39 lines
2.2 KiB
Markdown
39 lines
2.2 KiB
Markdown
# Backlighting
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<!-- FIXME: Describe how backlighting works in QMK -->
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## Backlight Keycodes
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These keycodes control the backlight. Most keyboards use this for single color in-switch lighting.
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|Key |Description |
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|---------|------------------------------------------|
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|`BL_TOGG`|Turn the backlight on or off |
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|`BL_STEP`|Cycle through backlight levels |
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|`BL_ON` |Set the backlight to max brightness |
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|`BL_OFF` |Turn the backlight off |
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|`BL_INC` |Increase the backlight level |
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|`BL_DEC` |Decrease the backlight level |
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|`BL_BRTG`|Toggle backlight breathing |
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Note that for backlight breathing, you need to have `#define BACKLIGHT_BREATHING` in your config.h.
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## Configuration Options in `config.h`
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* `BACKLIGHT_PIN B7` defines the pin that controlls the LEDs. Unless you design your own keyboard, you don't need to set this.
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* `BACKLIGHT_LEVELS 3` defines the number of brightness levels (maximum 15 excluding off).
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* `BACKLIGHT_BREATHING` if defined, enables backlight breathing. Note that this is only available if `BACKLIGHT_PIN` is B5, B6 or B7.
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* `BREATHING_PERIOD 6` defines the length of one backlight "breath" in seconds.
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## Notes on Implementation
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To change the brightness when using pins B5, B6 or B7, the PWM (Pulse Width Modulation) functionality of the on-chip timer is used.
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The timer is a counter that counts up to a certain TOP value (`0xFFFF` set in ICR1) before resetting to 0.
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We also set an OCR1x register.
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When the counter reaches the value stored in that register, the PWM pin drops to low.
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The PWM pin is pulled high again when the counter resets to 0.
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Therefore, OCR1x basically sets the duty cycle of the LEDs and as such the brightness where `0` is the darkest and `0xFFFF` the brightest setting.
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To enable the breathing effect, we register an interrupt handler to be called whenever the counter resets (with `ISR(TIMER1_OVF_vect)`).
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In this handler, which gets called roughly 244 times per second, we compute the desired brightness using a precomputed brightness curve.
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To disable breathing, we can just disable the respective interrupt vector and reset the brightness to the desired level.
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