USB Class Codes

                   

USB defines class code information that is used to identify a device’s functionality and to nominally load a device driver based on that functionality. The information is contained in three bytes with the names Base Class, SubClass, and Protocol. (Note that ‘Base Class’ is used in this description to identify the first byte of the Class Code triple. That terminology is not used in the USB Specification). There are two places on a device where class code information can be placed.One place is in the Device Descriptor, and the other is in Interface Descriptors. Some defined class codes are allowed to be used only in a Device Descriptor, others can be used in both Device and Interface Descriptors, and some can only be used in Interface Descriptors. The table below shows the currently defined set of  Base Class values, what the generic usage is, and where that Base Class can be used (either Device or Interface Descriptors or both).

 

Base Class

Descriptor Usage

Description

00h

Device

Use class information in the Interface Descriptors

01h

Interface

Audio  

02h

Both

Communications and CDC Control

03h

Interface

HID (Human Interface Device)

05h

Interface

Physical

06h

Interface

Image

07h

Interface

Printer

08h

Interface

Mass Storage

09h

Device

Hub

0Ah

Interface

CDC-Data

0Bh

Interface

Smart Card

0Dh

Interface

Content Security

0Eh

Interface

Video

0Fh

Interface

Personal Healthcare

DCh

Both

Diagnostic Device

E0h

Interface

Wireless Controller

EFh

Both

Miscellaneous

FEh

Interface

Application Specific

FFh

Both

Vendor Specific

Base Class 00h (Device)

This base class is defined to be used in Device Descriptors to indicate that class information should be determined from the Interface Descriptors in the device. There is one class code definition in this base class.  All other values are reserved.

 

This value is also used in Interface Descriptors to indicate a null class code triple.

 

Base Class

SubClass

Protocol

Meaning

00h

00h

00h

Use class code info from Interface Descriptors