Keyboard switch matrices are arranged in rows and columns. Without a matrix circuit, each switch would require its own wire directly to the controller.
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.
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:
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.
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:
A problem arises when you start pressing more than one key at a time. Looking at our matrix again, it should become pretty obvious:
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
The data we get from that is:
col0: 0b11
col1: 0b11
│└row0
└row1
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;
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 ─────┴────────┘
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: `>|`
Now when we press the three keys, invoking what would be a ghosting scenario:
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 ─────┴────────┘
Things act as they should! Which will get us the following data:
col0: 0b01
col1: 0b11
│└row0
└row1
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.
