This page gives an in-depth description of the USB Device Classes that are covered in the USB Middleware Component:

Audio Device Class (ADC) is used to exchange streaming audio data between the USB Host and the USB Device. The USB Component supports the ADC Class for USB Device applications only.
Communication Device Class (CDC) implements virtual communication ports. The USB Component supports the CDC Class for USB Device (ACM and NCM) and USB Host (ACM only) applications.
Human Interface Device (HID) is typically used to implement a keyboard, joystick or mouse. However, the HID class can be also used for simple data exchange. The USB Component supports the HID Class for USB Device and USB Host applications.
Mass Storage Class (MSC) devices are not limited to an USB memory stick. Various devices can be connected to an USB Host using MSC: SD cards, internal or external Flash memory and even a simple RAM disk. The USB Component supports the MSC Class for USB Device and USB Host applications.
Custom Class devices can be used to implement any USB Device Class. The USB Component supports the Custom Class for USB Device and USB Host applications.

ADC: Audio Device Class

USB peripherals that support the Audio Device Class send or receive audio, voice, and other sound-related functionality (such as control data for the audio equipment). These audio functions often come with other functions such as video or storage. Together, they make up a composite device (e.g. a DVD-ROM player that can provide video, audio, data storage and transport control functionality). Audio class devices use isochronous data transfer for audio streams. Digitally encoded music data can be delivered using bulk transfers.

A detailed description about ADC is provided by the USB Implementers Forum (USB-IF).

Note

The ADC implementation in the USB Component supports:
- A subset of the Audio Device Class Release v1.0 specification
- One streaming OUT and/or one streaming IN interface, each configurable in 1 channel (mono) or 2 channels (stereo) mode
- Asynchronous synchronization type
- Full-speed isochronous transfer type (only with one isochronous OUT and/or one isochronous IN endpoint)
- Type I PCM data Format
- One sampling frequency for each streaming interface
- Sample resolutions of 8 bits, 16 bits, 24 bits or 32 bits per sample
- Feature unit with mute and volume controls
The USB Component supports the ADC Class for USB Device applications only.
Further information on how to use ADC is given in the Reference section: ADC: Audio Device Class

CDC: Communication Device Class

The Communication Device Class (CDC) supports a wide range of devices that can perform telecommunications and networking functions. Examples for communications equipment are:

Telecommunications devices, such as analog phones and modems, ISDN terminal adapters, digital phones, as well as COM-port devices
Networking devices, such as ADSL and cable modems, as well as Ethernet adapters and hubs

A detailed description about CDC is provided by the USB Implementers Forum (USB-IF).

CDC Class Features

A communications device has three basic tasks:

Device management (controlling an configuring a specific device and notifying the USB Host of certain events)
Call management (establishing and terminating telephone calls or other connections)
Data transmission (sending and receiving application data)

The CDC implementation in the USB Component features:

Emulation of a Virtual COM-port using the ACM (Abstract Control Model) subclass of CDC.
Emulation of network connectivity using the RDNIS protocol using the ACM (Abstract Control Model) subclass of CDC. This enables network connections over USB between a Windows host PC and an embedded device, as well as USB Device RNDIS to Ethernet Bridge applications.
Emulation of an Ethernet adapter using the NCM (Network Control Model) subclass of CDC (only available for USB Device). Using CDC (NCM), you can create Ethernet-over-USB (for Linux hosts) applications on Linux-based host systems.

Note

The following endpoint configurations are supported:

One interrupt IN endpoint for notifications to the USB Host
One bulk IN and one bulk OUT endpoint for data transfer

Control Transfers

The documents available at Communications Device Class describe the nine available request types for the Abstract Control Model (ACM). All requests specified for the Network Control Model (NCM) by the documents in ECM120.pdf and NCM10.pdf are supported. are not supported.

USB Descriptors

The following descriptors are required in an USB CDC ACM Device:

Standard Device Descriptor
Standard Configuration Descriptor
Interface Association Descriptor
CDC Header Functional Descriptor
CDC Union Functional Descriptor
Call Management Functional Descriptor
Abstract Control Management Functional Descriptor
Standard Interface Descriptor for the CDC Class communication interface
Standard Endpoint Descriptor for Interrupt IN endpoint
Standard Interface Descriptor for the CDC Class data interface
Standard Endpoint Descriptors for Bulk IN and Bulk OUT endpoints

The following descriptors are required in an USB CDC NCM Device:

Standard Device Descriptor
Standard Configuration Descriptor
Interface Association Descriptor
CDC Header Functional Descriptor
CDC Union Functional Descriptor
CDC Ethernet Networking Functional Descriptor
NCM Functional Descriptor
Standard Interface Descriptor for the CDC Class communication interface
Standard Endpoint Descriptor for Interrupt IN endpoint
Standard Interface Descriptor for the CDC Class data interface
Standard Endpoint Descriptors for Bulk IN and Bulk OUT endpoints

The necessary descriptors are automatically generated by the USB Middleware Component. The page USB Descriptors provides more information on the topic.

Note

Further information on how to use CDC is given in the Reference section:

HID: Human Interface Device Class

The Human Interface Device Class (HID) is mainly used for devices that allow human control over a PC. Using these devices, the host is able to react on human input (e.g. movements of a mouse or key presses). This response has to happen quickly, so that the computer user does not notice a significant delay between his action and the expected feedback. Typical examples for HID class devices are:

Keyboards and pointing devices (such as mouse devices, joysticks and trackballs)
Front-panel controls (for example buttons, knobs, sliders, and switches)
Simulation devices (such as steering wheels, pedals, other VR input devices)
Remote controls and telephone keypads
Other low data-rate devices that provide for example environmental data (such as thermometers, energy meters or even bar-code readers)

A detailed description about HID is provided by the USB Implementers Forum (USB-IF).

HID Class Features

The HID class is not necessarily a human interface. But a device using the HID class has to be able to work within the limits of the HID class. These HID class implementation of the USB Component has the following features:

All data is exchanged in reports. These are fixed-length structures that sent or requested by the USB Host in control or interrupt transfers. Reports have a flexible format and can contain any type of data. Every HID device needs to have one input report in its report descriptor. Output and feature reports are optional.
An interrupt IN endpoint is required for sending input reports to the USB Host.
The maximum number of interrupt IN and OUT endpoints is limited to 1.
The interrupt OUT endpoint is optional.
As the HID device can send data at any point in time using the interrupt IN endpoint, the USB Host driver needs to make sure that the data is polled periodically.
The USB Component supports the HID Class for USB Device and USB Host applications.

Control Transfers

The HID specification defines six class-specific requests. They enable the USB Host to inquire about the capabilities and the current state of the device. Furthermore, the host can set the state of output and feature items. All six requests are transmitted using the control pipe.

The Get_Report request enables the USB Host to receive a report via the control pipe
The Set_Report request allows the USB Host to send a report to the device (for example for setting device states)
The device's current idle rate can be read using the Get_Idle request. The idle rate determines how often a device resends data that has not changed since the last report.
The idle rate can be set using the Set_Idle request. This is used to limit the reporting frequency of an interrupt IN endpoint. Default idle rates are 500 milliseconds for keyboards (delay before first repeat rate) and infinity for joysticks and mouse devices.
During booting of the USB Host, a simplified protocol can be used for communication. The Get_Protocol request reads which protocol is currently active. The Set_Protocol request switches between the boot protocol and the report protocol.

Interrupt Transfers

To ensure minimum data delay, interrupt endpoints are used. The bandwidth for interrupt transfers is guaranteed for every device after successful enumeration. Control endpoints may face data delivery delay in case the bus is busy.

USB Descriptors

The following descriptors are required in an USB HID Device:

Standard Device Descriptor
Standard Configuration Descriptor
Standard Interface Descriptor for the HID Class
Class-Specific HID Descriptor
Standard Endpoint Descriptor for Interrupt IN endpoint
Class-Specific Report Descriptor

The necessary descriptors are automatically generated by the USB Middleware Component. The report descriptor is built based on the settings in the USBD_Config_HID_x.h file. The number of reports and their maximum size are specified in this file. The page USB Descriptors provides more information on the topic.

Note

Further information on how to use HID is given in the Reference section:

MSC: Mass Storage Class

The Mass Storage Class (MSC) is mainly used for devices that allow access to their internal data storage. Typical examples for MSC class devices are:

External hard drives (HDD)
External optical drives (such as CD or DVD drives)
Portable Flash memory devices
Solid-state drives (SSD)
Digital cameras
Card readers
Mobile phones

A detailed description about MSC is provided by the USB Implementers Forum (USB-IF).

MSC Class Features

Not only hard disks use the MSC class. Any device that allows access to its internal data storage using MSC can be connected to the USB Bus as a mass storage device. The MSC class implementation in the USB Component has the following features:

It supports the bulk-only transport (BOT) protocol. The BOT protocol specifies that not only data is transferred using bulk transfer, but also command and status data.
The maximum number of bulk IN and OUT endpoints is fixed to 1.
USB Device supports multiple logical units (LUNs) which can operate separately, for example one unit could have an SD card as media and another one could have a RAM disk as media.
The USB Component supports the MSC Class for USB Device and USB Host applications.

Control Transfers

The MSC specification defines five class-specific requests to be transmitted over the default (control) pipe. Only two of them are supported by the bulk-only protocol:

The Bulk Only Mass Storage Reset request enables the USB Host to reset the MSC device and its associated interface.
The Get Max LUN request allows the USB Host determine the number of Logical Units (LUNs) that are supported by the device. LUNs are numbered starting from LUN 0 to LUN 15 (maximum).

Bulk Transfers

The bulk-only protocol divides a successful data transfer into three stages:

Command transport: the USB Host sends a command in the Command Block Wrapper (CBW) structure
Data transport: the data is sent to or from the USB Host
Status transport: the USB Device sends status information in the Command Status Wrapper (CSW) structure

USB Descriptors

The following descriptors are required in an USB MSC Device:

Standard Device Descriptor
Standard Configuration Descriptor
Standard Interface Descriptor for the MSC Class
Standard Endpoint Descriptors for Bulk IN and Bulk OUT endpoints
Standard String Descriptor

The necessary descriptors are automatically generated by the USB Middleware Component. The page USB Descriptors provides more information on the topic.

Note

Further information on how to use MSC is given in the Reference section:

Custom Class

USB Host supporting any Device Class

The Custom Class for USB Host has two use cases:

Implementing/supporting a standard Device class other than CDC, HID and MSC (also CDC, HID and MSC class handling can be overridden).
Implementing/supporting a vendor specific Device class.

Supporting Custom or Standard Class Devices requires the USB Host to be aware of the specific Class. On a PC, it is easy to add Class support by simply installing the required driver. However, in an embedded system this is not possible.

The Custom Class are a starting point for adding support for any USB Device Class. Furthermore, all Class Specific Requests need to be handled by the application running on the USB Host system. For more information on these requests, please consult the USB-IF Device Class Documents. The user code template file USBH_User_CustomClass.c contains all the functions that require adaptation.

The number of concurrent Custom Class Devices in a system can be configured using the USBH_Config_CustomClass.h configuration file.

USB Device implementing any Device Class

The Custom Class for USB Device has two use cases:

Implementing/supporting a standard Device class other than ADC, CDC, HID, and MSC.
Implementing/supporting a vendor specific Device class.

The Custom Class gives full control of the USB handling. It is designed to react on any kind of event on any of the assigned endpoints and to handle any control message that may reach the system. The application has the full capability of dealing with control messages for whatever USB Device class. Message handling control can be taken over from the USB Device Core. The user code template file USBD_User_Device.c contains all functions used for Device level Control Endpoint 0 (STANDARD, CLASS and VENDOR requests) message interception and handling, whereas file USBD_User_CustomClass.c contains all the functions used for Custom Class Control Endpoint 0 CLASS requests and Custom Class endpoint events handling.

To create the Custom Class the configuration file providing up to four interfaces with up to 8 endpoints has been provided.

USB Descriptors

Every USB Device requires certain descriptors. The following list is not complete but gives an impression about the various descriptors that may be required:

Standard Device Descriptor
Standard Configuration Descriptor
Standard Interface Descriptor for the Class that is implemented
Standard Endpoint Descriptors for IN and OUT endpoints
Standard String Descriptor

The necessary descriptors are automatically generated by the USB Middleware Component from the Class, Subclass, and Protocol Codes and the Interface String specified in the USBD_Config_CustomClass_n.h file. The page USB Descriptors provides more information on the topic.

Note

Further information on how to use the Custom Class in a USB Device is given in the Reference section: