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](understanding_qmk.md) will help you understand what is going on at a more fundamental level.
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.
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.
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.
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.
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.
*`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 needs to perform the low-level initialisation of all row and column pins. By default this will initialise the input/output state of each of the GPIO pins listed in `MATRIX_ROW_PINS` and `MATRIX_COL_PINS`, based on whether or not the keyboard is set up for `ROW2COL`, `COL2ROW`, or `DIRECT_PINS`. Should the keyboard designer override this function, no initialisation of pin state will occur within QMK itself, instead deferring to the keyboard's override.
* These three functions need to perform the low-level retrieval of matrix state of relevant input pins, based on the matrix type. Only one of the functions should be implemented, if needed. By default this will iterate through `MATRIX_ROW_PINS` and `MATRIX_COL_PINS`, configuring the inputs and outputs based on whether or not the keyboard is set up for `ROW2COL`, `COL2ROW`, or `DIRECT_PINS`. Should the keyboard designer override this function, no manipulation of matrix GPIO pin state will occur within QMK itself, instead deferring to the keyboard's override.
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.
```c
void keyboard_post_init_user(void) {
// Call the post init code.
rgblight_enable_noeeprom(); // enables Rgb, without saving settings
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.
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](https://github.com/qmk/qmk_firmware/issues/new) or [chat with us on Discord](https://discord.gg/Uq7gcHh).
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.
This function gets called at the end of all QMK processing, before starting the next iteration. You can safely assume that QMK has dealt with the last matrix scan at the time that these functions are invoked -- layer states have been updated, USB reports have been sent, LEDs have been updated, and displays have been drawn.
Similar to `matrix_scan_*`, these are called as often as the MCU can handle. To keep your board responsive, it's suggested to do as little as possible during these function calls, potentially throtting their behaviour if you do indeed require implementing something special.
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.
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.
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!
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:
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.
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:
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](feature_bootmagic.md) functionallity. For example, if you want to set rgb layer indication by default, and save the default valued.
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.
