Some info on USB and usb-keyboards

USB -- Universal Serial Bus

USB is a communications architecture that gives a personal
computer (PC) the ability to interconnect a variety of devices using a simple four-wire cable. The USB is actually a two-wire serial communication link that runs at either 1.5 or 12 megabits per second (mbs). USB protocols can configure devices at startup or when they are plugged in at run time.

These devices are broken into  various device classes. Each device class defines the common behaviour and protocol for devices that serve similar functions.
The HID class consists primarily of devices that are used by humans to control
the operation of computer systems. Typical examples of HID class devices
include:

• Keyboards and pointing devices—for example, standard mouse devices, trackballs, and joysticks.
• Front-panel controls—for example: knobs, switches, buttons, and sliders.
• Controls that might be found on devices such as telephones, VCR remote controls, games or simulation devices—for example: data gloves, throttles, steering wheels, and rudder pedals.
• Devices that may not require human interaction but provide data in a similar format to HID class devices—for example, bar-code readers, thermometers, or voltmeters.

HID1_11 PDF
==>page no. 9
====>Protocol codes

A variety of protocols are supported HID devices. The bInterfaceProtocol
member of an Interface descriptor only has meaning if the bInterfaceSubClass
member declares that the device supports a boot interface, otherwise it is 0.

Code for a USB Keyboard Driver from scratch

Code and Instructions to make this work is available on my github profile.

Much more information and many links are available there..