Customizing Functionality

How to Customize Your Keyboard's Behavior

For a lot of people a custom keyboard is about more than sending button presses to your computer. You want to be able to do things that are more complex than simple button presses and macros. QMK has hooks that allow you to inject code, override functionality, and otherwise customize how your keyboard behaves in different situations.

This page does not assume any special knowledge about QMK, but reading Understanding QMK will help you understand what is going on at a more fundamental level.

A Word on Core vs Keyboards vs Keymap :id=a-word-on-core-vs-keyboards-vs-keymap

We have structured QMK as a hierarchy:

  • Core (_quantum)

    • Keyboard/Revision (_kb)

      • Keymap (_user)

Each of the functions described below can be defined with a _kb() suffix or a _user() suffix. We intend for you to use the _kb() suffix at the Keyboard/Revision level, while the _user() suffix should be used at the Keymap level.

When defining functions at the Keyboard/Revision level it is important that your _kb() implementation call _user() before executing anything else- otherwise the keymap level function will never be called.

Custom Keycodes

By far the most common task is to change the behavior of an existing keycode or to create a new keycode. From a code standpoint the mechanism for each is very similar.

Defining a New Keycode

The first step to creating your own custom keycode(s) is to enumerate them. This means both naming them and assigning a unique number to that keycode. Rather than limit custom keycodes to a fixed range of numbers QMK provides the SAFE_RANGE macro. You can use SAFE_RANGE when enumerating your custom keycodes to guarantee that you get a unique number.

Here is an example of enumerating 2 keycodes. After adding this block to your keymap.c you will be able to use FOO and BAR inside your keymap.

enum my_keycodes {

Programming the Behavior of Any Keycode :id=programming-the-behavior-of-any-keycode

When you want to override the behavior of an existing key, or define the behavior for a new key, you should use the process_record_kb() and process_record_user() functions. These are called by QMK during key processing before the actual key event is handled. If these functions return true QMK will process the keycodes as usual. That can be handy for extending the functionality of a key rather than replacing it. If these functions return false QMK will skip the normal key handling, and it will be up to you to send any key up or down events that are required.

These function are called every time a key is pressed or released.

Example process_record_user() Implementation

This example does two things. It defines the behavior for a custom keycode called FOO, and it supplements our Enter key by playing a tone whenever it is pressed.

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
switch (keycode) {
case FOO:
if (record->event.pressed) {
// Do something when pressed
} else {
// Do something else when release
return false; // Skip all further processing of this key
case KC_ENTER:
// Play a tone when enter is pressed
if (record->event.pressed) {
return true; // Let QMK send the enter press/release events
return true; // Process all other keycodes normally

process_record_* Function Documentation

  • Keyboard/Revision: bool process_record_kb(uint16_t keycode, keyrecord_t *record)

  • Keymap: bool process_record_user(uint16_t keycode, keyrecord_t *record)

The keycode argument is whatever is defined in your keymap, eg MO(1), KC_L, etc. You should use a block to handle these events.

The record argument contains information about the actual press:

keyrecord_t record {
keyevent_t event {
keypos_t key {
uint8_t col
uint8_t row
bool pressed
uint16_t time

LED Control

QMK provides methods to read 5 of the LEDs defined in the HID spec:

  • Num Lock

  • Caps Lock

  • Scroll Lock

  • Compose

  • Kana

There are two ways to get the lock LED state:

  • by implementing bool led_update_kb(led_t led_state) or _user(led_t led_state); or

  • by calling led_t host_keyboard_led_state()

!> host_keyboard_led_state() may already reflect a new value before led_update_user() is called.

Two more deprecated functions exist that provide the LED state as a uint8_t:

  • uint8_t led_set_kb(uint8_t usb_led) and _user(uint8_t usb_led)

  • uint8_t host_keyboard_leds()


This function will be called when the state of one of those 5 LEDs changes. It receives the LED state as a struct parameter.

By convention, return true from led_update_user() to get the led_update_kb() hook to run its code, and return false when you would prefer not to run the code in led_update_kb().

Some examples include:

  • overriding the LEDs to use them for something else like layer indication

    • return false because you do not want the _kb() function to run, as it would override your layer behavior.

  • play a sound when an LED turns on or off.

    • return true because you want the _kb function to run, and this is in addition to the default LED behavior.

?> Because the led_set_* functions return void instead of bool, they do not allow for overriding the keyboard LED control, and thus it's recommended to use led_update_* instead.

Example led_update_kb() Implementation

bool led_update_kb(led_t led_state) {
bool res = led_update_user(led_state);
if(res) {
// writePin sets the pin high for 1 and low for 0.
// In this example the pins are inverted, setting
// it low/0 turns it on, and high/1 turns the LED off.
// This behavior depends on whether the LED is between the pin
// and VCC or the pin and GND.
writePin(B0, !led_state.num_lock);
writePin(B1, !led_state.caps_lock);
writePin(B2, !led_state.scroll_lock);
writePin(B3, !led_state.compose);
writePin(B4, !led_state.kana);
return res;

Example led_update_user() Implementation

This incomplete example would play a sound if Caps Lock is turned on or off. It returns true, because you also want the LEDs to maintain their state.

float caps_on[][2] = SONG(CAPS_LOCK_ON_SOUND);
float caps_off[][2] = SONG(CAPS_LOCK_OFF_SOUND);
bool led_update_user(led_t led_state) {
static uint8_t caps_state = 0;
if (caps_state != led_state.caps_lock) {
led_state.caps_lock ? PLAY_SONG(caps_on) : PLAY_SONG(caps_off);
caps_state = led_state.caps_lock;
return true;

led_update_* Function Documentation

  • Keyboard/Revision: bool led_update_kb(led_t led_state)

  • Keymap: bool led_update_user(led_t led_state)


Call this function to get the last received LED state as a led_t. This is useful for reading the LED state outside led_update_*, e.g. in matrix_scan_user().

Setting Physical LED State

Some keyboard implementations provide convenience methods for setting the state of the physical LEDs.

Ergodox Boards

The Ergodox implementations provide ergodox_right_led_1/2/3_on/off() to turn individual LEDs on or off, as well as ergodox_right_led_on/off(uint8_t led) to turn them on or off by their index.

In addition, it is possible to specify the brightness level of all LEDs with ergodox_led_all_set(uint8_t n); of individual LEDs with ergodox_right_led_1/2/3_set(uint8_t n); or by index with ergodox_right_led_set(uint8_t led, uint8_t n).

Ergodox boards also define LED_BRIGHTNESS_LO for the lowest brightness and LED_BRIGHTNESS_HI for the highest brightness (which is the default).

Keyboard Initialization Code

There are several steps in the keyboard initialization process. Depending on what you want to do, it will influence which function you should use.

These are the three main initialization functions, listed in the order that they're called.

  • keyboard_pre_init_* - Happens before most anything is started. Good for hardware setup that you want running very early.

  • matrix_init_* - Happens midway through the firmware's startup process. Hardware is initialized, but features may not be yet.

  • keyboard_post_init_* - Happens at the end of the firmware's startup process. This is where you'd want to put "customization" code, for the most part.

!> For most people, the keyboard_post_init_user function is what you want to call. For instance, this is where you want to set up things for RGB Underglow.

Keyboard Pre Initialization code

This runs very early during startup, even before the USB has been started.

Shortly after this, the matrix is initialized.

For most users, this shouldn't be used, as it's primarily for hardware oriented initialization.

However, if you have hardware stuff that you need initialized, this is the best place for it (such as initializing LED pins).

Example keyboard_pre_init_user() Implementation

This example, at the keyboard level, sets up B0, B1, B2, B3, and B4 as LED pins.

void keyboard_pre_init_user(void) {
// Call the keyboard pre init code.
// Set our LED pins as output

keyboard_pre_init_* Function Documentation

  • Keyboard/Revision: void keyboard_pre_init_kb(void)

  • Keymap: void keyboard_pre_init_user(void)

Matrix Initialization Code

This is called when the matrix is initialized, and after some of the hardware has been set up, but before many of the features have been initialized.

This is useful for setting up stuff that you may need elsewhere, but isn't hardware related nor is dependant on where it's started.

matrix_init_* Function Documentation

  • Keyboard/Revision: void matrix_init_kb(void)

  • Keymap: void matrix_init_user(void)

Keyboard Post Initialization code

This is ran as the very last task in the keyboard initialization process. This is useful if you want to make changes to certain features, as they should be initialized by this point.

Example keyboard_post_init_user() Implementation

This example, running after everything else has initialized, sets up the rgb underglow configuration.

void keyboard_post_init_user(void) {
// Call the post init code.
rgblight_enable_noeeprom(); // enables Rgb, without saving settings
rgblight_sethsv_noeeprom(180, 255, 255); // sets the color to teal/cyan without saving
rgblight_mode_noeeprom(RGBLIGHT_MODE_BREATHING + 3); // sets mode to Fast breathing without saving

keyboard_post_init_* Function Documentation

  • Keyboard/Revision: void keyboard_post_init_kb(void)

  • Keymap: void keyboard_post_init_user(void)

Matrix Scanning Code

Whenever possible you should customize your keyboard by using process_record_*() and hooking into events that way, to ensure that your code does not have a negative performance impact on your keyboard. However, in rare cases it is necessary to hook into the matrix scanning. Be extremely careful with the performance of code in these functions, as it will be called at least 10 times per second.

Example matrix_scan_* Implementation

This example has been deliberately omitted. You should understand enough about QMK internals to write this without an example before hooking into such a performance sensitive area. If you need help please open an issue or chat with us on Discord.

matrix_scan_* Function Documentation

  • Keyboard/Revision: void matrix_scan_kb(void)

  • Keymap: void matrix_scan_user(void)

This function gets called at every matrix scan, which is basically as often as the MCU can handle. Be careful what you put here, as it will get run a lot.

You should use this function if you need custom matrix scanning code. It can also be used for custom status output (such as LEDs or a display) or other functionality that you want to trigger regularly even when the user isn't typing.

Keyboard Idling/Wake Code

If the board supports it, it can be "idled", by stopping a number of functions. A good example of this is RGB lights or backlights. This can save on power consumption, or may be better behavior for your keyboard.

This is controlled by two functions: suspend_power_down_* and suspend_wakeup_init_*, which are called when the system board is idled and when it wakes up, respectively.

Example suspend_power_down_user() and suspend_wakeup_init_user() Implementation

void suspend_power_down_user(void) {
void suspend_wakeup_init_user(void) {

Keyboard suspend/wake Function Documentation

  • Keyboard/Revision: void suspend_power_down_kb(void) and void suspend_wakeup_init_user(void)

  • Keymap: void suspend_power_down_kb(void) and void suspend_wakeup_init_user(void)

Layer Change Code :id=layer-change-code

This runs code every time that the layers get changed. This can be useful for layer indication, or custom layer handling.

Example layer_state_set_* Implementation

This example shows how to set the RGB Underglow lights based on the layer, using the Planck as an example.

layer_state_t layer_state_set_user(layer_state_t state) {
switch (get_highest_layer(state)) {
case _RAISE:
rgblight_setrgb (0x00, 0x00, 0xFF);
case _LOWER:
rgblight_setrgb (0xFF, 0x00, 0x00);
case _PLOVER:
rgblight_setrgb (0x00, 0xFF, 0x00);
case _ADJUST:
rgblight_setrgb (0x7A, 0x00, 0xFF);
default: // for any other layers, or the default layer
rgblight_setrgb (0x00, 0xFF, 0xFF);
return state;

Use the IS_LAYER_ON_STATE(state, layer) and IS_LAYER_OFF_STATE(state, layer) macros to check the status of a particular layer.

Outside of layer_state_set_* functions, you can use the IS_LAYER_ON(layer) and IS_LAYER_OFF(layer) macros to check global layer state.

layer_state_set_* Function Documentation

  • Keyboard/Revision: layer_state_t layer_state_set_kb(layer_state_t state)

  • Keymap: layer_state_t layer_state_set_user(layer_state_t state)

The state is the bitmask of the active layers, as explained in the Keymap Overview

Persistent Configuration (EEPROM)

This allows you to configure persistent settings for your keyboard. These settings are stored in the EEPROM of your controller, and are retained even after power loss. The settings can be read with eeconfig_read_kb and eeconfig_read_user, and can be written to using eeconfig_update_kb and eeconfig_update_user. This is useful for features that you want to be able to toggle (like toggling rgb layer indication). Additionally, you can use eeconfig_init_kb and eeconfig_init_user to set the default values for the EEPROM.

The complicated part here, is that there are a bunch of ways that you can store and access data via EEPROM, and there is no "correct" way to do this. However, you only have a DWORD (4 bytes) for each function.

Keep in mind that EEPROM has a limited number of writes. While this is very high, it's not the only thing writing to the EEPROM, and if you write too often, you can potentially drastically shorten the life of your MCU.

  • If you don't understand the example, then you may want to avoid using this feature, as it is rather complicated.

Example Implementation

This is an example of how to add settings, and read and write it. We're using the user keymap for the example here. This is a complex function, and has a lot going on. In fact, it uses a lot of the above functions to work!

In your keymap.c file, add this to the top:

typedef union {
uint32_t raw;
struct {
bool rgb_layer_change :1;
} user_config_t;
user_config_t user_config;

This sets up a 32 bit structure that we can store settings with in memory, and write to the EEPROM. Using this removes the need to define variables, since they're defined in this structure. Remember that bool (boolean) values use 1 bit, uint8_t uses 8 bits, uint16_t uses up 16 bits. You can mix and match, but changing the order can cause issues, as it will change the values that are read and written.

We're using rgb_layer_change, for the layer_state_set_* function, and use keyboard_post_init_user and process_record_user to configure everything.

Now, using the keyboard_post_init_user code above, you want to add eeconfig_read_user() to it, to populate the structure you've just created. And you can then immediately use this structure to control functionality in your keymap. And It should look like:

void keyboard_post_init_user(void) {
// Call the keymap level matrix init.
// Read the user config from EEPROM
user_config.raw = eeconfig_read_user();
// Set default layer, if enabled
if (user_config.rgb_layer_change) {

The above function will use the EEPROM config immediately after reading it, to set the default layer's RGB color. The "raw" value of it is converted in a usable structure based on the "union" that you created above.

layer_state_t layer_state_set_user(layer_state_t state) {
switch (get_highest_layer(state)) {
case _RAISE:
if (user_config.rgb_layer_change) { rgblight_sethsv_noeeprom_magenta(); rgblight_mode_noeeprom(1); }
case _LOWER:
if (user_config.rgb_layer_change) { rgblight_sethsv_noeeprom_red(); rgblight_mode_noeeprom(1); }
case _PLOVER:
if (user_config.rgb_layer_change) { rgblight_sethsv_noeeprom_green(); rgblight_mode_noeeprom(1); }
case _ADJUST:
if (user_config.rgb_layer_change) { rgblight_sethsv_noeeprom_white(); rgblight_mode_noeeprom(1); }
default: // for any other layers, or the default layer
if (user_config.rgb_layer_change) { rgblight_sethsv_noeeprom_cyan(); rgblight_mode_noeeprom(1); }
return state;

This will cause the RGB underglow to be changed ONLY if the value was enabled. Now to configure this value, create a new keycode for process_record_user called RGB_LYR. Additionally, we want to make sure that if you use the normal RGB codes, that it turns off Using the example above, make it look this:

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
switch (keycode) {
case FOO:
if (record->event.pressed) {
// Do something when pressed
} else {
// Do something else when release
return false; // Skip all further processing of this key
case KC_ENTER:
// Play a tone when enter is pressed
if (record->event.pressed) {
return true; // Let QMK send the enter press/release events
case RGB_LYR: // This allows me to use underglow as layer indication, or as normal
if (record->event.pressed) {
user_config.rgb_layer_change ^= 1; // Toggles the status
eeconfig_update_user(user_config.raw); // Writes the new status to EEPROM
if (user_config.rgb_layer_change) { // if layer state indication is enabled,
layer_state_set(layer_state); // then immediately update the layer color
return false;
case RGB_MODE_FORWARD ... RGB_MODE_GRADIENT: // For any of the RGB codes (see quantum_keycodes.h, L400 for reference)
if (record->event.pressed) { //This disables layer indication, as it's assumed that if you're changing this ... you want that disabled
if (user_config.rgb_layer_change) { // only if this is enabled
user_config.rgb_layer_change = false; // disable it, and
eeconfig_update_user(user_config.raw); // write the setings to EEPROM
return true; break;
return true; // Process all other keycodes normally

And lastly, you want to add the eeconfig_init_user function, so that when the EEPROM is reset, you can specify default values, and even custom actions. To force an EEPROM reset, use the EEP_RST keycode or Bootmagic functionallity. For example, if you want to set rgb layer indication by default, and save the default valued.

void eeconfig_init_user(void) { // EEPROM is getting reset!
user_config.raw = 0;
user_config.rgb_layer_change = true; // We want this enabled by default
eeconfig_update_user(user_config.raw); // Write default value to EEPROM now
// use the non noeeprom versions, to write these values to EEPROM too
rgblight_enable(); // Enable RGB by default
rgblight_sethsv_cyan(); // Set it to CYAN by default
rgblight_mode(1); // set to solid by default

And you're done. The RGB layer indication will only work if you want it to. And it will be saved, even after unplugging the board. And if you use any of the RGB codes, it will disable the layer indication, so that it stays on the mode and color that you set it to.

'EECONFIG' Function Documentation

  • Keyboard/Revision: void eeconfig_init_kb(void), uint32_t eeconfig_read_kb(void) and void eeconfig_update_kb(uint32_t val)

  • Keymap: void eeconfig_init_user(void), uint32_t eeconfig_read_user(void) and void eeconfig_update_user(uint32_t val)

The val is the value of the data that you want to write to EEPROM. And the eeconfig_read_* function return a 32 bit (DWORD) value from the EEPROM.