QMK Demultiplexer Matrix Scanning

/*  https://www.reddit.com/r/olkb/comments/vthz1k/help_with_my_custom_matrix_for_demultiplexer/
 *  Code to scan a keyboard matrix using three 74HC238 demultiplexer chips.
 */


/* This should go into config.h
 *
 * #define MATRIX_ROWS 11
 * #define MATRIX_COLS 20
 *
 * #define MATRIX_ROW_PINS {B3, A10, B15, B14, B9, B12, B4, B5, B6, B7, B8}
 * #define MATRIX_COL_PINS {NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN, NO_PIN}
 *
 * #define DMUX_SELECT_PINS {A5, C13, C14}
 * #define NUM_DMUX_SELECT_PINS 3
 *
 * #define DMUX_ENABLE_PINS {C15, A0}
 * #define NUM_DMUX_ENABLE_PINS 2
 *
 */


// In matrix.c ----------------------------------------------------------------

/* This keyboard uses three 74HC238 demultiplexer chips to scan the columns of its matrix.
 * Columns 0-7  will use the first chip, enabled by a pullup resistor.
 * Columns 8-15 will use the second chip by writing to its enable pin, disabling the other 2 chips.
 * Columns 8-19 will use the third chip by writing to its enable pin, disabling the other 2 chips.
 *
 * The select pins are then used to produce output at the specified column.
 */

#include <stdint.h>
#include <stdbool.h>
#include "quantum.h"
#include "wait.h"
#include "matrix.h"


// Arrays of the pins to be used
static const pin_t row_pins[MATRIX_ROWS] = ROW_PINS;
static const pin_t dmux_select_pins[NUM_DMUX_SELECT_PINS] = DMUX_SELECT_PINS;
static const pin_t dmux_enable_pins[NUM_DMUX_ENABLE_PINS] = DMUX_ENABLE_PINS;


// Initialize pins to be used for matrix scanning
void matrix_init_custom(void) {
    // Set all row_pins to inputs, with pulldown resistors to detect HIGH values from columns
    for (int row = 0; row < MATRIX_ROWS; row++) {
        setPinInputLow(row_pins[row]);
    }

    // Set all dmux_select_pins to outputs, and write LOW
    for (int pin = 0; pin < NUM_DMUX_SELECT_PINS; pin++) {
        setPinOutput(dmux_select_pins[pin]);
        writePinLow(dmux_select_pins[pin]);
    }

    // Set all dmux_enable_pins to outputs, and write LOW
    for (int pin = 0; pin < NUM_DMUX_ENABLE_PINS; pin++) {
        setPinOutput(dmux_enable_pins[pin]);
        writePinLow(dmux_enable_pins[pin]);
    }
}


/* In binary, the first 3 bits of a number refer to its value between 0-7
 * These are written to the dmux_select_pins.
 *
 * The 4th bit is active for columns 8-15, in which case the first dmux_enable_pin is set
 * The 5th bit is active for columns 16-19, in which case the second dmux_enable_pin is set
 */

// Write dmux pins in order to produce output at specified column
static void select_column(uint8_t col_index) {
    writePin(dmux_select_pins[0], col_index & 0b001);  // is 1st bit of col_index true?
    writePin(dmux_select_pins[1], col_index & 0b010);  // is 2nd bit of col_index true?
    writePin(dmux_select_pins[2], col_index & 0b100);  // is 3rd bit of col_index true?

    writePin(dmux_enable_pins[0], col_index & 8);      // is 4th bit of col_index true?
    writePin(dmux_enable_pins[1], col_index & 16);     // is 5th bit of col_index true?

    wait_us(0.015);  // Wait 0.015 microseconds to propagate the signal
}


static bool read_rows_on_col(matrix_row_t current_matrix[], uint8_t current_col) {
    bool matrix_changed = false;

    // For each row
    for (uint8_t row_index = 0; row_index < MATRIX_ROWS; row_index++) {
        // Store last value of row prior to reading
        matrix_row_t last_row_value    = current_matrix[row_index];
        matrix_row_t current_row_value = last_row_value;

        // Check row pin state
        if (readPin(row_pins[row_index]) == 1) {
            // HIGH signal from column means a key is pressed, set column bit
            current_row_value |= (MATRIX_ROW_SHIFTER << current_col);
        } else {
            // No signal from column means no key is pressed, clear column bit
            current_row_value &= ~(MATRIX_ROW_SHIFTER << current_col);
        }

        // Determine if the matrix changed state, update current_matrix if needed
        if ((last_row_value != current_row_value)) {
            matrix_changed = true;
            current_matrix[row_index] = current_row_value;
        }
    }

    return matrix_changed;  // True if any rows had a change, otherwise false
}


bool matrix_scan_custom(matrix_row_t current_matrix[]) {
    bool matrix_has_changed = false;

    // For each column
    for (uint8_t col = 0; col < MATRIX_COLS; col++) {
        // Write dmux_pins for current column
        select_column(col);

        // Read the rows for this column.
        // If any rows have changed, matrix_has_changed will be true
        matrix_has_changed |= read_rows_on_col(current_matrix, col);
    }

    return matrix_has_changed;
}