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Overview

A HID device is composed of the following endpoints:

  • A pair of control IN and OUT endpoints called the default endpoint.
  • An interrupt IN endpoint.
  • An optional interrupt OUT endpoint.

describes the usage of the different endpoints:

Report

A host and a HID device exchange data using reports. A report contains formatted data giving information about controls and other physical entities of the HID device. A control is manipulable by the user and operates an aspect of the device. For instance, a control can be a button on a mouse or a keyboard, a switch, etc. Other entities inform the user about the state of certain device’s features. For instance, LEDs on a keyboard notify the user about the caps lock on, the numeric keypad active, etc.

The format and the use of a report data is understood by the host by analyzing the content of a Report descriptor. Analyzing the content is done by a parser. The Report descriptor describes the data provided by each control in a device. It is composed of items. An item is a piece of information about the device and consists of a 1-byte prefix and variable-length data. Refer to “Device Class Definition for Human Interface Devices (HID) Version 1.11”, section 5.6 and 6.2.2 for more details about the item format.

There are three principal types of items:

  • Main item defines or groups certain types of data fields.
  • Global item describes data characteristics of a control.
  • Local item describes data characteristics of a control.

Each item type is defined by different functions. An item function can also be called a tag. An item function can be seen as a sub-item that belongs to one of the three principal item types.  gives a brief overview of the item’s functions in each item type. For a complete description of the items in each category, refer to “Device Class Definition for Human Interface Devices (HID) Version 1.11”, section 6.2.2.

A control’s data must define at least the following items:

  • Input, Output or Feature Main items.
  • Usage Local item.
  • Usage Page Global item.
  • Logical Minimum Global item.
  • Logical Maximum Global item.
  • Report Size Global item.
  • Report Count Global item.

shows the representation of a Mouse Report descriptor content from a host HID parser perspective. The mouse has three buttons (left, right and wheel). The code presented in  is an example of code implementation corresponding to this mouse Report descriptor representation.

After analyzing the previous mouse Report descriptor content, the host’s HID parser is able to interpret the Input report data sent by the device with an interrupt IN transfer or in response to a GET_REPORT request. The Input report data corresponding to the mouse Report descriptor shown in  is presented in . The total size of the report data is 4 bytes. Different types of reports may be sent over the same endpoint. For the purpose of distinguishing the different types of reports, a 1-byte report ID prefix is added to the data report. If a report ID was used in the example of the mouse report, the total size of the report data would be 5 bytes.

A Physical descriptor indicates the part or parts of the body intended to activate a control or controls. An application may use this information to assign a functionality to the control of a device. A Physical descriptor is an optional class-specific descriptor and most devices have little gain for using it. Refer to “Device Class Definition for Human Interface Devices (HID) Version 1.11” section 6.2.3 for more details about this descriptor.

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