kmk_firmware/docs/en/scanners.md

Scanners

The default key scanner in KMK assumes a garden variety switch matrix, with one diode per switch to prevent ghosting. This doesn't cover all hardware designs though. With macro pads, for example, it is very common to not have a matrix topology at all. Boards like this aren't compatible with the default matrix scanner, so you will need to swap it out with an alternative scanner.

Keypad Scanners

The scanners in kmk.scanners.keypad wrap the keypad module that ships with CircuitPython and support the same configuration and tuning options as their upstream. You can find out more in the CircuitPython documentation.

keypad MatrixScanner

This is the default scanner used by KMK. It uses the CircuitPython builtin keypad.KeyMatrix.

from kmk.scanners.keypad import MatrixScanner

class MyKeyboard(KMKKeyboard):
    def __init__(self):
        # create and register the scanner
        self.matrix = MatrixScanner(
            # required arguments:
            column_pins=self.col_pins,
            row_pins=self.row_pins,
            # optional arguments with defaults:
            columns_to_anodes=DiodeOrientation.COL2ROW,
            interval=0.02,  # Debounce time in floating point seconds
            max_events=64
        )

keypad KeysScanner

The keypad.Keys scanner treats individual GPIO pins as discrete keys. To use this scanner, provide a sequence of pins that describes the layout of your board then include it in the initialization sequence of your keyboard class.

import board
from kmk.kmk_keyboard import KMKKeyboard
from kmk.scanners.keypad import KeysScanner


# GPIO to key mapping - each line is a new row.
_KEY_CFG = [
    board.SW3,  board.SW7,  board.SW11, board.SW15,
    board.SW2,  board.SW6,  board.SW10, board.SW14,
    board.SW1,  board.SW5,  board.SW9,  board.SW13,
    board.SW0,  board.SW4,  board.SW8,  board.SW12,
]


# Keyboard implementation class
class MyKeyboard(KMKKeyboard):
    def __init__(self):
        # create and register the scanner
        self.matrix = KeysScanner(
            # require argument:
            pins=_KEY_CFG,
            # optional arguments with defaults:
            value_when_pressed=False,
            pull=True,
            interval=0.02,  # Debounce time in floating point seconds
            max_events=64
        )

keypad ShiftRegisterKeys

This scanner can read keys attached to a parallel-in serial-out shift register like the 74HC165 or CD4021. Note that you may chain shift registers to load in as many values as you need.

from kmk.scanners.keypad import ShiftRegisterKeys

class MyKeyboard(KMKKeyboard):
    def __init__(self):
        # create and register the scanner
        self.matrix = ShiftRegisterKeys(
            # require arguments:
            clock=board.GP0,
            data=board.GP1,
            latch=board.GP2,
            key_count=8,
            # optional arguments with defaults:
            value_to_latch=True, # 74HC165: True, CD4021: False
            value_when_pressed=False,
            interval=0.02,  # Debounce time in floating point seconds
            max_events=64
        )

Digitalio Scanners

digitalio MatrixScanner

The digitalio Matrix can scan over, as the name implies, digitalio.DigitalInOut objects. That is especially useful if a matrix is build with IO-expanders.

from kmk.scanners.digitalio import MatrixScanner

class MyKeyboard(KMKKeyboard):
    def __init__(self):
        # create and register the scanner
        self.matrix = MatrixScanner(
            cols=self.col_pins,
            rows=self.row_pins,
            diode_orientation=self.diode_orientation,
            rollover_cols_every_rows=None, # optional
        )

Rotary Encoder Scanners

RotaryioEncoder

Matrix events from a quadrature ("rotary") encoder?

from kmk.scanners.encoder import RotaryioEncoder

class MyKeyboard(KMKKeyboard):
    def __init__(self):
        # create and register the scanner
        self.matrix = RotaryioEncoder(
            pin_a=board.GP0,
            pin_b=board.GP1,
            # optional
            divisor=4,
        )

Scanner base class

If you require a different type of scanner, you can create your own by providing a subclass of Scanner. This is a very simple interface, it only contains a single method, scan_for_changes(self) which returns a key report if one exists, or None otherwise.

Advanced Configuration

Multiple Scanners

Sometimes a single scanner doesn't cover all hardware configurations. For example: The bulk of the keyboard may be scanned with a matrix scanner, but a couple of additional keys are directly connected to GPIOs. In that case KMK allows you to define multiple scanners. The KMKKeyboard.matrix attribute can either be assigned a single scanner, or a list of scanners. KMK assumes that successive scanner keys are consecutive, and populates KMKKeyboard.coord_mapping accordingly; for convenience you may have to supply a coord_mapping that resembles your physical layout more closely (expanded below).

Example:

class MyKeyboard(KMKKeyboard):
    self.matrix = [
        MatrixScanner(...),
        KeysScanner(...),
        # etc...
    ]

Multiple Scanners coord_mapping and keymap changes

To add more scanners you need to add onto your coord_mapping.

Example:

coord_mapping with just one MatrixScanner on a 58 key split keyboard:

coord_mapping = [
     0,  1,  2,  3,  4,  5,         35, 34, 33, 32, 31, 30,
     6,  7,  8,  9, 10, 11,         41, 40, 39, 38, 37, 36,
    12, 13, 14, 15, 16, 17,         47, 46, 45, 44, 43, 42,
    18, 19, 20, 21, 22, 23, 29, 59, 53, 52, 51, 50, 49, 48,
            25, 26, 27, 28,         58, 57, 56, 55, 
    ]

coord_mapping using MatrixScanner and RotaryioEncoder on the same 58 key split keyboard with an encoder on each half:

coord_mapping = [
     0,  1,  2,  3,  4,  5,         37, 36, 35, 34, 33, 32,
     6,  7,  8,  9, 10, 11,         43, 42, 41, 40, 39, 38,
    12, 13, 14, 15, 16, 17,         49, 48, 47, 46, 45, 44,
    18, 19, 20, 21, 22, 23, 29, 61, 55, 54, 53, 52, 51, 50,
            25, 26, 27, 28,         60, 59, 58, 57,
            30, 31,                         62, 63 
    ]

On the top left side of a standard split keyboard coord_mapping, right below that you see a split keyboard where RotaryioEncoder and MatrixScanner (the default scanner) are used. In the single scanner example, we used to count from 0 to 29 while the top right side starts at 30. With the addition of the encoder scanner, the left side has 2 additional keys making it count up to 31 and the right side would then start at 32 and count to 63. This means that keys 30, 31, 62, and 63 are for encoders. Notice that all of the encoders are at the end of the array, because we put the encoder scanner after the matrix scanner in keyboard.matrix. Therefore, we need to add 4 more key codes in the corresponding places of our keyboard.keymap, they will be used for the encoders.