qmk_firmware/docs/feature_rgblight.md

RGB Lighting

QMK has the ability to control RGB LEDs attached to your keyboard. This is commonly called underglow, due to the LEDs often being mounted on the bottom of the keyboard, producing a nice diffused effect when combined with a translucent case.

Some keyboards come with RGB LEDs preinstalled. Others must have them installed after the fact. See the Hardware Modification section for information on adding RGB lighting to your keyboard.

Currently QMK supports the following addressable LEDs (however, the white LED in RGBW variants is not supported):

• WS2811, WS2812, WS2812B, WS2812C, etc.
• SK6812, SK6812MINI, SK6805

These LEDs are called "addressable" because instead of using a wire per color, each LED contains a small microchip that understands a special protocol sent over a single wire. The chip passes on the remaining data to the next LED, allowing them to be chained together. In this way, you can easily control the color of the individual LEDs.

Usage

On keyboards with onboard RGB LEDs, it is usually enabled by default. If it is not working for you, check that your rules.mk includes the following:

RGBLIGHT_ENABLE = yes

At minimum you must define the data pin your LED strip is connected to, and the number of LEDs in the strip, in your config.h. If your keyboard has onboard RGB LEDs, and you are simply creating a keymap, you usually won't need to modify these.

Define	Description
RGB_DI_PIN	The pin connected to the data pin of the LEDs
RGBLED_NUM	The number of LEDs connected
RGBLED_SPLIT	(Optional) For split keyboards, the number of LEDs connected on each half directly wired to RGB_DI_PIN

Then you should be able to use the keycodes below to change the RGB lighting to your liking.

Color Selection

QMK uses Hue, Saturation, and Value to select colors rather than RGB. The color wheel below demonstrates how this works.

Changing the Hue cycles around the circle.
Changing the Saturation moves between the inner and outer sections of the wheel, affecting the intensity of the color.
Changing the Value sets the overall brightness.

Keycodes

Key	Aliases	Description
RGB_TOG		Toggle RGB lighting on or off
RGB_MODE_FORWARD	RGB_MOD	Cycle through modes, reverse direction when Shift is held
RGB_MODE_REVERSE	RGB_RMOD	Cycle through modes in reverse, forward direction when Shift is held
RGB_HUI		Increase hue, decrease hue when Shift is held
RGB_HUD		Decrease hue, increase hue when Shift is held
RGB_SAI		Increase saturation, decrease saturation when Shift is held
RGB_SAD		Decrease saturation, increase saturation when Shift is held
RGB_VAI		Increase value (brightness), decrease value when Shift is held
RGB_VAD		Decrease value (brightness), increase value when Shift is held
RGB_MODE_PLAIN	RGB_M_P	Static (no animation) mode
RGB_MODE_BREATHE	RGB_M_B	Breathing animation mode
RGB_MODE_RAINBOW	RGB_M_R	Rainbow animation mode
RGB_MODE_SWIRL	RGB_M_SW	Swirl animation mode
RGB_MODE_SNAKE	RGB_M_SN	Snake animation mode
RGB_MODE_KNIGHT	RGB_M_K	"Knight Rider" animation mode
RGB_MODE_XMAS	RGB_M_X	Christmas animation mode
RGB_MODE_GRADIENT	RGB_M_G	Static gradient animation mode
RGB_MODE_RGBTEST	RGB_M_T	Red, Green, Blue test animation mode

Configuration

Your RGB lighting can be configured by placing these #defines in your config.h:

Define	Default	Description
RGBLIGHT_HUE_STEP	10	The number of steps to cycle through the hue by
RGBLIGHT_SAT_STEP	17	The number of steps to increment the saturation by
RGBLIGHT_VAL_STEP	17	The number of steps to increment the brightness by
RGBLIGHT_LIMIT_VAL	255	The maximum brightness level
RGBLIGHT_SLEEP	Not defined	If defined, the RGB lighting will be switched off when the host goes to sleep
RGBLIGHT_SPLIT	Not defined	If defined, synchronization functionality for split keyboards is added

Effects and Animations

Not only can this lighting be whatever color you want, if RGBLIGHT_EFFECT_xxxx or RGBLIGHT_ANIMATIONS is defined, you also have a number of animation modes at your disposal:

Mode number symbol	Additional number	Description
RGBLIGHT_MODE_STATIC_LIGHT	None	Solid color (this mode is always enabled)
RGBLIGHT_MODE_BREATHING	0,1,2,3	Solid color breathing
RGBLIGHT_MODE_RAINBOW_MOOD	0,1,2	Cycling rainbow
RGBLIGHT_MODE_RAINBOW_SWIRL	0,1,2,3,4,5	Swirling rainbow
RGBLIGHT_MODE_SNAKE	0,1,2,3,4,5	Snake
RGBLIGHT_MODE_KNIGHT	0,1,2	Knight
RGBLIGHT_MODE_CHRISTMAS	None	Christmas
RGBLIGHT_MODE_STATIC_GRADIENT	0,1,..,9	Static gradient
RGBLIGHT_MODE_RGB_TEST	None	RGB Test
RGBLIGHT_MODE_ALTERNATING	None	Alternating
RGBLIGHT_MODE_TWINKLE	0,1,2,3,4,5	Twinkle

Check out this video for a demonstration.

Note: For versions older than 0.6.117, The mode numbers were written directly. In quantum/rgblight.h there is a contrast table between the old mode number and the current symbol.

Effect and Animation Toggles

Use these defines to add or remove animations from the firmware. When you are running low on flash space, it can be helpful to disable animations you are not using.

Define	Default	Description
RGBLIGHT_ANIMATIONS	Not defined	Enable all additional animation modes.
RGBLIGHT_EFFECT_ALTERNATING	Not defined	Enable alternating animation mode.
RGBLIGHT_EFFECT_BREATHING	Not defined	Enable breathing animation mode.
RGBLIGHT_EFFECT_CHRISTMAS	Not defined	Enable christmas animation mode.
RGBLIGHT_EFFECT_KNIGHT	Not defined	Enable knight animation mode.
RGBLIGHT_EFFECT_RAINBOW_MOOD	Not defined	Enable rainbow mood animation mode.
RGBLIGHT_EFFECT_RAINBOW_SWIRL	Not defined	Enable rainbow swirl animation mode.
RGBLIGHT_EFFECT_RGB_TEST	Not defined	Enable RGB test animation mode.
RGBLIGHT_EFFECT_SNAKE	Not defined	Enable snake animation mode.
RGBLIGHT_EFFECT_STATIC_GRADIENT	Not defined	Enable static gradient mode.
RGBLIGHT_EFFECT_TWINKLE	Not defined	Enable twinkle animation mode.

Effect and Animation Settings

The following options are used to tweak the various animations:

Define	Default	Description
RGBLIGHT_EFFECT_BREATHE_CENTER	Not defined	If defined, used to calculate the curve for the breathing animation. Valid values are 1.0 to 2.7
RGBLIGHT_EFFECT_BREATHE_MAX	255	The maximum brightness for the breathing mode. Valid values are 1 to 255
RGBLIGHT_EFFECT_CHRISTMAS_INTERVAL	1000	How long to wait between light changes for the "Christmas" animation, in milliseconds
RGBLIGHT_EFFECT_CHRISTMAS_STEP	2	The number of LEDs to group the red/green colors by for the "Christmas" animation
RGBLIGHT_EFFECT_KNIGHT_LED_NUM	RGBLED_NUM	The number of LEDs to have the "Knight" animation travel
RGBLIGHT_EFFECT_KNIGHT_LENGTH	3	The number of LEDs to light up for the "Knight" animation
RGBLIGHT_EFFECT_KNIGHT_OFFSET	0	The number of LEDs to start the "Knight" animation from the start of the strip by
RGBLIGHT_RAINBOW_SWIRL_RANGE	255	Range adjustment for the rainbow swirl effect to get different swirls
RGBLIGHT_EFFECT_SNAKE_LENGTH	4	The number of LEDs to light up for the "Snake" animation
RGBLIGHT_EFFECT_TWINKLE_LIFE	75	Adjusts how quickly each LED brightens and dims when twinkling (in animation steps)
RGBLIGHT_EFFECT_TWINKLE_PROBABILITY	1/127	Adjusts how likely each LED is to twinkle (on each animation step)

Example Usage to Reduce Memory Footprint

1. Remove RGBLIGHT_ANIMATIONS from config.h.
2. Selectively add the animations you want to enable. The following would enable two animations and save about 4KiB:

 #undef RGBLED_NUM
-#define RGBLIGHT_ANIMATIONS
+#define RGBLIGHT_EFFECT_STATIC_GRADIENT
+#define RGBLIGHT_EFFECT_RAINBOW_SWIRL
 #define RGBLED_NUM 12
 #define RGBLIGHT_HUE_STEP 8
 #define RGBLIGHT_SAT_STEP 8

Animation Speed

You can also modify the speeds that the different modes animate at:

Here is a quick demo on Youtube (with NPKC KC60) (https://www.youtube.com/watch?v=VKrpPAHlisY).

// How long (in milliseconds) to wait between animation steps for each of the "Solid color breathing" animations
const uint8_t RGBLED_BREATHING_INTERVALS[] PROGMEM = {30, 20, 10, 5};

// How long (in milliseconds) to wait between animation steps for each of the "Cycling rainbow" animations
const uint8_t RGBLED_RAINBOW_MOOD_INTERVALS[] PROGMEM = {120, 60, 30};

// How long (in milliseconds) to wait between animation steps for each of the "Swirling rainbow" animations
const uint8_t RGBLED_RAINBOW_SWIRL_INTERVALS[] PROGMEM = {100, 50, 20};

// How long (in milliseconds) to wait between animation steps for each of the "Snake" animations
const uint8_t RGBLED_SNAKE_INTERVALS[] PROGMEM = {100, 50, 20};

// How long (in milliseconds) to wait between animation steps for each of the "Knight" animations
const uint8_t RGBLED_KNIGHT_INTERVALS[] PROGMEM = {127, 63, 31};

// How long (in milliseconds) to wait between animation steps for each of the "Twinkle" animations
const uint8_t RGBLED_TWINKLE_INTERVALS[] PROGMEM = {50, 25, 10};

// These control which hues are selected for each of the "Static gradient" modes
const uint8_t RGBLED_GRADIENT_RANGES[] PROGMEM = {255, 170, 127, 85, 64};

Lighting Layers

By including #define RGBLIGHT_LAYERS in your config.h file you can enable lighting layers. These make it easy to use your underglow LEDs as status indicators to show which keyboard layer is currently active, or the state of caps lock, all without disrupting any animations. Here's a video showing an example of what you can do.

Defining Lighting Layers :id=defining-lighting-layers

By default, 8 layers are possible. This can be expanded to as many as 32 by overriding the definition of RGBLIGHT_MAX_LAYERS in config.h (e.g. #define RGBLIGHT_MAX_LAYERS 32). Please note, if you use a split keyboard, you will need to flash both sides of the split after changing this. Also, increasing the maximum will increase the firmware size, and will slow sync on split keyboards.

To define a layer, we modify keymap.c to list out LED ranges and the colors we want to overlay on them using an array of rgblight_segment_t using the RGBLIGHT_LAYER_SEGMENTS macro. We can define multiple layers and enable/disable them independently:

// Light LEDs 6 to 9 and 12 to 15 red when caps lock is active. Hard to ignore!
const rgblight_segment_t PROGMEM my_capslock_layer[] = RGBLIGHT_LAYER_SEGMENTS(
    {6, 4, HSV_RED},       // Light 4 LEDs, starting with LED 6
    {12, 4, HSV_RED}       // Light 4 LEDs, starting with LED 12
);
// Light LEDs 9 & 10 in cyan when keyboard layer 1 is active
const rgblight_segment_t PROGMEM my_layer1_layer[] = RGBLIGHT_LAYER_SEGMENTS(
    {9, 2, HSV_CYAN}
);
// Light LEDs 11 & 12 in purple when keyboard layer 2 is active
const rgblight_segment_t PROGMEM my_layer2_layer[] = RGBLIGHT_LAYER_SEGMENTS(
    {11, 2, HSV_PURPLE}
);
// etc..

We combine these layers into an array using the RGBLIGHT_LAYERS_LIST macro, and assign it to the rgblight_layers variable during keyboard setup. Note that you can only define up to 8 lighting layers. Any extra layers will be ignored. Since the different lighting layers overlap, the order matters in the array, with later layers taking precedence:

// Now define the array of layers. Later layers take precedence
const rgblight_segment_t* const PROGMEM my_rgb_layers[] = RGBLIGHT_LAYERS_LIST(
    my_capslock_layer,
    my_layer1_layer,    // Overrides caps lock layer
    my_layer2_layer     // Overrides other layers
);

void keyboard_post_init_user(void) {
    // Enable the LED layers
    rgblight_layers = my_rgb_layers;
}

Note: For split keyboards with two controllers, both sides need to be flashed when updating the contents of rgblight_layers.

Enabling and disabling lighting layers :id=enabling-lighting-layers

Everything above just configured the definition of each lighting layer. We can now enable and disable the lighting layers whenever the state of the keyboard changes:

layer_state_t layer_state_set_user(layer_state_t state) {
    // Both layers will light up if both kb layers are active
    rgblight_set_layer_state(1, layer_state_cmp(state, 1));
    rgblight_set_layer_state(2, layer_state_cmp(state, 2));
    return state;
}

bool led_update_user(led_t led_state) {
    rgblight_set_layer_state(0, led_state.caps_lock);
    return true;
}

Lighting layer blink :id=lighting-layer-blink

By including #define RGBLIGHT_LAYER_BLINK in your config.h file you can turn a lighting layer on for a specified duration. Once the specified number of milliseconds has elapsed the layer will be turned off. This is useful, e.g., if you want to acknowledge some action (e.g. toggling some setting):

const rgblight_segment_t PROGMEM _yes_layer[] = RGBLIGHT_LAYER_SEGMENTS( {9, 6, HSV_GREEN} );
const rgblight_segment_t PROGMEM _no_layer[] = RGBLIGHT_LAYER_SEGMENTS( {9, 6, HSV_RED} );

const rgblight_segment_t* const PROGMEM _rgb_layers[] =
    RGBLIGHT_LAYERS_LIST( _yes_layer, _no_layer );

void keyboard_post_init_user(void) {
    rgblight_layers = _rgb_layers;
}

// Note we user post_process_record_user because we want the state
// after the flag has been flipped...
void post_process_record_user(uint16_t keycode, keyrecord_t *record) {
    switch (keycode) {
        case DEBUG:
            rgblight_blink_layer(debug_enable ? 0 : 1, 500);
            break;

        case NK_TOGG:
        case NK_ON:
        case NK_OFF:
            rgblight_blink_layer(keymap_config.nkro ? 0 : 1, 500);
            break;
    }
}

Functions

If you need to change your RGB lighting in code, for example in a macro to change the color whenever you switch layers, QMK provides a set of functions to assist you. See rgblight.h for the full list, but the most commonly used functions include:

Utility Functions

Function	Description
sethsv(hue, sat, val, ledbuf)	Set ledbuf to the given HSV value
sethsv_raw(hue, sat, val, ledbuf)	Set ledbuf to the given HSV value without RGBLIGHT_LIMIT_VAL check
setrgb(r, g, b, ledbuf)	Set ledbuf to the given RGB value where r/g/b

Low level Functions

Function	Description
rgblight_set()	Flash out led buffers to LEDs
rgblight_set_clipping_range(pos, num)	Set clipping Range. see Clipping Range

Example:

sethsv(HSV_WHITE, (LED_TYPE *)&led[0]); // led 0
sethsv(HSV_RED,   (LED_TYPE *)&led[1]); // led 1
sethsv(HSV_GREEN, (LED_TYPE *)&led[2]); // led 2
rgblight_set(); // Utility functions do not call rgblight_set() automatically, so they need to be called explicitly.

Effects and Animations Functions

effect range setting

Function	Description
rgblight_set_effect_range(pos, num)	Set Effects Range

direct operation

Function	Description
rgblight_setrgb_at(r, g, b, index)	Set a single LED to the given RGB value, where r/g/b are between 0 and 255 and index is between 0 and RGBLED_NUM (not written to EEPROM)
rgblight_sethsv_at(h, s, v, index)	Set a single LED to the given HSV value, where h/s/v are between 0 and 255, and index is between 0 and RGBLED_NUM (not written to EEPROM)
rgblight_setrgb_range(r, g, b, start, end)	Set a continuous range of LEDs to the given RGB value, where r/g/b are between 0 and 255 and start(included) and stop(excluded) are between 0 and RGBLED_NUM (not written to EEPROM)
rgblight_sethsv_range(h, s, v, start, end)	Set a continuous range of LEDs to the given HSV value, where h/s/v are between 0 and 255, and start(included) and stop(excluded) are between 0 and RGBLED_NUM (not written to EEPROM)
rgblight_setrgb(r, g, b)	Set effect range LEDs to the given RGB value where r/g/b are between 0 and 255 (not written to EEPROM)
rgblight_setrgb_master(r, g, b)	Set the LEDs on the master side to the given RGB value, where r/g/b are between 0 and 255 (not written to EEPROM)
rgblight_setrgb_slave(r, g, b)	Set the LEDs on the slave side to the given RGB value, where r/g/b are between 0 and 255 (not written to EEPROM)
rgblight_sethsv_master(h, s, v)	Set the LEDs on the master side to the given HSV value, where h/s/v are between 0 and 255 (not written to EEPROM)
rgblight_sethsv_slave(h, s, v)	Set the LEDs on the slave side to the given HSV value, where h/s/v are between 0 and 255 (not written to EEPROM)

Example:

rgblight_sethsv(HSV_WHITE, 0); // led 0
rgblight_sethsv(HSV_RED,   1); // led 1
rgblight_sethsv(HSV_GREEN, 2); // led 2
// The above functions automatically calls rgblight_set(), so there is no need to call it explicitly.
// Note that it is inefficient to call repeatedly.

effect mode change

Function	Description
rgblight_mode(x)	Set the mode, if RGB animations are enabled
rgblight_mode_noeeprom(x)	Set the mode, if RGB animations are enabled (not written to EEPROM)
rgblight_step()	Change the mode to the next RGB animation in the list of enabled RGB animations
rgblight_step_noeeprom()	Change the mode to the next RGB animation in the list of enabled RGB animations (not written to EEPROM)
rgblight_step_reverse()	Change the mode to the previous RGB animation in the list of enabled RGB animations
rgblight_step_reverse_noeeprom()	Change the mode to the previous RGB animation in the list of enabled RGB animations (not written to EEPROM)

effects mode disable/enable

Function	Description
rgblight_toggle()	Toggle effect range LEDs between on and off
rgblight_toggle_noeeprom()	Toggle effect range LEDs between on and off (not written to EEPROM)
rgblight_enable()	Turn effect range LEDs on, based on their previous state
rgblight_enable_noeeprom()	Turn effect range LEDs on, based on their previous state (not written to EEPROM)
rgblight_disable()	Turn effect range LEDs off
rgblight_disable_noeeprom()	Turn effect range LEDs off (not written to EEPROM)

hue, sat, val change

Function	Description
rgblight_increase_hue()	Increase the hue for effect range LEDs. This wraps around at maximum hue
rgblight_increase_hue_noeeprom()	Increase the hue for effect range LEDs. This wraps around at maximum hue (not written to EEPROM)
rgblight_decrease_hue()	Decrease the hue for effect range LEDs. This wraps around at minimum hue
rgblight_decrease_hue_noeeprom()	Decrease the hue for effect range LEDs. This wraps around at minimum hue (not written to EEPROM)
rgblight_increase_sat()	Increase the saturation for effect range LEDs. This wraps around at maximum saturation
rgblight_increase_sat_noeeprom()	Increase the saturation for effect range LEDs. This wraps around at maximum saturation (not written to EEPROM)
rgblight_decrease_sat()	Decrease the saturation for effect range LEDs. This wraps around at minimum saturation
rgblight_decrease_sat_noeeprom()	Decrease the saturation for effect range LEDs. This wraps around at minimum saturation (not written to EEPROM)
rgblight_increase_val()	Increase the value for effect range LEDs. This wraps around at maximum value
rgblight_increase_val_noeeprom()	Increase the value for effect range LEDs. This wraps around at maximum value (not written to EEPROM)
rgblight_decrease_val()	Decrease the value for effect range LEDs. This wraps around at minimum value
rgblight_decrease_val_noeeprom()	Decrease the value for effect range LEDs. This wraps around at minimum value (not written to EEPROM)
rgblight_sethsv(h, s, v)	Set effect range LEDs to the given HSV value where h/s/v are between 0 and 255
rgblight_sethsv_noeeprom(h, s, v)	Set effect range LEDs to the given HSV value where h/s/v are between 0 and 255 (not written to EEPROM)

layer functions

Function	Description
rgblight_get_layer_state(i)	Returns true if lighting layer i is enabled
rgblight_set_layer_state(i, is_on)	Enable or disable lighting layer i based on value of bool is_on

query

Function	Description
rgblight_get_mode()	Get current mode
rgblight_get_hue()	Get current hue
rgblight_get_sat()	Get current sat
rgblight_get_val()	Get current val

Colors

These are shorthands to popular colors. The RGB ones can be passed to the setrgb functions, while the HSV ones to the sethsv functions.

RGB	HSV
RGB_WHITE	HSV_WHITE
RGB_RED	HSV_RED
RGB_CORAL	HSV_CORAL
RGB_ORANGE	HSV_ORANGE
RGB_GOLDENROD	HSV_GOLDENROD
RGB_GOLD	HSV_GOLD
RGB_YELLOW	HSV_YELLOW
RGB_CHARTREUSE	HSV_CHARTREUSE
RGB_GREEN	HSV_GREEN
RGB_SPRINGGREEN	HSV_SPRINGGREEN
RGB_TURQUOISE	HSV_TURQUOISE
RGB_TEAL	HSV_TEAL
RGB_CYAN	HSV_CYAN
RGB_AZURE	HSV_AZURE
RGB_BLUE	HSV_BLUE
RGB_PURPLE	HSV_PURPLE
RGB_MAGENTA	HSV_MAGENTA
RGB_PINK	HSV_PINK

rgblight_setrgb(RGB_ORANGE);
rgblight_sethsv_noeeprom(HSV_GREEN);
rgblight_setrgb_at(RGB_GOLD, 3);
rgblight_sethsv_range(HSV_WHITE, 0, 6);

These are defined in rgblight_list.h. Feel free to add to this list!

Changing the order of the LEDs

If you want to make the logical order of LEDs different from the electrical connection order, you can do this by defining the RGBLIGHT_LED_MAP macro in your config.h.

Normally, the contents of the LED buffer are output to the LEDs in the same order.

By defining RGBLIGHT_LED_MAP as in the example below, you can specify the LED with addressing in reverse order of the electrical connection order.

// config.h

#define RGBLED_NUM 4
#define RGBLIGHT_LED_MAP { 3, 2, 1, 0 }

For keyboards that use the RGB LEDs as a backlight for each key, you can also define it as in the example below.

// config.h

#define RGBLED_NUM 30

/* RGB LED Conversion macro from physical array to electric array */
#define LED_LAYOUT( \
    L00, L01, L02, L03, L04, L05,  \
    L10, L11, L12, L13, L14, L15,  \
    L20, L21, L22, L23, L24, L25,  \
    L30, L31, L32, L33, L34, L35,  \
    L40, L41, L42, L43, L44, L45 ) \
  { \
    L05, L04, L03, L02, L01, L00,   \
    L10, L11, L12, L13, L14, L15,   \
    L25, L24, L23, L22, L21, L20,   \
    L30, L31, L32, L33, L34, L35,   \
    L46, L45, L44, L43, L42, L41    \
  }

/* RGB LED logical order map */
/* Top->Bottom, Right->Left */
#define RGBLIGHT_LED_MAP LED_LAYOUT( \
  25, 20, 15, 10,  5,  0,       \
  26, 21, 16, 11,  6,  1,       \
  27, 22, 17, 12,  7,  2,       \
  28, 23, 18, 13,  8,  3,       \
  29, 24, 19, 14,  9,  4 )

Clipping Range

Using the rgblight_set_clipping_range() function, you can prepare more buffers than the actual number of LEDs, and output some of the buffers to the LEDs. This is useful if you want the split keyboard to treat left and right LEDs as logically contiguous.

You can set the Clipping Range by executing the following code.

// some source
rgblight_set_clipping_range(3, 4);

In addition to setting the Clipping Range, you can use RGBLIGHT_LED_MAP together.

// config.h
#define RGBLED_NUM 8
#define RGBLIGHT_LED_MAP { 7, 6, 5, 4, 3, 2, 1, 0 }

// some soruce
  rgblight_set_clipping_range(3, 4);

Hardware Modification

If your keyboard lacks onboard underglow LEDs, you may often be able to solder on an RGB LED strip yourself. You will need to find an unused pin to wire to the data pin of your LED strip. Some keyboards may break out unused pins from the MCU to make soldering easier. The other two pins, VCC and GND, must also be connected to the appropriate power pins.