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falshing pro micro DFU-QMK bootloader and seting fuses so its not stuck in DFU mode

no cap needed

avrdude.exe -p atmega32u4 -c stk500v1 -b 19200 -U lfuse:w:0x5e:m -U hfuse:w:0x99:m -U efuse:w:0xf3:m -U flash:w:"bootloader/promicro_dfu.hex":i -P PROGRAMMER_COM_PORT

env setup

programming layout, has to be run for each side

make handwired/dactyl_promicro:default:dfu-split-left
make handwired/dactyl_promicro:default:dfu-split-right

main side is defined by

#define MASTER_LEFT in config.h

and the master becomes the side to which the USB is connected to.

Required Hardware

Apart from diodes and key switches for the keyboard matrix in each half, you will need:

  • 2 Arduino Pro Micros. You can find these on AliExpress for ≈3.50USD each.
  • 2 TRRS sockets and 1 TRRS cable, or 2 TRS sockets and 1 TRS cable

Alternatively, you can use any sort of cable and socket that has at least 3 wires. If you want to use I2C to communicate between halves, you will need a cable with at least 4 wires and 2x 4.7kΩ pull-up resistors

Optional Hardware

A speaker can be hooked-up to either side to the 5 (C6) pin and GND, and turned on via AUDIO_ENABLE.


left right

The 3 wires of the TRS/TRRS cable need to connect GND, VCC, and digital pin 3 (i.e. PD0 on the ATmega32u4) between the two Pro Micros.

Next, wire your key matrix to any of the remaining 17 IO pins of the pro micro and modify the matrix.c accordingly.

The wiring for serial:

serial wiring

The wiring for i2c:

i2c wiring

The pull-up resistors may be placed on either half. It is also possible to use 4 resistors and have the pull-ups in both halves, but this is unnecessary in simple use cases.

You can change your configuration between serial and i2c by modifying your config.h file.

Notes on Software Configuration

the keymaps in here are for the 4x5 layout of the keyboard only.


To flash your firmware take a look at: Flashing Instructions and Bootloader Information

Choosing which board to plug the USB cable into (choosing Master)

Because the two boards are identical, the firmware has logic to differentiate the left and right board.

It uses two strategies to figure things out: looking at the EEPROM (memory on the chip) or looking if the current board has the usb cable.

The EEPROM approach requires additional setup (flashing the eeprom) but allows you to swap the usb cable to either side.

The USB cable approach is easier to setup and if you just want the usb cable on the left board, you do not need to do anything extra.

Setting the left hand as master

If you always plug the usb cable into the left board, nothing extra is needed as this is the default. Comment out EE_HANDS and comment out I2C_MASTER_RIGHT or MASTER_RIGHT if for some reason it was set.

Setting the right hand as master

If you always plug the usb cable into the right board, add an extra flag to your config.h


Setting EE_hands to use either hands as master

If you define EE_HANDS in your config.h, you will need to set the EEPROM for the left and right halves.

The EEPROM is used to store whether the half is left handed or right handed. This makes it so that the same firmware file will run on both hands instead of having to flash left and right handed versions of the firmware to each half. To flash the EEPROM file for the left half run:

make handwired/dactyl_promicro:default:dfu-split-left
make handwired/dactyl_promicro:default:dfu-split-right

After you have flashed the EEPROM, you then need to set EE_HANDS in your config.h, rebuild the hex files and reflash.

Note that you need to program both halves, but you have the option of using different keymaps for each half. You could program the left half with a QWERTY layout and the right half with a Colemak layout using bootmagic's default layout option. Then if you connect the left half to a computer by USB the keyboard will use QWERTY and Colemak when the right half is connected.

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