HID USB Minidriver

HID evolved from the USB specification, but is now a general purpose input device model. Nonetheless, HID fits neatly into the USB model. The HID class is one of the classes defined in the USB Specifications.

At present, the USB bus is one of the few places where HID devices can be found. The HID class driver talks to the USB device stack through the HidUsb.sys driver.

A HID USB device has the standard USB device, configuration, interface, and endpoint descriptors. In addition, it has the class-specific HID, Report, and Physical descriptors described in the previous chapter.

The standard USB device descriptor has zeroes for its class, subclass, and protocol. The standard interface descriptor has constant USB_DEVICE_CLASS_HUMAN_INTERFACE (0x03) in its bInterfaceClass field, iInterfaceSubClass is set to 1 if the device supports a boot protocol, described in the following text. In this case, bInterfaceProtocol is 1 for a keyboard and 2 for a mouse.

A USB request for the Configuration descriptor usually returns the HID descriptor before the endpoint descriptors^[57]. The report and physical descriptors must be specifically asked for, using the following codes.

A HID USB device uses the default control pipe endpoint 0 and one interrupt pipe. The control pipe is used for standard USB purposes and for sending and receiving reports, etc. The interrupt pipe is used to return new input reports.

USB HID devices support the notion of an idle rate. Whenever an interrupt request is received from the host, the USB HID device can either return a report or a NAK to indicate that nothing has changed. In the former case, no input state may have changed and so host processing of the report will waste time. The idle rate determines how often the HID USB device returns a report. If at any time during the idle rate period, an input report changes, the HID USB device does return an input report at the next interrupt. The recommended idle rate for keyboards is 500ms and infinity for joysticks and mice.

The following HID-specific class requests are supported on the default pipe. GET_REPORT and SET_REPORT get or send a specific input, output, or feature report. An idle rate can be defined using SET_IDLE and read using GET_IDLE. SET_PROTOCOL and GET_PROTOCOL are used for boot devices, as described in the following text.