TMK Firmware

How you upload the firmware to your Arduino, depends on your computer’s operating system. As I prefer to work with macOS, this guide uses tools and commands that work fine on a current High Sierra v10.13 machine.

USB Cable

You need a Micro-USB cable to connect the Arduino Pro Micro. The cable is required for uploading a new firmware and using the keyboard afterward. You can just order any ordinary cable from the Shopping List, or look for a vendor that makes awesome-looking custom cables.

Do you work for a company that offers custom USB cables and you can provide a discount code? I would love to feature your business and services here! Please let me know.

If you use the Shrink Kit, you can use already use the Mini-USB cable to flash the Arduino. The connection works for using the keyboard, as well as uploading a new firmware to it.


To flash your Arduino with software, and for compiling the firmware, you must install a few tools like avr-gcc and avrdude. Both are available on Homebrew, but I recommend the CrossPack by ObDev.

CrossPack for AVR

DMG, 42 MB

CrossPack for AVR Development is distributed under the terms of the GNU General Public License version 2, except AVR Libc which is distributed under a more liberal license. See the manual for the full text of these licenses.


The firmware is based on TMK and customized by di0ib to work with the Arduino Pro Micro on the board. All sources for the keyboard are available on GitHub using the GPL v2 license.

$ > git clone
$ > cd tmk_keyboard/keyboard/68Keys

The repository already includes a default keyboard configuration that you can use right away available in the keymap_68Keys.c file.

#include "keymap_common.h"

const uint8_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
      GRV,  1,    2,    3,     4,    5,    6,    7,    8,    9,    0,     MINS, EQL,  BSPC,   ESC,  PGUP,
      TAB,  Q,    W,    E,     R,    T,    Y,    U,    I,    O,    P,     LBRC, RBRC, BSLS,   DEL,  PGDN,
      FN0,  A,    S,    D,     F,    G,    H,    J,    K,    L,    SCLN,  QUOT, ENT, 
      LSFT, Z,    X,    C,     V,    B,    N,    M,    COMM, DOT,  SLSH,        RSFT,         UP,
      LCTL, LALT, LGUI,                 SPC,                 RGUI, RALT,  RCTL,         LEFT, DOWN, RGHT
      GRV,  F1,   F2,   F3,    F4,   F5,   F6,   F7,   F8,   F9,   F10,   F11,  F12,  TRNS,   TRNS, HOME,
      TRNS, TRNS, TRNS,                 TRNS,                TRNS, TRNS,  APP,          MPRV, VOLD, MNXT

const action_t PROGMEM fn_actions[] = {

Basically, I moved ESC to the right side and replaced Caps Lock on my keyboard with FN. Holding FN enables the second keyboard layer, which includes media controls and additional arrow keys. Most keys do not have an extra function, adding TRNS just passes through the normal key.

Compile your Firmware

After you have installed all needed dependencies, compile the firmware with your custom keyboard layout:

$ > make clean
$ > make all


   text	   data	    bss	    dec	    hex	filename
  22930	     56	    230	  23216	   5ab0	68Keys.elf

When the command finishes without errors, you end up with a few 68Keys.* files. In the next step will upload the 68Keys.hex file to your Arduino board.

Upload Firmware

Connect the Micro-USB cable to your Arduino Pro Micro and your computer. The board should be listed using ls /dev/tty.* . If you cannot find your board, first check if the green light on the Arduino is on.

$ > ls /dev/tty.*

crw-rw-rw-  1 root  wheel   21,   4 Jan 24 12:06 /dev/tty.Bluetooth-Incoming-Port
crw-rw-rw-  1 root  wheel   21,   2 Jan 24 12:06 /dev/tty.MALS
crw-rw-rw-  1 root  wheel   21,   0 Jan 24 12:06 /dev/tty.SOC
crw-rw-rw-  1 root  wheel   21,   0 Jan 24 12:06 /dev/tty.usbmodem1441

If your computer detected the Arduino board correctly, it’s time to upload your custom firmware to it. You can use the Push Button to enter the Bootload on your Arduino. Press it twice directly after re-connecting the board to your computer and you have 8 seconds to start with uploading your firmware.

$ > avrdude \
    -patmega32u4 -cavr109 -b57600 -D -V \
    -Uflash:w:68Keys.hex:i -P/dev/tty.usbmodem1441

On some systems, the number following usbmodem increases every time you re-connect the board. On others, it stays the same or just switches to a different number when the Bootloader is enabled. Just run ls /dev/tty.* after you connected the board or enabled the Bootloader to figure this out.

After you hit enter and the Arduino is ready to receive the firmware, avrdude will start to transfer the data and show you the progress:

Connecting to programmer: ...

Found programmer: Id = "CATERIN"; type = S

Programmer supports the following devices:
    Device code: 0x44

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.01s

avrdude: Device signature = 0x1e9587
avrdude: reading input file "68Keys.hex"
avrdude: writing flash (22948 bytes):

Writing | ################################################## | 100% 1.78s

avrdude: 22948 bytes of flash written

avrdude: safemode: Fuses OK (H:CB, E:D8, L:FF)

avrdude done.
Thank you.

Make sure everything works

You can install lsusb using Homebrew to list all connected USB devices. If you uploaded the firmware to the Arduino, you computer should list the keyboard with its name.

$ > lsusb

Bus 020 Device 004: ID feed:0a0c feed  The Keyboard
Bus 000 Device 001: ID 1d6b:ISPT Linux Foundation USB 3.0 Bus
Bus 001 Device 001: ID 1d6b:CIAR Linux Foundation USB 3.1 Bus
Bus 000 Device 001: ID 1d6b:CIAR Linux Foundation USB 3.1 Bus

Finish the Sandwich Case

After the successful upload of your firmware, you should finally be able to use the keyboard after re-connecting it to your computer. If everything works as expected, it’s time to close the Sandwich Case and you’re done!

Make sure the Micro-USB cable is connected to the Arduino board and use the M3 Screws and Standoffs to close the Sandwich Case.

Custom 68% Mechanical Keyboard from


Woow, that was a ride! Thanks for reading this guide, I hope you enjoyed building your new mechanical keyboard. I would love to hear your feedback and see the results of your work, please share them on twitter. What key caps did you use for your keyboard? The SA Grade set is a perfect match! 😍

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68 Keys