Overview

This project is intended to provide a clean and reliable mechanism for programming a microcontroller to emulate a USB keyboard, using this set up:

  +--------------------+  USB                                (RasPi Zero)
  |      PC/Laptop     |<-----> UART-USB Bridge         +-------------------+
  | (Serial Interface) |         (ex: CP2102)           | Target System USB |
  +--------------------+             ^                  +-------------------+
                                     |                             ^
                         Tx -> Pin 7 |                             |
                         Rx -> Pin 8 |                             | USB -> USB
                                     |  (Arduino Uno or Pro Mini)  |
                              +----- V --------------------------- V -+
                              |  MCU Board IO Pins <-> MCU Board USB  |
                              +---------------------------------------+

The MCU will relay keystrokes from the UART to the USB interface, handling the scan code mapping so that the USB port appears as a keyboard (or some type of HID) to the OS on the target system.

Details

This program allows an Arduino Uno to simulate a USB keyboard on it's USB port. The Uno reads it's input from a Software Serial connection, translates to the corresponding USB scan code, constructs the IEEE standardized keystroke data buffer, and sends that data buffer to the target system via the USB port.

The input connection to the Arduino is from a computer running a serial terminal program, using a UART-USB bridge adapter connected to the computers USB port. The adapters Rx/Tx pins are wired to the appropriate digital I/O pins on the Arduino, and the ground pin of the bridge adapter bonded to the ground pin on the Arduino. The shared ground is needed to prevent Arduino from picking up noise on the serial connection from a floating ground and treating it as valid traffic-- and writing lots of garbage to the output... It's best to just bond all of the grounds from each board and avoid problems.

Programming Steps

Connect the Arduino:

  (Arduino 7) -> (UART Tx) 
  (Arduino 8) -> (UART Rx)
  (Arduino Ground) -> (UART Ground) -> (RasPi Ground)
  (Arduino USB) -> (RasPi USB)

An easy way to tell if things worked is to plug the USB back into the same PC used to program the MCU, so both the USB and UART are connected, and open a serial terminal emulator and connect to the UART device using unbuffered key mode. The first key stroke should start a feedback loop and print forever. The OS should also recognize the USB device as a keyboard, or at least as some type of HID.

To Use as keyboard, flash with keyboard firmware:

bash-$ ./etc/program_keyboard.sh
[...]

or do it manaully:

bash-$ make clean
[...]
bash-$ make
[...]
bash-$ dfu-programmer atmega16u2 erase
Checking memory from 0x0 to 0x2FFF...  Not blank at 0x1.
Erasing flash...  Success
bash-$ dfu-programmer atmega16u2 flash kbd.hex 
Checking memory from 0x0 to 0xFFF...  Empty.
0%                            100%  Programming 0x1000 bytes...
[>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>]  Success
0%                            100%  Reading 0x3000 bytes...
[>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>]  Success
Validating...  Success
0x1000 bytes written into 0x3000 bytes memory (33.33%).
bash-$ dfu-programmer atmega16u2 reset

To restore the standard arduino USB firmware:

bash-$ ./etc/program_usb.sh
[...]

or do it manually:

bash-$ dfu-programmer atmega16u2 erase
Checking memory from 0x0 to 0x2FFF...  Not blank at 0x1.
Erasing flash...  Success
bash-$ dfu-programmer atmega16u2 flash usb.hex 
Checking memory from 0x0 to 0xFFF...  Empty.
0%                            100%  Programming 0x1000 bytes...
[>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>]  Success
0%                            100%  Reading 0x3000 bytes...
[>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>]  Success
Validating...  Success
0x1000 bytes written into 0x3000 bytes memory (33.33%).
bash-$ dfu-programmer atmega16u2 reset