Human Interface Devices (HID) represent a versatile device class designed for generic USB drivers, accommodating a wide array of peripherals such as keyboards, mice, and gaming controllers. Before the advent of HID, device compatibility relied on rigid protocols primarily tailored for mice and keyboards. Innovation in hardware either necessitated the overloading of existing protocol data or the creation of proprietary, non-standard hardware accompanied by specialized drivers. HID revolutionized this landscape by delivering support for boot mode devices while fostering innovation through extensible, standardized, and readily programmable interfaces, making Human Interface Device Driver development more streamlined.
HID’s adaptability extends to a multitude of devices beyond traditional input tools, encompassing alphanumeric displays, barcode scanners, speakers, headsets, auxiliary screens, sensors, and numerous other peripherals. Furthermore, hardware manufacturers widely adopt HID for their proprietary devices, leveraging its flexibility and broad compatibility.
Initially conceived for USB, HID was architected to be bus-agnostic from its inception. It was optimized for devices demanding low latency and bandwidth, yet provided the flexibility to specify data rates within the underlying transport mechanism. The USB Implementers Forum (USB-IF) officially ratified the specification for HID over USB in 1996. Subsequently, support for HID across various other transport protocols rapidly emerged. Detailed information regarding currently supported transports can be found in HID Transports Supported in Windows. Moreover, custom transport drivers are permitted, enabling vendor-specific transport implementations and further expanding the applicability of human interface device driver technology.
Understanding Core HID Concepts
HID fundamentally revolves around two key concepts: the report descriptor and reports themselves. Reports are the actual data packets exchanged between a device and a software client. Conversely, the report descriptor meticulously outlines the format and semantic meaning of the data that the device is capable of transmitting and receiving, crucial for effective human interface device driver implementation.
Delving into Reports
Applications and HID devices communicate by exchanging data through reports. These reports are categorized into three distinct types, each serving a specific purpose in the communication flow:
| Report Type | Description |
|---|---|
| Input Report | Data transmitted from the HID device to the host application, typically triggered by a change in control state. |
| Output Report | Data sent from the application to the HID device, commonly used to control device features like keyboard LEDs. |
| Feature Report | Data that applications can read and write on demand, typically used for device configuration and settings. |
Each top-level collection defined within a report descriptor can incorporate zero or more reports of each of these types, allowing for flexible and comprehensive human interface device driver designs.
Navigating Usage Tables
The USB-IF, a key standardization body, maintains and publishes HID usage tables. These tables are integral components of report descriptors, defining the functionalities and capabilities of HID devices. HID usage tables provide a structured list of “Usages,” each accompanied by a description elucidating the intended meaning and application of a specific element within the report descriptor. For instance, a dedicated usage is defined for the left button of a mouse, enabling a human interface device driver to interpret mouse inputs correctly. The report descriptor utilizes these usages to specify the precise location within a report where an application can ascertain the current state of the mouse’s left button. To enhance organization and clarity, usage tables are segmented into namespaces known as usage pages. Each usage page groups together a set of related usages. The combination of a usage and its corresponding usage page constitutes the usage ID, which uniquely identifies a specific usage within the broader context of HID usage tables, essential knowledge for developers working on a human interface device driver.
