This reference design from Texas Instruments shows how to build a USB keyboard design using a MSP430 microcontroller. The article contains the full design: the keyboard matrix, schematic, and the firmware needed to get it going.
This application report describes the implementation of a USB keyboard with the following characteristics:
• 101 keys, 2 LEDs: standard HID keyboard and LED usage
• 16×8 matrix: allows for easy customization of different keyboard layouts
• Composite USB device: In addition to the keyboard interface, it includes an HID-datapipe back-channel which can be used to transmit any custom data
• HID boot protocol support, allowing keyboard to be used to interface with a PC’s BIOS
• “Ghost” key handling in software, to prevent errors from multiple key presses
• Uses MSP430F550x/5510 low-cost USB family