116 lines
6.3 KiB
Markdown
116 lines
6.3 KiB
Markdown
# How a Keyboard Matrix Works
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Keyboard switch matrices are arranged in rows and columns. Without a matrix circuit, each switch would require its own wire directly to the controller.
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When the circuit is arranged in rows and columns, if a key is pressed, a column wire makes contact with a row wire and completes a circuit. The keyboard controller detects this closed circuit and registers it as a key press.
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The microcontroller will be set up via the firmware to send a logical 1 to the columns, one at a time, and read from the rows, all at once - this process is called matrix scanning. The matrix is a bunch of open switches that, by default, don't allow any current to pass through - the firmware will read this as no keys being pressed. As soon as you press one key down, the logical 1 that was coming from the column the keyswitch is attached to gets passed through the switch and to the corresponding row - check out the following 2x2 example:
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```
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Column 0 being scanned Column 1 being scanned
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x x
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col0 col1 col0 col1
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| | | |
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row0 ---(key0)---(key1) row0 ---(key0)---(key1)
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| | | |
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row1 ---(key2)---(key3) row1 ---(key2)---(key3)
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```
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The `x` 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 `x`. For one keyswitch, keep in mind that one side of the contacts is connected to its row, and the other, its column.
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When we press `key0`, `col0` gets connected to `row0`, so the values that the firmware receives for that row is `0b01` (the `0b` 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'll use this notation to show when a keyswitch has been pressed, to show that the column and row are being connected:
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```
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Column 0 being scanned Column 1 being scanned
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x x
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col0 col1 col0 col1
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x row0 ---(-+-0)---(key1) row0 ---(-+-0)---(key1)
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row1 ---(key2)---(key3) row1 ---(key2)---(key3)
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```
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We can now see that `row0` has an `x`, so has the value of 1. As a whole, the data the firmware receives when `key0` is pressed is:
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```
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col0: 0b01
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col1: 0b00
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│└row0
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└row1
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```
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A problem arises when you start pressing more than one key at a time. Looking at our matrix again, it should become pretty obvious:
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```
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Column 0 being scanned Column 1 being scanned
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x x
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col0 col1 col0 col1
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x row0 ---(-+-0)---(-+-1) x row0 ---(-+-0)---(-+-1)
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x row1 ---(key2)---(-+-3) x row1 ---(key2)---(-+-3)
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Remember that this ^ is still connected to row1
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```
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The data we get from that is:
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```
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col0: 0b11
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col1: 0b11
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│└row0
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└row1
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```
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Which isn'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'll represent a dioded matrix like this;
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```
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Column 0 being scanned Column 1 being scanned
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x x
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col0 col1 col0 col1
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│ │ | │
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(key0) (key1) (key0) (key1)
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! │ ! │ ! | ! │
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row0 ─────┴────────┘ │ row0 ─────┴────────┘ │
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│ │ | │
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(key2) (key3) (key2) (key3)
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! ! ! !
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row1 ─────┴────────┘ row1 ─────┴────────┘
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```
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In practical applications, the black line of the diode will be placed facing the row, and away from the keyswitch - the `!` 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: `>|`
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Now when we press the three keys, invoking what would be a ghosting scenario:
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```
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Column 0 being scanned Column 1 being scanned
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x x
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col0 col1 col0 col1
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│ │ │ │
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(┌─┤0) (┌─┤1) (┌─┤0) (┌─┤1)
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! │ ! │ ! │ ! │
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x row0 ─────┴────────┘ │ x row0 ─────┴────────┘ │
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│ │ │ │
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(key2) (┌─┘3) (key2) (┌─┘3)
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! ! ! !
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row1 ─────┴────────┘ x row1 ─────┴────────┘
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```
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Things act as they should! Which will get us the following data:
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```
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col0: 0b01
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col1: 0b11
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│└row0
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└row1
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```
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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.
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Further reading:
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- [Wikipedia article](https://en.wikipedia.org/wiki/Keyboard_matrix_circuit)
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- [Deskthority article](https://deskthority.net/wiki/Keyboard_matrix)
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- [Keyboard Matrix Help by Dave Dribin (2000)](https://www.dribin.org/dave/keyboard/one_html/)
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- [How Key Matrices Works by PCBheaven](https://pcbheaven.com/wikipages/How_Key_Matrices_Works/) (animated examples)
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- [How keyboards work - QMK documentation](how_keyboards_work)
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