QMK is nearly infinitely configurable. Wherever possible we err on the side of allowing users to customize their keyboard, even at the expense of code size. That level of flexibility makes for a daunting configuration experience, however.
There are two main types of configuration files in QMK- `config.h` and `rules.mk`. These files exist at various levels in QMK and all files of the same type are combined to build the final configuration. The levels, from lowest priority to highest priority, are:
* QMK Default
* Keyboard
* Folders (Up to 5 levels deep)
* Keymap
## QMK Default
Every available setting in QMK has a default. If that setting is not set at the Keyboard, Folder, or Keymap level this is the setting that will be used.
This level contains config options that should apply to the whole keyboard. Some settings won't change in revisions, or most keymaps. Other settings are merely defaults for this keyboard and can be overridden by folders and/or keymaps.
Some keyboards have folders and sub-folders to allow for different hardware configurations. Most keyboards only go 1 folder deep, but QMK supports structures up to 5 folders deep. Each folder can have its own `config.h` and `rules.mk` files that are incorporated into the final configuration.
This level contains all of the options for that particular keymap. If you wish to override a previous declaration, you can use `#undef <variable>` to undefine it, where you can then redefine it without an error.
This is a C header file that is one of the first things included, and will persist over the whole project (if included). Lots of variables can be set here and accessed elsewhere. The `config.h` file shouldn't be including other `config.h` files, or anything besides this:
* may be omitted by the keyboard designer if matrix reads are handled in an alternate manner. See [low-level matrix overrides](custom_quantum_functions.md?id=low-level-matrix-overrides) for more information.
* may be omitted by the keyboard designer if matrix reads are handled in an alternate manner. See [low-level matrix overrides](custom_quantum_functions.md?id=low-level-matrix-overrides) for more information.
* COL2ROW or ROW2COL - how your matrix is configured. COL2ROW means the black mark on your diode is facing to the rows, and between the switch and the rows.
* sets the I2C clock rate speed for keyboards using I2C. The default is `400000L`, except for keyboards using `split_common`, where the default is `100000L`.
* NKRO by default requires to be turned on, this forces it on during keyboard startup regardless of EEPROM setting. NKRO can still be turned off but will be turned on again if the keyboard reboots.
* If you're having issues finishing the sequence before it times out, you may need to increase the timeout setting. Or you may want to enable the `LEADER_PER_KEY_TIMING` option, which resets the timeout after each key is tapped.
* Disables keycode filtering for Mod-Tap and Layer-Tap keycodes. Eg, if you enable this, you would need to specify `MT(MOD_CTL, KC_A)` if you want to use `KC_A`.
* Sets the delay between `register_code` and `unregister_code`, if you're having issues with it registering properly (common on VUSB boards). The value is in milliseconds.
* Sets the delay for Tap Hold keys (`LT`, `MT`) when using `KC_CAPSLOCK` keycode, as this has some special handling on MacOS. The value is in milliseconds, and defaults to 80 ms if not defined. For macOS, you may want to set this to 200 or higher.
* Lets you define [lighting layers](feature_rgblight.md?id=lighting-layers) that can be toggled on or off. Great for showing the current keyboard layer or caps lock state.
* Adds ability to [blink](feature_rgblight.md?id=lighting-layer-blink) a lighting layer for a specified number of milliseconds (e.g. to acknowledge an action).
* Needed if both halves of the board have RGB LEDs wired directly to the RGB output pin on the controllers instead of passing the output of the left half to the input of the right half
* For using high/low pin to determine handedness, low = right hand, high = left hand. Replace `B7` with the pin you are using. This is optional, and if you leave `SPLIT_HAND_PIN` undefined, then you can still use the EE_HANDS method or MASTER_LEFT / MASTER_RIGHT defines like the stock Let's Split uses.
* The handedness is determined by using the intersection of the keyswitches in the key matrix, which does not exist. Normally, when this intersection is shorted (level low), it is considered left. If you define `#define SPLIT_HAND_MATRIX_GRID_LOW_IS_RIGHT`, it is determined to be right when the level is low.
*`#define EE_HANDS` (only works if `SPLIT_HAND_PIN` and `SPLIT_HAND_MATRIX_GRID` are not defined)
* If you want to specify a different pinout for the right half than the left half, you can define `MATRIX_ROW_PINS_RIGHT`/`MATRIX_COL_PINS_RIGHT`. Currently, the size of `MATRIX_ROW_PINS` must be the same as `MATRIX_ROW_PINS_RIGHT` and likewise for the definition of columns.
* may be omitted by the keyboard designer if matrix reads are handled in an alternate manner. See [low-level matrix overrides](custom_quantum_functions.md?id=low-level-matrix-overrides) for more information.
* If you want to specify a different direct pinout for the right half than the left half, you can define `DIRECT_PINS_RIGHT`. Currently, the size of `DIRECT_PINS` must be the same as `DIRECT_PINS_RIGHT`.
* Deadline for synchronizing data from master to slave when using the QMK-provided split transport.
*`#define SPLIT_TRANSPORT_MIRROR`
* Mirrors the master-side matrix on the slave when using the QMK-provided split transport.
*`#define SPLIT_LAYER_STATE_ENABLE`
* Ensures the current layer state is available on the slave when using the QMK-provided split transport.
*`#define SPLIT_LED_STATE_ENABLE`
* Ensures the current host indicator state (caps/num/scroll) is available on the slave when using the QMK-provided split transport.
*`#define SPLIT_MODS_ENABLE`
* Ensures the current modifier state (normal, weak, and oneshot) is available on the slave when using the QMK-provided split transport.
*`#define SPLIT_WPM_ENABLE`
* Ensures the current WPM is available on the slave when using the QMK-provided split transport.
*`#define SPLIT_TRANSACTION_IDS_KB .....`
*`#define SPLIT_TRANSACTION_IDS_USER .....`
* Allows for custom data sync with the slave when using the QMK-provided split transport. See [custom data sync between sides](feature_split_keyboard.md#custom-data-sync) for more information.
This is a [make](https://www.gnu.org/software/make/manual/make.html) file that is included by the top-level `Makefile`. It is used to set some information about the MCU that we will be compiling for as well as enabling and disabling certain features.
* Enables Link Time Optimization (LTO) when compiling the keyboard. This makes the process take longer, but it can significantly reduce the compiled size (and since the firmware is small, the added time is not noticeable).
However, this will automatically disable the legacy TMK Macros and Functions features, as these break when LTO is enabled. It does this by automatically defining `NO_ACTION_MACRO` and `NO_ACTION_FUNCTION`. (Note: This does not affect QMK [Macros](feature_macros.md) and [Layers](feature_layers.md).)
Use these to enable or disable building certain features. The more you have enabled the bigger your firmware will be, and you run the risk of building a firmware too large for your MCU.
* Enables split keyboard support (dual MCU like the let's split and bakingpy's boards) and includes all necessary files located at quantum/split_common
* Forces the keyboard to wait for a USB connection to be established before it starts up
* `NO_USB_STARTUP_CHECK`
* Disables usb suspend check after keyboard startup. Usually the keyboard waits for the host to wake it up before any tasks are performed. This is useful for split keyboards as one half will not get a wakeup call but must send commands to the master.
