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x x
col0 col1 col0 col1
| | | |
row0 ---(key0)---(key1) row0 ---(key0)---(key1)
| | | |
row1 ---(key2)---(key3) row1 ---(key2)---(key3)
</code></pre><p>The <code>x</code> represents that the column/row associated has a value of 1, or is HIGH. Here, we see that no keys are being pressed, so no rows get an <code>x</code>. For one keyswitch, keep in mind that one side of the contacts is connected to its row, and the other, its column.</p><p>When we press <code>key0</code>, <code>col0</code> gets connected to <code>row0</code>, so the values that the firmware receives for that row is <code>0b01</code> (the <code>0b</code> here means that this is a bit value, meaning all of the following digits are bits - 0 or 1 - and represent the keys in that column). We&#39;ll use this notation to show when a keyswitch has been pressed, to show that the column and row are being connected:</p><pre><code> Column 0 being scanned Column 1 being scanned
x x
col0 col1 col0 col1
| | | |
x row0 ---(-+-0)---(key1) row0 ---(-+-0)---(key1)
| | | |
row1 ---(key2)---(key3) row1 ---(key2)---(key3)
</code></pre><p>We can now see that <code>row0</code> has an <code>x</code>, so has the value of 1. As a whole, the data the firmware receives when <code>key0</code> is pressed is:</p><pre><code>col0: 0b01
col1: 0b00
│└row0
└row1
</code></pre><p>A problem arises when you start pressing more than one key at a time. Looking at our matrix again, it should become pretty obvious:</p><pre><code> Column 0 being scanned Column 1 being scanned
x x
col0 col1 col0 col1
| | | |
x row0 ---(-+-0)---(-+-1) x row0 ---(-+-0)---(-+-1)
| | | |
x row1 ---(key2)---(-+-3) x row1 ---(key2)---(-+-3)
Remember that this ^ is still connected to row1
</code></pre><p>The data we get from that is:</p><pre><code>col0: 0b11
col1: 0b11
│└row0
└row1
</code></pre><p>Which isn&#39;t accurate, since we only have 3 keys pressed down, not all 4. This behavior is called ghosting, and only happens in odd scenarios like this, but can be much more common on a bigger keyboard. The way we can get around this is by placing a diode after the keyswitch, but before it connects to its row. A diode only allows current to pass through one way, which will protect our other columns/rows from being activated in the previous example. We&#39;ll represent a dioded matrix like this;</p><pre><code> Column 0 being scanned Column 1 being scanned
x x
col0 col1 col0 col1
│ │ | │
(key0) (key1) (key0) (key1)
! │ ! │ ! | ! │
row0 ─────┴────────┘ │ row0 ─────┴────────┘ │
│ │ | │
(key2) (key3) (key2) (key3)
! ! ! !
row1 ─────┴────────┘ row1 ─────┴────────┘
</code></pre><p>In practical applications, the black line of the diode will be placed facing the row, and away from the keyswitch - the <code>!</code> in this case is the diode, where the gap represents the black line. A good way to remember this is to think of this symbol: <code>&gt;|</code></p><p>Now when we press the three keys, invoking what would be a ghosting scenario:</p><pre><code> Column 0 being scanned Column 1 being scanned
x x
col0 col1 col0 col1
│ │ │ │
(┌─┤0) (┌─┤1) (┌─┤0) (┌─┤1)
! │ ! │ ! │ ! │
x row0 ─────┴────────┘ │ x row0 ─────┴────────┘ │
│ │ │ │
(key2) (┌─┘3) (key2) (┌─┘3)
! ! ! !
row1 ─────┴────────┘ x row1 ─────┴────────┘
</code></pre><p>Things act as they should! Which will get us the following data:</p><pre><code>col0: 0b01
col1: 0b11
│└row0
└row1
</code></pre><p>The firmware can then use this correct data to detect what it should do, and eventually, what signals it needs to send to the OS.</p><p>Further reading:</p><ul><li><a href="https://en.wikipedia.org/wiki/Keyboard_matrix_circuit" target="_blank" rel="noreferrer">Wikipedia article</a></li><li><a href="https://deskthority.net/wiki/Keyboard_matrix" target="_blank" rel="noreferrer">Deskthority article</a></li><li><a href="https://www.dribin.org/dave/keyboard/one_html/" target="_blank" rel="noreferrer">Keyboard Matrix Help by Dave Dribin (2000)</a></li><li><a href="https://pcbheaven.com/wikipages/How_Key_Matrices_Works/" target="_blank" rel="noreferrer">How Key Matrices Works by PCBheaven</a> (animated examples)</li><li><a href="./how_keyboards_work">How keyboards work - QMK documentation</a></li></ul></div></div></main><footer class="VPDocFooter" data-v-39a288b8 data-v-09de1c0f><!--[--><!--]--><!----><nav class="prev-next" data-v-09de1c0f><div class="pager" data-v-09de1c0f><a class="VPLink link pager-link prev" href="/how_keyboards_work" data-v-09de1c0f><!--[--><span class="desc" data-v-09de1c0f>Previous page</span><span class="title" data-v-09de1c0f>How Keyboards Work</span><!--]--></a></div><div class="pager" data-v-09de1c0f><a class="VPLink link pager-link next" href="/understanding_qmk" data-v-09de1c0f><!--[--><span class="desc" data-v-09de1c0f>Next page</span><span class="title" data-v-09de1c0f>Understanding QMK</span><!--]--></a></div></nav></footer><!--[--><!--]--></div></div></div><!--[--><!--]--></div></div><!----><!--[--><!--]--></div></div>
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