A HID device needs a standard way to tell programmers what their device is: what Input. Output, and Feature reports it can send and receive. Discovering this information is known as getting a device's capabilities.

Each control or collection of controls is assigned a usage that describes its purpose. The HID Specifications list a large number of usages for a range of standard devices. (I have yet to see a device with a "Do Not Disturb" LED. Did you know that a magic carpet has standard controls?)

A usage is, in fact, represented by two bytes: a Usage Page and a Usage. Table 22.1 shows the main usage categories.

Table 22.1 Main Usage categories

Note that usages are used both to specify the general characteristics of a device (i.e., of an Application collection) and the specifics of each control or group of controls.

The Application collection usage defines the overall purpose of a report. For example, a keyboard usually has an Application collection with a Usage Page of "Generic Desktop" and a Usage of "keyboard". The hidusage.h file defines these as byte constants HID_USAGE_PAGE_GENERIC and HID_USAGE_GENERIC_KEYBOARD, respectively. In most Usage Pages, Usage values in the range 1 to 0x1F usually indicate the overall purpose of a device.

The Application collection usage sometimes implies what controls should be in the report. A keyboard usually has eight input single-bit values, eight reserved bits, five output bits, three padding output bits and six input 8-bit values. Although some clients rely on devices having specific controls, most should be able to cope with different devices (e.g., with more controls) or controls in different reports.

Each control also has a usage. The eight single-bit input values in a keyboard report represent the modifier keys (i.e., whether a Shift, Ctrl, or Alt key is pressed at the same time). These modifiers are defined in the "Keyboard" Usage Page (HID_USAGE_PAGE_KEYBOARD). The first single bit value represents the state of the left control key and has a Usage of HID_USAGE_ KEYBOARD_LCTRL.

The HID Specification defines Usage values in the "Keyboard" Usage Page that represent all the common keyboard key scan codes. PC-AT keyboards return usages in the range 0 to 101.

However, the left control key has a usage of 240, HID_USAGE_KEYBOARD_LCTRL As described previously, this is usually returned as a bit value to indicate whether the key is pressed or not.

The HID Specification lets you define several reports in a report descriptor, each with different report IDs. However most Report descriptors do not use report IDs at all. For example, the keyboard Application collection just defines one report with both input and output controls in it. However, a keyboard with a built-in pointing device may have two Application collections with two reports, a keyboard report, and a pointer report.