docs/feature_backlight.md

   1 # Backlighting
   2
   3 Many keyboards support backlit keys by way of individual LEDs placed through or underneath the keyswitches. This feature is distinct from both the [RGB underglow](feature_rgblight.md) and [RGB matrix](feature_rgb_matrix.md) features as it usually allows for only a single colour per switch, though you can obviously install multiple different single coloured LEDs on a keyboard.
   4
   5 QMK is able to control the brightness of these LEDs by switching them on and off rapidly in a certain ratio, a technique known as *Pulse Width Modulation*, or PWM. By altering the duty cycle of the PWM signal, it creates the illusion of dimming.
   6
   7 The MCU can only supply so much current to its GPIO pins. Instead of powering the backlight directly from the MCU, the backlight pin is connected to a transistor or MOSFET that switches the power to the LEDs.
   8
   9 ## Feature Configuration
  10
  11 Most keyboards have backlighting enabled by default if they support it, but if it is not working for you, check that your `rules.mk` includes the following:
  12
  13 ```makefile
  14 BACKLIGHT_ENABLE = yes
  15 ```
  16
  17 ## Keycodes
  18 Once enabled the following keycodes below can be used to change the backlight level.
  19
  20 |Key      |Description                               |
  21 |---------|------------------------------------------|
  22 |`BL_TOGG`|Turn the backlight on or off              |
  23 |`BL_STEP`|Cycle through backlight levels            |
  24 |`BL_ON`  |Set the backlight to max brightness       |
  25 |`BL_OFF` |Turn the backlight off                    |
  26 |`BL_INC` |Increase the backlight level              |
  27 |`BL_DEC` |Decrease the backlight level              |
  28 |`BL_BRTG`|Toggle backlight breathing                |
  29
  30 ## Backlight Functions
  31
  32 |Function  |Description                                                |
  33 |----------|-----------------------------------------------------------|
  34 |`backlight_toggle()`    |Turn the backlight on or off                 |
  35 |`backlight_enable()`    |Turn the backlight on                        |
  36 |`backlight_disable()`   |Turn the backlight off                       |
  37 |`backlight_step()`      |Cycle through backlight levels               |
  38 |`backlight_increase()`  |Increase the backlight level                 |
  39 |`backlight_decrease()`  |Decrease the backlight level                 |
  40 |`backlight_level(x)`    |Sets the backlight level to specified level  |
  41 |`get_backlight_level()` |Return the current backlight level           |
  42 |`is_backlight_enabled()`|Return whether the backlight is currently on |
  43
  44 ### Backlight Breathing Functions
  45
  46 |Function  |Description                                        |
  47 |----------|---------------------------------------------------|
  48 |`breathing_toggle()`  |Turn the backlight breathing on or off |
  49 |`breathing_enable()`  |Turns on backlight breathing           |
  50 |`breathing_disable()` |Turns off backlight breathing          |
  51
  52 ## Driver Configuration
  53
  54 To select which driver to use, configure your `rules.mk` with the following:
  55
  56 ```makefile
  57 BACKLIGHT_DRIVER = software # Valid driver values are 'pwm,software,no'
  58 ```
  59
  60 See below for help on individual drivers.
  61
  62 ## Common Driver Configuration
  63
  64 To change the behavior of the backlighting, `#define` these in your `config.h`:
  65
  66 |Define               |Default      |Description                                                                           |
  67 |---------------------|-------------|--------------------------------------------------------------------------------------|
  68 |`BACKLIGHT_LEVELS`   |`3`          |The number of brightness levels (maximum 31 excluding off)                            |
  69 |`BACKLIGHT_CAPS_LOCK`|*Not defined*|Enable Caps Lock indicator using backlight (for keyboards without dedicated LED)      |
  70 |`BACKLIGHT_BREATHING`|*Not defined*|Enable backlight breathing, if supported                                              |
  71 |`BREATHING_PERIOD`   |`6`          |The length of one backlight "breath" in seconds                                       |
  72 |`BACKLIGHT_ON_STATE` |`0`          |The state of the backlight pin when the backlight is "on" - `1` for high, `0` for low |
  73
  74 ### Backlight On State
  75
  76 Most backlight circuits are driven by an N-channel MOSFET or NPN transistor. This means that to turn the transistor *on* and light the LEDs, you must drive the backlight pin, connected to the gate or base, *high*.
  77 Sometimes, however, a P-channel MOSFET, or a PNP transistor is used. In this case, when the transistor is on, the pin is driven *low* instead.
  78
  79 This functionality is configured at the keyboard level with the `BACKLIGHT_ON_STATE` define.
  80
  81 ## AVR driver
  82
  83 On AVR boards, the default driver currently sniffs the configuration to pick the best scenario. The driver is configured by default, however the equivalent setting within rules.mk would be:
  84 ```makefile
  85 BACKLIGHT_DRIVER = pwm
  86 ```
  87
  88 ### Caveats
  89
  90 Hardware PWM is supported according to the following table:
  91
  92 |Backlight Pin|AT90USB64/128|ATmega16/32U4|ATmega16/32U2|ATmega32A|ATmega328P|
  93 |-------------|-------------|-------------|-------------|---------|----------|
  94 |`B1`         |             |             |             |         |Timer 1   |
  95 |`B2`         |             |             |             |         |Timer 1   |
  96 |`B5`         |Timer 1      |Timer 1      |             |         |          |
  97 |`B6`         |Timer 1      |Timer 1      |             |         |          |
  98 |`B7`         |Timer 1      |Timer 1      |Timer 1      |         |          |
  99 |`C4`         |Timer 3      |             |             |         |          |
 100 |`C5`         |Timer 3      |             |Timer 1      |         |          |
 101 |`C6`         |Timer 3      |Timer 3      |Timer 1      |         |          |
 102 |`D4`         |             |             |             |Timer 1  |          |
 103 |`D5`         |             |             |             |Timer 1  |          |
 104
 105 All other pins will use software PWM. If the [Audio](feature_audio.md) feature is disabled or only using one timer, the backlight PWM can be triggered by a hardware timer:
 106
 107 |Audio Pin|Audio Timer|Software PWM Timer|
 108 |---------|-----------|------------------|
 109 |`C4`     |Timer 3    |Timer 1           |
 110 |`C5`     |Timer 3    |Timer 1           |
 111 |`C6`     |Timer 3    |Timer 1           |
 112 |`B5`     |Timer 1    |Timer 3           |
 113 |`B6`     |Timer 1    |Timer 3           |
 114 |`B7`     |Timer 1    |Timer 3           |
 115
 116 When both timers are in use for Audio, the backlight PWM will not use a hardware timer, but will instead be triggered during the matrix scan. In this case, breathing is not supported, and the backlight might flicker, because the PWM computation may not be called with enough timing precision.
 117
 118 ### AVR Configuration
 119
 120 To change the behavior of the backlighting, `#define` these in your `config.h`:
 121
 122 |Define               |Default      |Description                                                                                                  |
 123 |---------------------|-------------|-------------------------------------------------------------------------------------------------------------|
 124 |`BACKLIGHT_PIN`      |`B7`         |The pin that controls the LEDs. Unless you are designing your own keyboard, you shouldn't need to change this|
 125 |`BACKLIGHT_PINS`     |*Not defined*|experimental: see below for more information                                                                 |
 126 |`BACKLIGHT_LEVELS`   |`3`          |The number of brightness levels (maximum 31 excluding off)                                                   |
 127 |`BACKLIGHT_CAPS_LOCK`|*Not defined*|Enable Caps Lock indicator using backlight (for keyboards without dedicated LED)                             |
 128 |`BACKLIGHT_BREATHING`|*Not defined*|Enable backlight breathing, if supported                                                                     |
 129 |`BREATHING_PERIOD`   |`6`          |The length of one backlight "breath" in seconds                                                              |
 130 |`BACKLIGHT_ON_STATE` |`1`          |The state of the backlight pin when the backlight is "on" - `1` for high, `0` for low                        |
 131
 132 ### Backlight On State
 133
 134 Most backlight circuits are driven by an N-channel MOSFET or NPN transistor. This means that to turn the transistor *on* and light the LEDs, you must drive the backlight pin, connected to the gate or base, *high*.
 135 Sometimes, however, a P-channel MOSFET, or a PNP transistor is used. In this case, when the transistor is on, the pin is driven *low* instead.
 136
 137 This functionality is configured at the keyboard level with the `BACKLIGHT_ON_STATE` define.
 138
 139 ### Multiple backlight pins
 140
 141 Most keyboards have only one backlight pin which control all backlight LEDs (especially if the backlight is connected to an hardware PWM pin).
 142 In software PWM, it is possible to define multiple backlight pins. All those pins will be turned on and off at the same time during the PWM duty cycle.
 143 This feature allows to set for instance the Caps Lock LED (or any other controllable LED) brightness at the same level as the other LEDs of the backlight. This is useful if you have mapped LCTRL in place of Caps Lock and you need the Caps Lock LED to be part of the backlight instead of being activated when Caps Lock is on.
 144
 145 To activate multiple backlight pins, you need to add something like this to your user `config.h`:
 146
 147 ```c
 148 #define BACKLIGHT_LED_COUNT 2
 149 #undef BACKLIGHT_PIN
 150 #define BACKLIGHT_PINS { F5, B2 }
 151 ```
 152
 153 ### Hardware PWM Implementation
 154
 155 When using the supported pins for backlighting, QMK will use a hardware timer configured to output a PWM signal. This timer will count up to `ICRx` (by default `0xFFFF`) before resetting to 0.
 156 The desired brightness is calculated and stored in the `OCRxx` register. When the counter reaches this value, the backlight pin will go low, and is pulled high again when the counter resets.
 157 In this way `OCRxx` essentially controls the duty cycle of the LEDs, and thus the brightness, where `0x0000` is completely off and `0xFFFF` is completely on.
 158
 159 The breathing effect is achieved by registering an interrupt handler for `TIMER1_OVF_vect` that is called whenever the counter resets, roughly 244 times per second.
 160 In this handler, the value of an incrementing counter is mapped onto a precomputed brightness curve. To turn off breathing, the interrupt handler is simply disabled, and the brightness reset to the level stored in EEPROM.
 161
 162 ### Timer Assisted PWM Implementation
 163
 164 When `BACKLIGHT_PIN` is not set to a hardware backlight pin, QMK will use a hardware timer configured to trigger software interrupts. This time will count up to `ICRx` (by default `0xFFFF`) before resetting to 0.
 165 When resetting to 0, the CPU will fire an OVF (overflow) interrupt that will turn the LEDs on, starting the duty cycle.
 166 The desired brightness is calculated and stored in the `OCRxx` register. When the counter reaches this value, the CPU will fire a Compare Output match interrupt, which will turn the LEDs off.
 167 In this way `OCRxx` essentially controls the duty cycle of the LEDs, and thus the brightness, where `0x0000` is completely off and `0xFFFF` is completely on.
 168
 169 The breathing effect is the same as in the hardware PWM implementation.
 170
 171 ## ARM Driver
 172
 173 While still in its early stages, ARM backlight support aims to eventually have feature parity with AVR. The driver is configured by default, however the equivalent setting within rules.mk would be:
 174 ```makefile
 175 BACKLIGHT_DRIVER = pwm
 176 ```
 177
 178 ### Caveats
 179
 180 Currently only hardware PWM is supported, not timer assisted, and does not provide automatic configuration.
 181
 182 ?> Backlight support for STMF072 has had limited testing, YMMV. If unsure, set `BACKLIGHT_ENABLE = no` in your rules.mk.
 183
 184 ### ARM Configuration
 185
 186 To change the behavior of the backlighting, `#define` these in your `config.h`:
 187
 188 |Define                  |Default      |Description                                                                                                  |
 189 |------------------------|-------------|-------------------------------------------------------------------------------------------------------------|
 190 |`BACKLIGHT_PIN`         |`B7`         |The pin that controls the LEDs. Unless you are designing your own keyboard, you shouldn't need to change this|
 191 |`BACKLIGHT_PWM_DRIVER`  |`PWMD4`      |The PWM driver to use, see ST datasheets for pin to PWM timer mapping. Unless you are designing your own keyboard, you shouldn't need to change this|
 192 |`BACKLIGHT_PWM_CHANNEL` |`3`          |The PWM channel to use, see ST datasheets for pin to PWM channel mapping. Unless you are designing your own keyboard, you shouldn't need to change this|
 193 |`BACKLIGHT_PAL_MODE`    |`2`          |The pin alternative function to use, see ST datasheets for pin AF mapping. Unless you are designing your own keyboard, you shouldn't need to change this|
 194
 195 ## Software PWM Driver :id=software-pwm-driver
 196
 197 Emulation of PWM while running other keyboard tasks, it offers maximum hardware compatibility without extra platform configuration. The tradeoff is the backlight might jitter when the keyboard is busy. To enable, add this to your rules.mk:
 198 ```makefile
 199 BACKLIGHT_DRIVER = software
 200 ```
 201
 202 ### Software PWM Configuration
 203
 204 To change the behavior of the backlighting, `#define` these in your `config.h`:
 205
 206 |Define           |Default      |Description                                                                                                  |
 207 |-----------------|-------------|-------------------------------------------------------------------------------------------------------------|
 208 |`BACKLIGHT_PIN`  |`B7`         |The pin that controls the LEDs. Unless you are designing your own keyboard, you shouldn't need to change this|
 209 |`BACKLIGHT_PINS` |*Not defined*|experimental: see below for more information                                                                 |
 210
 211 ### Multiple backlight pins
 212
 213 Most keyboards have only one backlight pin which control all backlight LEDs (especially if the backlight is connected to an hardware PWM pin).
 214 In software PWM, it is possible to define multiple backlight pins. All those pins will be turned on and off at the same time during the PWM duty cycle.
 215 This feature allows to set for instance the Caps Lock LED (or any other controllable LED) brightness at the same level as the other LEDs of the backlight. This is useful if you have mapped LCTRL in place of Caps Lock and you need the Caps Lock LED to be part of the backlight instead of being activated when Caps Lock is on.
 216
 217 To activate multiple backlight pins, you need to add something like this to your user `config.h`:
 218
 219 ```c
 220 #undef BACKLIGHT_PIN
 221 #define BACKLIGHT_PINS { F5, B2 }
 222 ```
 223
 224 ## Custom Driver
 225
 226 To enable, add this to your rules.mk:
 227
 228 ```makefile
 229 BACKLIGHT_DRIVER = custom
 230 ```
 231
 232 When implementing the custom driver API, the provided keyboard hooks are as follows:
 233
 234 ```c
 235 void backlight_init_ports(void) {
 236     // Optional - Run on startup
 237     //          - usually you want to configure pins here
 238 }
 239 void backlight_set(uint8_t level) {
 240     // Optional - Run on level change
 241     //          - usually you want to respond to the new value
 242 }
 243
 244 void backlight_task(void) {
 245     // Optional - Run periodically
 246     //          - long running actions here can cause performance issues
 247 }
 248 ```