This repository is a fork of the official Adafruit repository. My keyboards use external LED indicators, so I have made changes that allow them to display the bootloader status. The original code supports blinking only a single LED, whereas my version adds the ability to work with multiple indicators. If you have any questions unrelated to indicators, you should probably contact the creators of the original repository.

I do not describe some details that are not important to me. I am only interested in building UF2 bootloaders for nRF52 locally in Fedora or using GitHub Actions.

• nice_nano_LED
• nice_nano_RGB
• kabarga
• PNCATEHO MK Dose
• mouse

The mouse config was originally published here.

In most cases, you just need to connect the keyboard to your computer, double-press the MCU reset button, and copy the UF2 bootloader file to the newly appeared device. If you need to flash the bootloader using a programmer, follow this guide.

You should only continue if you are looking to develop bootloader for your own. You must have have a J-Link available to "unbrick" your device.

To create your own bootloader variant for n!n, copy the src/boards/nice_nano_LED folder and rename it, for example, to kabarga. Then, edit the board.h file inside this folder.

Set LED_PRIMARY_PIN and LED_SECONDARY_PIN to the appropriate LED pins. Also, replace the following lines:

#ifndef _NICENANO_LED_H  
#define _NICENANO_LED_H  

with

#ifndef _KABARGA_H  
#define _KABARGA_H  

At the bottom of the file, you can change the device name.

Finally, build your bootloader using the command:

BOARD=kabarga all

The lighting parameters are configured in the src/boards/boards.c file, starting from line 365.

To create your own bootloader variant for n!n, copy the src/boards/nice_nano_RGB folder and rename it, for example, to kabarga_RGB. Then, edit the board.h file inside this folder.

Set LED_NEOPIXEL and LED_NEOPIXEL to the appropriate LED pins. Set the NEOPIXELS_NUMBER parameter (number of LEDs) and BOARD_RGB_BRIGHTNESS (their brightness).

Also, replace the following lines:

#ifndef _NICENANO_RGB_H  
#define _NICENANO_RGB_H  

with

#ifndef _KABARGA_RGB_H  
#define _KABARGA_RGB_H  

At the bottom of the file, you can change the device name.

Finally, build your bootloader using the command:

BOARD=kabarga-rgb all

The lighting parameters are configured in the src/boards/boards.c file, starting from line 396.

• Fork this repository.
• Make the necessary changes.
• Commit and push.
• Enable GitHub Actions on the corresponding tab.
• Create an empty release with the tag 0.9.2.
• Wait for GitHub Actions to finish building your bootloader.

I use Fedora, but the commands will be similar on other distributions.

sudo dnf install python3-pip gcc-arm-none-eabi make arm-none-eabi-newlib arm-none-eabi-gcc-cs git --skip-unavailable    
pip3 install --user adafruit-nrfutil intelhex

Firstly clone this repo with following commands

cd $HOME
git clone https://github.com/aroum/nRF52_Bootloader_custom_LED
cd nRF52_Bootloader_custom_LED
git submodule update --init

Then build it with make BOARD={board} all, for example:

make BOARD=feather_nrf52840_express all

For the list of supported boards, run make without BOARD= :

$ make
You must provide a BOARD parameter with 'BOARD='
Supported boards are: feather_nrf52840_express feather_nrf52840_express pca10056
Makefile:90: *** BOARD not defined.  Stop

Here, {board} matches the name of the configuration folder in src/boards/{board}.

The bootloader file can be found here: _build/build-kabarga/update-kabarga_bootloader-XXX.uf2

If you get the following error ...

lib/sdk11/components/libraries/bootloader_dfu/bootloader_settings.c: In function 'bootloader_mbr_addrs_populate':
lib/sdk11/components/libraries/bootloader_dfu/bootloader_settings.c:45:7: error: array subscript 0 is outside array bounds of 'const uint32_t[0]' {aka 'const long unsigned int[]'} [-Werror=array-bounds=]
45 | if (*(const uint32_t *)MBR_BOOTLOADER_ADDR == 0xFFFFFFFF)
compilation terminated due to -Wfatal-errors.
cc1: all warnings being treated as errors
make: *** [Makefile:412: _build/build-feather_nrf52840_express/bootloader_settings.o] Error 1`

You are probably using arm-none-eabi-gcc version 14 or higher. You can check it with the following command:

arm-none-eabi-gcc --version

If the version is 15 or higher, you need to modify one line in the Makefile. Replace:

filter 12.% 13.% 14.%,$

with:

filter 12.% 13.% 14.% 15.%,$

Here is an example command to do this automatically:

sed -i 's/ifneq (,$(filter 12.% 13.% 14.%,$(shell \$(CC) -dumpversion 2>\/dev\/nul
l)))/ifneq (,$(filter 12.% 13.% 14.% 15.%,$(shell \$(CC) -dumpversion 2>\/dev\/null)))/g' "Makefile"

original issues

The P0.18 pin on the nRF52 series microcontrollers is typically configured as the dedicated Pin Reset input. However, its functionality is configurable via the CONFIG_GPIO_AS_PINRESET register, allowing it to be used as a standard General Purpose I/O (GPIO) pin.

This document details the procedure and constraints involved in enabling P0.18 as a GPIO pin for use in custom firmware, such as ZMK.

To use P0.18 as a standard GPIO pin, the CONFIG_GPIO_AS_PINRESET definition must be explicitly disabled in the bootloader source code.

Note: This modification requires direct source code editing and a hardware programmer (e.g., J-Link, ST-Link) to flash the resulting bootloader image, as this option is not typically accessible through standard configuration files.

The following changes must be applied to the bootloader's build configuration files:

src/yourboard/board.h

#ifdef CONFIG_GPIO_AS_PINRESET
#undef CONFIG_GPIO_AS_PINRESET
#endif

Makefile

Locate and remove the following line:

CFLAGS += -DCONFIG_GPIO_AS_PINRESET

/lib/tinyusb/hw/bsp/nrf/family.mk

Locate and remove the following line:

  -DCONFIG_GPIO_AS_PINRESET

Once CONFIG_GPIO_AS_PINRESET is disabled, P0.18 can be configured within the bootloader to function as a DFU (Device Firmware Update) entry switch.

Mechanism: If P0.18 is tied to ground when power is applied to the microcontroller, the system will enter the flashing mode instead of running the application firmware.

When configured as a GPIO, P0.18 becomes available for general purposes within ZMK, such as:

• Switching Bluetooth profiles.
• Battery charge indication.
• Triggering deep sleep mode.

Using P0.18 as a DFU trigger imposes a significant constraint on its ability to wake the microcontroller from deep sleep:

• Conflict: If P0.18 is configured as a simple external wake-up pin (shorting it to ground to wake the MCU), the action of grounding the pin will instead initiate DFU mode, preventing the application from resuming.
• Exception (Matrix): This issue does not apply if P0.18 is used as a column or row pin within a keyboard matrix, as the matrix logic handles the pin state.

To fully utilize P0.18 for deep sleep wake-up while retaining the modified bootloader:

• Use the &bootloader node configuration within ZMK's device tree.
• The ZMK application can then programmatically override and disable the DFU trigger functionality associated with P0.18.
• This allows the pin to be used reliably as a full-featured GPIO, including waking the MCU from deep sleep.

On certain custom PCBs, such as the n!n v2, the P0.14, P0.16, and P0.18 pins are connected together for PCB routing simplicity. Developers should be aware of this hardware connection when assigning functions to any of these pins, as their state may influence each other.

• Due to the deep modifications made to the bootloader's configuration registers, the following deployment steps are mandatory:
• Local Compilation: The modified bootloader must be compiled locally to ensure the changes are correctly integrated.
• Using a Programmer: Flashing must be performed using a hardware programmer (e.g., J-Link, ST-Link).
• File Selection: The required .hex file is located in the build/yourboard/ directory. Select the file with the longest filename (this typically corresponds to the final, complete bootloader image).
• UF2 Restriction: Flashing the bootloader via .uf2 files will not correctly rewrite the necessary configuration registers, thus failing to enable P0.18 as a GPIO pin. A hardware programmer must be used.