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Keymap framework - how to define your keymap

NOTE: This is not final version, may be inconsistent with source code and changed occasionally for a while.

0. Keymap and layers

The keymap is an array composed of one or more layers. Each layer is an array of keycodes, defining actions for each physical key. Layers can be activated and deactivated independently. Multiple layers may be active at once, resulting in the currently-active layer state. Each layer has an index between 0-31. As active layers are stacked together, higher layers take precedence over lower layers.

Keymap: 32 Layers                   Layer: Keycode matrix
-----------------                   ---------------------
stack of layers                     array_of_keycode[row][column]
       ____________ precedence               _______________________
      /           / | high                  / ESC / F1  / F2  / F3   ....
  31 /___________// |                      /-----/-----/-----/-----
  30 /___________// |                     / TAB /  Q  /  W  /  E   ....
  29 /___________/  |                    /-----/-----/-----/-----
   :   _:_:_:_:_:__ |               :   /LCtrl/  A  /  S  /  D   ....
   :  / : : : : : / |               :  /  :     :     :     :
   2 /___________// |               2 `--------------------------
   1 /___________// |               1 `--------------------------
   0 /___________/  V low           0 `--------------------------

Note: The keymap array is limited to 32 layers.

0.1 Layer state

The current keymap layer state is determined by two parameters: the default layer, and the individual layer states. Changing the default layer is useful for switching key layouts completely; for example, switching to Dvorak, Colemak or Workman instead of QWERTY. Individual layer states, on the other hand, can be used to overlay the base layer with other functions such as navigation keys, function keys (F1-F12), media keys or other actions.

Because the default layer is really just a special case affecting the overall layer state, it is important to first understand how the layer state is determined.

0.1.1 The layer state

The layer state indicates the current on/off status of all layers. It is defined in the firmware by a 32-bit integer, layer_state, which stores each layer's on/off status in a single bit: 0 for off, 1 for on. As layers are activated and deactivated, their respective bits are flipped, changing the value of layer_state.

Overlay feature layer
---------------------      bit|status
       ____________        ---+------
  31  /           /        31 |   0
  30 /___________// -----> 30 |   1
  29 /___________/  -----> 29 |   1
   :                        : |   :
   :   ____________         : |   :
   2  /           /         2 |   0
,->1 /___________/  ----->  1 |   1
|  0                        0 |   0
|                                 +
`--- default_layer = 1            |
     layer_state   = 0x60000002 <-'

0.1.2 The default layer

The default layer is the base keymap layer (0-31) which is always active and considered the "bottom" of the stack. When the firmware boots, the default layer is the only active layer. It is set to layer 0 by default, though this can be changed in config.h via Boot Magic settings.

Initial state of Keymap          Change base layout
-----------------------          ------------------

  31                               31
  30                               30
  29                               29
   :                                :
   :                                :   ____________
   2   ____________                 2  /           /
   1  /           /              ,->1 /___________/
,->0 /___________/               |  0
|                                |
`--- default_layer = 0           `--- default_layer = 1
     layer_state   = 0x00000001       layer_state   = 0x00000002

Note that the default_layer_state variable only determines the lowest value to which layer_state may be set, and that default_layer_state is used by the core firmware when determining the starting value of layer_state before applying changes. In other words, the default layer will always be set to on in layer_state.

The default layer is defined in the firmware by the default_layer_state variable, which is identical in format to the layer_state variable explained above. The value may be changed using the following functions:

  • default_layer_state_set(state) sets the state to the specified 32-bit integer value.
  • AND/OR/XOR functions set the state based on a boolean logic comparison between the current state and the specified 32-bit integer value:
    • default_layer_state_and(state)
    • default_layer_state_or(state)
    • default_layer_state_xor(state)

For example, to set layer 3 as the default layer:

// convert 3 to a 32-bit unsigned long value, and set the default layer
default_layer_state_set(1UL<<3);

0.2 Layer Precedence and Transparency

Note that higher layers have priority in the layer stack. The firmware starts at the topmost active layer, and works down to the bottom to find an active keycode. Once the search encounters any keycode other than KC_TRNS (transparent) on an active layer, the search is halted and the remaining lower layers aren't examined, even if they are active.

Note: a layer must be activated before it may be included in the stack search.

KC_TRNS is a special placeholder which can be used on overlay layers. This allows for the creation of "partial" layers which fall back on the lower layers, eliminating a good deal of repetition in keymap files.

0.3 Keymap Example

The keymap is defined in the uint8_t keymaps[] array, a 2-dimensional array of rows and columns corresponding to positions in the keyboard matrix. But most often the layers are defined using C macros to allow for easier reading and editing of the keymap files. To use complex actions you need to define Fn action in the action_t fn_actions[] array.

This is a keymap example for the HHKB keyboard. This example has three layers: the QWERTY base layer, and two overlay layers for cursor and mousekey control, respectively. In this example,

Fn0 is a momentary layer switching key--you can use keys on the Cursor layer while holding the key.

Fn1 is a momentary layer switching key with tapping function--tapping the key as one would normally use it, sends the semicolon ';' keycode, while holding the key down switches layers.

Fn2 is a toggle layer switch key--pressing the key toggles the layer on until you press it again.

You can find other keymap definitions in file keymap.c located on project directories.

const uint8_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
    /* 0: Qwerty
     * ,-----------------------------------------------------------.
     * |Esc|  1|  2|  3|  4|  5|  6|  7|  8|  9|  0|  -|  =|  \|  `|
     * |-----------------------------------------------------------|
     * |Tab  |  Q|  W|  E|  R|  T|  Y|  U|  I|  O|  P|  [|  ]|Backs|
     * |-----------------------------------------------------------|
     * |Contro|  A|  S|  D|  F|  G|  H|  J|  K|  L|Fn1|  '|Enter   |
     * |-----------------------------------------------------------|
     * |Shift   |  Z|  X|  C|  V|  B|  N|  M|  ,|  .|  /|Shift |Fn0|
     * `-----------------------------------------------------------'
     *       |Gui|Alt  |Space                  |Alt  |Fn2|
     *       `-------------------------------------------'
     */
    KEYMAP(ESC, 1,   2,   3,   4,   5,   6,   7,   8,   9,   0,   MINS,EQL, BSLS,GRV, \
           TAB, Q,   W,   E,   R,   T,   Y,   U,   I,   O,   P,   LBRC,RBRC,BSPC, \
           LCTL,A,   S,   D,   F,   G,   H,   J,   K,   L,   FN1, QUOT,ENT, \
           LSFT,Z,   X,   C,   V,   B,   N,   M,   COMM,DOT, SLSH,RSFT,FN0, \
                LGUI,LALT,          SPC,                RALT,FN2),
    /* 1: Cursor(HHKB mode)
     * ,-----------------------------------------------------------.
     * |Pwr| F1| F2| F3| F4| F5| F6| F7| F8| F9|F10|F11|F12|Ins|Del|
     * |-----------------------------------------------------------|
     * |Caps |   |   |   |   |   |   |   |Psc|Slk|Pus|Up |   |Backs|
     * |-----------------------------------------------------------|
     * |Contro|VoD|VoU|Mut|   |   |  *|  /|Hom|PgU|Lef|Rig|Enter   |
     * |-----------------------------------------------------------|
     * |Shift   |   |   |   |   |   |  +|  -|End|PgD|Dow|Shift |   |
     * `-----------------------------------------------------------'
     *      |Gui |Alt  |Space                  |Alt  |Gui|
     *      `--------------------------------------------'
     */
    KEYMAP(PWR, F1,  F2,  F3,  F4,  F5,  F6,  F7,  F8,  F9,  F10, F11, F12, INS, DEL, \
           CAPS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,PSCR,SLCK,PAUS,UP,  TRNS,BSPC, \
           LCTL,VOLD,VOLU,MUTE,TRNS,TRNS,PAST,PSLS,HOME,PGUP,LEFT,RGHT,ENT, \
           LSFT,TRNS,TRNS,TRNS,TRNS,TRNS,PPLS,PMNS,END, PGDN,DOWN,RSFT,TRNS, \
                LGUI,LALT,          SPC,                RALT,RGUI),
    /* 2: Mousekey
     * ,-----------------------------------------------------------.
     * |Esc| F1| F2| F3| F4| F5| F6| F7| F8| F9|F10|F11|F12|Ins|Del|
     * |-----------------------------------------------------------|
     * |Tab  |   |   |   |   |   |MwL|MwD|MwU|MwR|   |   |   |Backs|
     * |-----------------------------------------------------------|
     * |Contro|   |   |   |   |   |McL|McD|McU|McR|   |   |Return  |
     * |-----------------------------------------------------------|
     * |Shift   |   |   |   |   |Mb3|Mb2|Mb1|Mb4|Mb5|   |Shift |   |
     * `-----------------------------------------------------------'
     *      |Gui |Alt  |Mb1                    |Alt  |   |
     *      `--------------------------------------------'
     * Mc: Mouse Cursor / Mb: Mouse Button / Mw: Mouse Wheel
     */
    KEYMAP(ESC, F1,  F2,  F3,  F4,  F5,  F6,  F7,  F8,  F9,  F10, F11, F12, INS, DEL, \
           TAB, TRNS,TRNS,TRNS,TRNS,TRNS,WH_L,WH_D,WH_U,WH_R,TRNS,TRNS,TRNS,BSPC, \
           LCTL,TRNS,ACL0,ACL1,ACL2,TRNS,MS_L,MS_D,MS_U,MS_R,TRNS,QUOT,ENT, \
           LSFT,TRNS,TRNS,TRNS,TRNS,BTN3,BTN2,BTN1,BTN4,BTN5,SLSH,RSFT,TRNS, \
                LGUI,LALT,          BTN1,               RALT,TRNS),
};

const action_t PROGMEM fn_actions[] = {
    ACTION_LAYER_MOMENTARY(1),                  // FN0
    ACTION_LAYER_TAP_KEY(2, KC_SCLN),           // FN1
    ACTION_LAYER_TOGGLE(2),                     // FN2
};

1. Keycode

See common/keycode.h or keycode table below for the detail. Keycode is internal 8bit code to indicate action performed on key in keymap. Keycode has KC_ prefixed symbol respectively. Most of keycodes like KC_A have simple action registers key to host on press and unregister on release, while some of other keycodes has some special actions like Fn keys, Media control keys, System control keys and Mousekeys.

In KEYMAP() macro you should omit prefix part KC_ of keycode to keep keymap compact. For example, just use A instead you place KC_A in KEYMAP(). Some keycodes has 4-letter short name in addition to descriptive name, you'll prefer short one in KEYMAP().

1.0 Other key

  • KC_NO for no action
  • KC_TRNS for layer transparency (See above)

1.1 Normal key

  • KC_A to KC_Z, KC_1 to KC_0 for alpha numeric key
  • KC_MINS, KC_EQL, KC_GRV, KC_RBRC, KC_LBRC, KC_COMM, KC_DOT, KC_BSLS, KC_SLSH, KC_SCLN, KC_QUOT
  • KC_ESC, KC_TAB, KC_SPC, KC_BSPC, KC_ENT, KC_DEL, KC_INS
  • KC_UP, KC_DOWN, KC_RGHT, KC_LEFT, KC_PGUP, KC_PGDN, KC_HOME, KC_END
  • KC_CAPS, KC_NLCK, KC_SLCK, KC_PSCR, KC_PAUS, KC_APP, KC_F1 to KC_F24
  • KC_P1 to KC_P0, KC_PDOT, KC_PCMM, KC_PSLS, KC_PAST, KC_PMNS, KC_PPLS, KC_PEQL, KC_PENT for keypad.

1.2 Modifier

There are 8 modifiers which has discrimination between left and right.

  • KC_LCTL and KC_RCTL for Control
  • KC_LSFT and KC_RSFT for Shift
  • KC_LALT and KC_RALT for Alt
  • KC_LGUI and KC_RGUI for Windows key or Command key in Mac

1.3 Mousekey

  • KC_MS_U, KC_MS_D, KC_MS_L, KC_MS_R for mouse cursor
  • KC_WH_U, KC_WH_D, KC_WH_L, KC_WH_R for mouse wheel
  • KC_BTN1, KC_BTN2, KC_BTN3, KC_BTN4, KC_BTN5 for mouse buttons

1.4 System & Media key

  • KC_PWR, KC_SLEP, KC_WAKE for Power, Sleep, Wake
  • KC_MUTE, KC_VOLU, KC_VOLD for audio volume control
  • KC_MNXT, KC_MPRV, KC_MSTP, KC_MPLY, KC_MSEL for media control
  • KC_MAIL, KC_CALC, KC_MYCM for application launch
  • KC_WSCH, KC_WHOM, KC_WBAK, KC_WFWD, KC_WSTP, KC_WREF, KC_WFAV for web browser operation

1.5 Fn key

KC_FNnn are keycodes for Fn key which not given any actions at the beginning unlike most of keycodes has its own inborn action. To use these keycodes in KEYMAP() you need to assign action you want at first. Action of Fn key is defined in action_t fn_actions[] and its index of the array is identical with number part of KC_FNnn. Thus KC_FN0 keycode indicates the action defined in first element of the array. 32 Fn keys can be defined at most.

1.6 Keycode Table

See keycode table in doc/keycode.txt for description of keycodes.

In regard to implementation side most of keycodes are identical with [HID usage]HID_usage sent to host for real and some virtual keycodes are defined to support special actions. [HID_usage]: http://www.usb.org/developers/hidpage/Hut1_12v2.pdf

2. Action

See common/action_code.h. Action is a 16bit code and defines function to perform on events of a key like press, release, holding and tapping.

Most of keys just register 8bit scancode to host, but to support other complex features needs 16bit extended action codes internally. However, using 16bit action codes in keymap results in double size in memory compared to using just keycodes. To avoid this waste 8bit keycodes are used in KEYMAP() instead of action codes.

You can just use keycodes of Normal key, Modifier, Mousekey and System & Media key in keymap to indicate corresponding actions instead of using action codes. While to use other special actions you should use keycode of Fn key defined in fn_actions[].

2.1 Key Action

This is a simple action that registers scancodes(HID usage in fact) to host on press event of key and unregister on release.

Parameters

  • mods: { MOD_LCTL, MOD_LSFT, MOD_LALT, MOD_LGUI, MOD_RCTL, MOD_RSFT, MOD_RALT, MOD_RGUI }
  • key: keycode

2.1.1 Normal key and Modifier

This action usually won't be used expressly in keymap because you can just use keycodes in KEYMAP() instead.

You can define these actions on 'A' key and 'left shift' modifier with:

ACTION_KEY(KC_A)
ACTION_KEY(KC_LSFT)

2.1.2 Modified key

This action is comprised of modifiers and a key.

Modified keys can be defined as below. Say you want to assign a key to Shift + 1 to get character '!' or Alt + Tab to switch application windows.

ACTION_MODS_KEY(MOD_LSFT, KC_1)
ACTION_MODS_KEY(MOD_LALT, KC_TAB)
ACTION_MODS_KEY(MOD_LALT | MOD_LSFT, KC_TAB)

These are identical to examples above.

ACTION_KEY(MOD_LSFT | KC_1)
ACTION_KEY(MOD_LALT | KC_TAB)
ACTION_KEY(MOD_LSFT | MOD_LALT | KC_TAB)

2.1.3 Multiple Modifiers

Registers multiple modifiers with pressing a key. To specify multiple modifiers use |.

ACTION_MODS(MOD_LALT | MOD_LSFT)
ACTION_MODS(MOD_LALT | MOD_LSFT | MOD_LCTL)

These are identical to examples above.

ACTION_KEY(MOD_LALT | MOD_LSFT, KC_NO)
ACTION_KEY(MOD_LALT | MOD_LSFT | MOD_LCTL, KC_NO)

2.1.3 Modifier with Tap key(Dual role)

Works as a modifier key while holding, but registers a key on tap(press and release quickly).

ACTION_MODS_TAP_KEY(MOD_RCTL, KC_ENT)

2.2 Layer Action

These actions operate layers of keymap.

Parameters

You can specify a target layer of action and when the action is executed. Some actions take a bit value for bitwise operation.

  • layer: 0-31
  • on: { ON_PRESS | ON_RELEASE | ON_BOTH }
  • bits: 5-bit: 1-bit for mask and 4-bit for operand

2.2.1 Default Layer

Default Layer is a layer which always is valid and referred to when actions is not defined on other overlay layers.

This sets Default Layer to given parameter layer and activate it.

ACTION_DEFAULT_LAYER_SET(layer)

2.2.2 Momentary

Turns on layer momentarily while holding, in other words it activates when key is pressed and deactivate when released.

ACTION_LAYER_MOMENTARY(layer)

2.2.3 Toggle Switch

Turns on layer with first type(press and release) and turns off with next.

ACTION_LAYER_TOGGLE(layer)

2.2.4 Momentary Switch with tap key

Turns on layer momentary while holding, but registers key on tap(press and release quickly).

ACTION_LAYER_TAP_KEY(layer, key)

2.2.5 Momentary Switch with tap toggle

Turns on layer momentary while holding and toggles it with serial taps.

ACTION_LAYER_TAP_TOGGLE(layer)

2.2.6 Invert state of layer

Inverts current state of layer. If the layer is on it becomes off with this action.

ACTION_LAYER_INVERT(layer, on)

2.2.7 Turn On layer

Turns on layer state.

ACTION_LAYER_ON(layer, on)

Turns on layer state on press and turns off on release.

ACTION_LAYER_ON_OFF(layer)

2.2.8 Turn Off layer

Turns off layer state.

ACTION_LAYER_OFF(layer, on)

Turns off layer state on press and activates on release.

ACTION_LAYER_OFF_ON(layer)

2.2.9 Set layer

Turn on layer only. layer_state = (1<<layer) [layer: 0-31]

ACTION_LAYER_SET(layer, on)

Turns on layer only and clear all layer on release..

ACTION_LAYER_SET_CLEAR(layer)

2.2.10 Bitwise operation

Performs bitwise operation(AND, OR, XOR, SET) against layer state.

ACTION_LAYER_BIT_AND(part, bits, on)
ACTION_LAYER_BIT_OR(part, bits, on)
ACTION_LAYER_BIT_XOR(part, bits, on)
ACTION_LAYER_BIT_SET(part, bits, on)

part parameter indicates 0-based index(0-7) of where breaking 32-bit layer_state into eight nibbles(4-bit unit).

bs

part            7    6    5    4    3    2    1    0
layer_state     0000 0000 0000 0000 0000 0000 0000 0000
                msb                                 lsb

bits parameter is 5-bit value and consists of two portions, most significant bit(m) controls mask and other 4 bits(abcd) are operand of bit operation.

                43210
bits            mdcba

These parameters works as following code.

uint32_t layer_state;
uint8_t shift = part*4;
uint32_t mask = (bits&0x10) ? ~((uint32_t)0xf<<shift) : 0;
switch (<bitop>) {
case BIT_AND:
    layer_state = layer_state & (((bits&0xf)<<shift)|mask);
    break;
case BIT_OR:
    layer_state = layer_state | (((bits&0xf)<<shift)|mask);
    break;
case BIT_XOR:
    layer_state = layer_state ^ (((bits&0xf)<<shift)|mask);
    break;
case BIT_SET:
    layer_state = layer_state <bitop> (((bits&0xf)<<shift)|mask);
    break;

Default Layer also has bitwise operations, they are executed when key is released.

ACTION_DEFAULT_LAYER_BIT_AND(part, bits)
ACTION_DEFAULT_LAYER_BIT_OR(part, bits)
ACTION_DEFAULT_LAYER_BIT_XOR(part, bits)
ACTION_DEFAULT_LAYER_BIT_SET(part, bits)

2.3 Macro action

Macro actions allow you to register a complex sequence of keystrokes when a key is pressed, where macros are simple sequences of keypresses.

ACTION_MACRO(id)
ACTION_MACRO_TAP(id)

id is an 8-bit user-defined value the macro getter function can use to pick the specific macro.

2.3.1 Implementing Macro getter function

To implement macro functions, the macro lookup list must be implemented:

const macro_t *action_get_macro(keyrecord_t *record, uint8_t id, uint8_t opt);

The function must always return a valid macro, and default implementation of action_get_macro always returns MACRO_NONE which has no effect.

2.3.1.1 Limitations

Similar to the Function Action system, the selector functions is passed a keyrecord_t object, so it can inspect the key state (e.g. different macros on key press or release), and key itself.

Unlike the Function Action system,macros are pre-recorded key sequences, so you can only select from a list. If you want to use dynamic macros then you should look at the more complex function action system.

2.3.2 Implementing/Defining Macro sequences

Macros are of the form (must be wrapped by the MACRO function, and end with an END mark)

MACRO( ..., END )

Within each macro, the following commands can be used:

  • I() change interval of stroke.
  • D() press key
  • U() release key
  • T() type key(press and release)
  • W() wait
  • SM() store modifier state
  • RM() restore modifier state
  • CM() clear modifier state

e.g.:

MACRO( D(LSHIFT), D(D), END )  // hold down LSHIFT and D - will print 'D'
MACRO( U(D), U(LSHIFT), END )  // release U and LSHIFT keys (an event.pressed == False counterpart for the one above)
MACRO( I(255), T(H), T(E), T(L), T(L), W(255), T(O), END ) // slowly print out h-e-l-l---o

2.3.2 Examples

in keymap.c, define action_get_macro

const macro_t *action_get_macro(keyrecord_t *record, uint8_t id, uint8_t opt)
{
    switch (id) {
        case 0:
            return (record->event.pressed ?
                    MACRO( I(0), T(H), T(E), T(L), T(L), W(255), T(O), END ) :
                    MACRO_NONE );
        case 1:
            return (record->event.pressed ?
                    MACRO( D(LALT), D(TAB), END ) :
                    MACRO( U(TAB), END ));
    }
    return MACRO_NONE;
}

in keymap.c, bind items in fn_actions to the macro function

const action_t PROGMEM fn_actions[] = {
     [0] = ACTION_MACRO(0), // will print 'hello' for example
     [1] = ACTION_MACRO(1),
};

2.4 Function action

TBD

There are two type of action, normal Function and tappable Function. These actions call user defined function with id, opt, and key event information as arguments.

2.4.1 Function

To define normal Function action in keymap use this.

ACTION_FUNCTION(id, opt)

2.4.2 Function with tap

To define tappable Function action in keymap use this.

ACTION_FUNCTION_TAP(id, opt)

2.4.3 Implement user function

Function actions can be defined freely with C by user in callback function:

void action_function(keyrecord_t *record, uint8_t id, uint8_t opt);

This C function is called every time key is operated, argument id selects action to be performed and opt can be used for option. Function id can be 0-255 and opt can be 0-15.

keyrecord_t is comprised of key event and tap count. keyevent_t indicates which and when key is pressed or released. From tap_count you can know tap state, 0 means no tap. These information will be used in user function to decide how action of key is performed.

typedef struct {
    keyevent_t  event;
    uint8_t     tap_count;
} keyrecord_t;

typedef struct {
    key_t    key;
    bool     pressed;
    uint16_t time;
} keyevent_t;

typedef struct {
    uint8_t col;
    uint8_t row;
} key_t;

TODO: sample implementation See keyboard/hhkb/keymap.c for sample.

2.5 Backlight Action

These actions control the backlight.

2.5.1 Change backlight level

Increase backlight level.

ACTION_BACKLIGHT_INCREASE()

Decrease backlight level.

ACTION_BACKLIGHT_DECREASE()

Step through backlight levels.

ACTION_BACKLIGHT_STEP()

Turn a specific backlight level on or off.

ACTION_BACKLIGHT_LEVEL(1)

2.5.2 Turn on / off backlight

Turn the backlight on and off without changing level.

ACTION_BACKLIGHT_TOGGLE()

3. Layer switching Example

There are some ways to switch layer with 'Layer' actions.

3.1 Momentary switching

Momentary switching changes layer only while holding Fn key.

This action makes 'Layer 1' active(valid) on key press event and inactive on release event. Namely you can overlay a layer on lower layers or default layer temporarily with this action.

ACTION_LAYER_MOMENTARY(1)

Note that after switching on press the actions on destination layer(Layer 1) are performed. Thus you shall need to place an action to go back on destination layer, or you will be stuck in destination layer without way to get back. Usually you need to place same action or 'KC_TRNS` on destination layer to get back.

3.2 Toggle switching

Toggle switching performed after releasing a key. With this action you can keep staying on the destination layer until you type the key again to return.

This performs toggle switching action of 'Layer 2'.

ACTION_LAYER_TOGGLE(2)

3.3 Momentary switching with Tap key

These actions switch a layer only while holding a key but register the key on tap. Tap means to press and release a key quickly.

ACTION_LAYER_TAP_KEY(2, KC_SCLN)

With this you can place a layer switching action on normal key like ';' without losing its original key register function. This action allows you to have layer switching action without necessity of a dedicated key. It means you can have it even on home row of keyboard.

3.4 Momentary switching with Tap Toggle

This switches layer only while holding a key but toggle layer with several taps. Tap means to press and release key quickly.

ACTION_LAYER_TAP_TOGGLE(1)

Number of taps can be configured with TAPPING_TOGGLE in config.h, 5 by default.

3.5 Momentary switching with Modifiers

This registers modifier key(s) simultaneously with layer switching.

ACTION_LAYER_MODS(2, MOD_LSFT | MOD_LALT)

You can combine four modifiers at most but cannot use both left and right modifiers at a time, either left or right modifiers only can be allowed.

4. Tapping

Tapping is to press and release a key quickly. Tapping speed is determined with setting of TAPPING_TERM, which can be defined in config.h, 200ms by default.

4.1 Tap Key

This is a feature to assign normal key action and modifier including layer switching to just same one physical key. This is a kind of Dual role key. It works as modifier when holding the key but registers normal key when tapping.

Modifier with tap key:

ACTION_MODS_TAP_KEY(MOD_RSFT, KC_GRV)

Layer switching with tap key:

ACTION_LAYER_TAP_KEY(2, KC_SCLN)

4.2 Tap Toggle

This is a feature to assign both toggle layer and momentary switch layer action to just same one physical key. It works as momentary layer switch when holding a key but toggle switch with several taps.

ACTION_LAYER_TAP_TOGGLE(1)

4.3 Oneshot Modifier

This runs onetime effects which modify only on just one following key. It works as normal modifier key when holding down while oneshot modifier when tapping.

ACTION_MODS_ONESHOT(MOD_LSFT)

Say you want to type 'The', you have to push and hold Shift key before type 't' then release it before type 'h' and 'e', otherwise you'll get 'THe' or 'the' unintentionally. With Oneshot Modifier you can tap Shift then type 't', 'h' and 'e' normally, you don't need to holding Shift key properly here. This mean you can release Shift before 't' is pressed down.

Oneshot effect is cancel unless following key is pressed down within ONESHOT_TIMEOUT of config.h. No timeout when it is 0 or not defined.

4.4 Tap Toggle Mods

Similar to layer tap toggle, this works as a momentary modifier when holding, but toggles on with several taps. A single tap will 'unstick' the modifier again.

ACTION_MODS_TAP_TOGGLE(MOD_LSFT)

5. Legacy Keymap

This was used in prior version and still works due to legacy support code in common/keymap.c. Legacy keymap doesn't support many of features that new keymap offers. It is not recommended to use Legacy Keymap for new project.

To enable Legacy Keymap support define this macro in config.h.

#define USE_LEGACY_KEYMAP

Legacy Keymap uses two arrays fn_layer[] and fn_keycode[] to define Fn key. The index of arrays corresponds with postfix number of Fn key. Array fn_layer[] indicates destination layer to switch and fn_keycode[] has keycodes to send when tapping Fn key.

In following setting example, Fn0, Fn1 and Fn2 switch layer to 1, 2 and 2 respectively. Fn2 registers Space key when tapping while Fn0 and Fn1 doesn't send any key.

const uint8_t PROGMEM fn_layer[] = {
    1,              // Fn0
    2,              // Fn1
    2,              // Fn2
};

const uint8_t PROGMEM fn_keycode[] = {
    KC_NO,          // Fn0
    KC_NO,          // Fn1
    KC_SPC,         // Fn2
};

6. Terminology

TBD

keymap

is comprised of multiple layers.

layer

is matrix of keycodes.

key

is physical button on keyboard or logical switch on software.

keycode

is codes used on firmware.

action

is a function assigned on a key.

layer transparency

Using transparent keycode one layer can refer key definition on other lower layer.

layer precedence

Top layer has higher precedence than lower layers.

tapping

is to press and release a key quickly.

Fn key

is key which executes a special action like layer switching, mouse key, macro or etc.

dual role key

http://en.wikipedia.org/wiki/Modifier_key#Dual-role_keys