181 lines
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<h1><a href="../Features/index.html">Features</a> <svg class="Page__header--separator" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 477.175 477.175"><path d="M360.73 229.075l-225.1-225.1c-5.3-5.3-13.8-5.3-19.1 0s-5.3 13.8 0 19.1l215.5 215.5-215.5 215.5c-5.3 5.3-5.3 13.8 0 19.1 2.6 2.6 6.1 4 9.5 4 3.4 0 6.9-1.3 9.5-4l225.1-225.1c5.3-5.2 5.3-13.8.1-19z"/></svg> <a href="../Features/Backlight.html">Backlight</a></h1>
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<ul class="TableOfContents">
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<li>
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<p><a href="#page_Backlighting">Backlighting</a></p>
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<ul class="TableOfContents">
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<li>
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<p><a href="#page_Backlight-Keycodes">Backlight Keycodes</a></p>
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<li>
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<p><a href="#page_Configuration-Options-in">Configuration Options in <code>config.h</code></a></p>
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</li>
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<li>
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<p><a href="#page_Notes-on-Implementation">Notes on Implementation</a></p>
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</li>
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</ul>
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</li>
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</ul>
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<h1 id="page_Backlighting">Backlighting</h1>
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<!-- FIXME: Describe how backlighting works in QMK -->
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<h2 id="page_Backlight-Keycodes">Backlight Keycodes</h2>
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<p>These keycodes control the backlight. Most keyboards use this for single color in-switch lighting.</p>
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<table>
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<thead>
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<tr>
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<th>Key</th>
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<th>Description</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td><code>BL_TOGG</code></td>
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<td>Turn the backlight on or off</td>
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</tr>
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<tr>
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<td><code>BL_STEP</code></td>
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<td>Cycle through backlight levels</td>
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</tr>
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<tr>
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<td><code>BL_ON</code></td>
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<td>Set the backlight to max brightness</td>
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</tr>
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<tr>
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<td><code>BL_OFF</code></td>
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<td>Turn the backlight off</td>
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</tr>
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<tr>
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<td><code>BL_INC</code></td>
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<td>Increase the backlight level</td>
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</tr>
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<tr>
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<td><code>BL_DEC</code></td>
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<td>Decrease the backlight level</td>
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</tr>
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<tr>
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<td><code>BL_BRTG</code></td>
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<td>Toggle backlight breathing</td>
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</tr>
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</tbody>
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</table>
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<p>Note that for backlight breathing, you need to have <code>#define BACKLIGHT_BREATHING</code> in your config.h.</p>
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<h2 id="page_Configuration-Options-in">Configuration Options in <code>config.h</code></h2>
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<ul>
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<li>
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<code>BACKLIGHT_PIN B7</code> defines the pin that controlls the LEDs. Unless you design your own keyboard, you don't need to set this.</li>
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<li>
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<code>BACKLIGHT_LEVELS 3</code> defines the number of brightness levels (maximum 15 excluding off).</li>
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<li>
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<code>BACKLIGHT_BREATHING</code> if defined, enables backlight breathing. Note that this is only available if <code>BACKLIGHT_PIN</code> is B5, B6 or B7.</li>
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<li>
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<code>BREATHING_PERIOD 6</code> defines the length of one backlight "breath" in seconds.</li>
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</ul>
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<h2 id="page_Notes-on-Implementation">Notes on Implementation</h2>
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<p>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 (<code>0xFFFF</code> 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 <code>0</code> is the darkest and <code>0xFFFF</code> the brightest setting.</p>
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<p>To enable the breathing effect, we register an interrupt handler to be called whenever the counter resets (with <code>ISR(TIMER1_OVF_vect)</code>).
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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.</p>
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