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Understanding USB HID

In computing, the USB HID / USB Human Interface Device class is a part of the comprehensive USB specification for computer/electronic peripherals that specifies a device class (a type of hardware) for human interface devices such as keyboards, mice, game controllers and display devices.

The USB HID class is defined in a number of documents provided by the USB Implementers Forum's Device Working Group. The primary document used to describe the USB HID class is the Device Class Definition for HID 1.11.

The USB HID class describes devices used with nearly every modern computer and incorporated in many modern "smart" electronic equipment. Since there are many basic communication functions are predefined (abstracted) in the USB HID class, hardware manufacturers should adhere to the USB HID class specifications and expect it to work with any software that also meets these specifications. This allows standardized devices operate interchangeable across wide range of equipment, no matter how different they are in their original nature.

The same HID protocol is used unmodified in Bluetooth Human Interface Devices. The Bluetooth profile specification only points readers to the USB HID documentation. In this sense those devices also belong to the USB HID class.

Examples of such devices:

• Keyboards
  Keyboards are some of the most popular USB HID class devices. The USB HID class keyboard is normally designed with an IN endpoint that communicates keystrokes to the computer and an OUT endpoint that communicates the status of the keyboard's LEDs from the computer to the keyboard. The PC 97 standard requires that a computer's BIOS must detect and work with USB HID class keyboards that are designed to be used during the boot process.
  
• Mice
  Computer mice are almost equally popular USB HID class devices. USB HID mice can range from single-button simple devices to multi-button compound devices. Most modern operating systems ship with drivers for standard HID mice designs (the most common modern mouse design has two dedicated buttons and a mouse wheel that doubles as the third button); mice with extended functionality require custom drivers from the manufacturer.
  
• Game controllers
  Modern game controllers and joysticks are often USB HID class devices. Unlike legacy game port devices, USB HID class game devices do not normally require proprietary drivers to function. Nearly all game devices will function using onboard drivers as long as the device is designed around the drivers and the USB HID class specifications.
  
• Other devices
  The USB HID class specifications allow for myriad other devices under the USB HID class. Some examples are automobile simulation controllers, exercise machines, telephony devices, thermometers, audio controls and medical instrumentation. Even uninterruptible power supplies and software protection dongles declare themselves under this class, despite the fact they often have no human interface at all. Any device can be a USB HID class device as long as a designer meets the USB HID class logical specifications. This is not to say that there is no need to ship drivers for these devices, nor that an operating system will immediately recognize the device. This only means that the device can declare itself under the human interface device class.

One of the benefits of a well-defined specification like the USB HID class is the abundance of device drivers available in most modern operating systems. The USB HID class devices and their basic functions are defined in USB-IF documentation without any specific software in mind. Because of these generic descriptions, it is easy for operating system designers to include functioning drivers for devices such as keyboards, mice, and other generic human interface devices. The inclusion of these generic drivers allows for faster deployment of devices and easier installation by end-users.

USB HID Logical Specifications

The USB human interface device class can be used to describe both device and interface classes. The interface class is used when a USB device can contain more than one function. It is possible, therefore, to have USB devices with two different interfaces at the same time (for example, a USB telephone may use a keypad covered by the HID class and a speaker covered by the USB audio device class).

The interface devices are also defined with subclass descriptors. The subclass descriptor is used to declare a device bootable. A boot device meets a minimum adherence to a basic protocol and will be recognized by a computer's BIOS.

Each USB HID interface communicates with the host using either a control pipe or an interrupt pipe. Isochronous and bulk pipes are not used in HID class devices. Both IN and OUT control transfers are required for enumeration; only an IN interrupt transfer is required for HID reports. OUT interrupt transfers are optional in HID-class devices.

The USB HID class requires that every device describes how it will communicate with the host device in order to accurately predict and define all current and future human interface devices. During enumeration the device describes how its reports are to be structured so that the host device can properly prepare to receive this information.

When user operates HID device, the device produces a piece of data called report. Computer learns what happened by polling device from time to time, parsing received reports and changes program flow accordingly. Devices operate with many different types of information – for example, keyboard has many buttons and sends key codes, mouse has just a few buttons so it sends just the state of those buttons, but is also capable to report its’ X and Y coordinates, while a steering wheel-type game controller sends wheel and pedal positions along with button presses. At the same time, various data can be sent from a computer to the device – LEDs on a keyboard or force-feedback on joystick or game controller, just to name a few. Simple devices, like mouse or keyboard, usually generate single report, while more complex devices often generate several.

A report is simple data structure, in most cases less than 10 bytes long. Format of this report is contained in much bigger and complex data structure called report descriptor. Report descriptor outlines what is contained in each byte (sometimes even each bit) of the report, type of data, units of measurement, range of values and other good stuff. Therefore, the format of report can be (and often is) determined by parsing report descriptor. The format and contents of report descriptors are well documented. The USB.org website has HID Page containing many useful documents, the main two being Device Class Definition for Human Interface Devices and HID Usage Tables. These two documents give good picture of what kind of information may be expected from HID device. In addition to this, many web resources exist presenting topic of HID formats in more humane way – googling for “HID format”, “HID report”, etc., produces plenty of links to HID-related content.

The host periodically polls the device's interrupt IN endpoint during operation. When the device has data to send it forms a report and sends it as a reply to the poll token. Common devices such as keyboards and mice send reports that are compliant with standards set by the USB Implementers Forum (USB-IF). When a vendor makes a custom USB HID class device, the reports formed by the device need to match the report description given during enumeration and the driver installed on the host system. In this way it is possible for the USB HID class to be extremely flexible.

USB API

There are two levels of APIs related to USB HID: the USB level and the operating system level. At the USB level, there is a protocol for devices to announce their capabilities and the operating system to parse the data it gets. The operating system then offers a higher-level view to applications, which do not need to include support for individual devices but for classes of devices. This abstraction layer allows a game to work with any USB controller, for example, even ones created after the game.

For more information, visit these sites:

• USB.org: The homepage of the USB Implementers Forum, Inc.
• USB in a Nutshell 
• Microsoft Related HID Documentation: The USB-IF's collection of Microsoft's HID documentation.
• USB-IF HID Tools: The USB-IF's page devoted to human interface devices. Includes all approved documentation.
• Lakeview Research HID Page: A collection of articles about, and example code for USB HID devices.
• PC System Guides: Microsoft's PC System specifications (e.g. PC '97, PC '98).

For Arduino related content, visit/download:

• USB HID Descriptor Parser Arduino Sketch