClass field is not set to FFH, all values are reserved for

assignment by the USB-IF

SubClass1bDeviceSubClass5

Class code (assigned by the USB-IF)

If this field is reset to zero, each interface within a configuration specifies its

own class information and the various interfaces operate independently.

If this field is set to a value between 1 and FEH, the device supports different

class specifications on different interfaces and the interfaces may not operate

independently. This value identifies the class definition used for the aggregate

interfaces.

If this field is set to FFH, the device class is vendor-specific

Class1bDeviceClass4

USB Specification Release Number in Binary-Coded Decimal (i.e., 2.10 is 210H)

This field identifies the release of the USB Specification with which the device and

its descriptors are compliant.

BCD2bcdUSB2

DEVICE Descriptor Type

Constant1bDescriptorType1

Size of this descriptor in bytes

Number1bLength0

DescriptionValueSizeFieldOffset

Now that we understand how the host requests data from a device and how the

device responds to the request by passing along the descriptors that are stored in

memory, we can take a deeper look at the descriptors. One example of a descriptor

is the Device Descriptor and is transferred to the host as a result of the

Get_Descriptor(DEVICE) standard request. The table in the slide shows the fields

associated with the device descriptor. The first field identifies the length of the data

including the size field. For the device descriptor the size field is always 18 bytes

(0x12). The next is the descriptor type field which identifies the data as the device

descriptor. Many different fields follow including what revision of the USB

specification is supported by the endpoint device, the vendor ID and Product ID, as

well as the maximum packet size supported for the endpoint. Take a look at the

table on this slide and the next to see how the fields are defined for the device

descriptor.