KeypadMatrix Scanning

4×4 Keypad · GPIO Rows/Cols · Pull-up · Debounce

// What you'll learn

Why keypads use a matrix (saves GPIO pins)

How column-by-column scanning identifies a keypress

Pull-up resistors — why floating inputs cause ghost presses

Software debouncing to prevent multiple triggers

GitHub Repo ↗ ← All Projects

// Click keys — live matrix scanner

// PuTTY Serial Console

Waiting for keypress...

—

Active Col (LOW)

—

Detected Row

// 01 — Core Concepts

The Matrix Trick

🔢

Why a Matrix?

A 4×4 keypad has 16 keys. Wiring each key directly would need 16 GPIO pins. A matrix only needs 4 rows + 4 cols = 8 pins for 16 keys. For a 100-key keyboard, that's 10+10 = 20 pins vs 100. The trade-off: software must scan.

8 pins = 16 keys

🔍

Column Scanning

One column at a time is pulled LOW. All other columns stay HIGH. Then all 4 row pins are read. If a row reads LOW, a key in that column AND row is pressed — the intersection identifies the exact key.

gpio_put(col, 0)

⬆️

Pull-up Resistors

Without a pull-up, an unconnected GPIO floats — it can read random 0s and 1s, causing ghost keypresses. The Pico's internal pull-ups hold row pins at HIGH (1) by default. A key press shorts the row to the LOW column.

gpio_pull_up()

// 02 — Scanning Algorithm

How Column Scan Works

// Column 0 Active → Row Scan

          Active col → LOW (0)

          Other cols → HIGH (1)

          Read all rows → if row == LOW → key found!

keypad.c

#define NUM_ROWS 4
#define NUM_COLS 4

const uint8_t rowPins[4] = {10,11,12,13};
const uint8_t colPins[4] = {18,19,20,21};

char keyMap[NUM_ROWS][NUM_COLS] = {
    {'1','2','3','A'},
    {'4','5','6','B'},
    {'7','8','9','C'},
    {'*','0','#','D'}
};

void initializeKeypad() {
    // Columns: OUTPUT, start HIGH
    for (int i=0; i<NUM_COLS; i++) {
        gpio_init(colPins[i]);
        gpio_set_dir(colPins[i], GPIO_OUT);
        gpio_put(colPins[i], 1);
    }
    // Rows: INPUT with pull-up
    for (int i=0; i<NUM_ROWS; i++) {
        gpio_init(rowPins[i]);
        gpio_set_dir(rowPins[i], GPIO_IN);
        gpio_pull_up(rowPins[i]);
    }
}

char readKeypad() {
    for (int col=0; col<NUM_COLS; col++) {
        gpio_put(colPins[col], 0); // activate col
        for (int row=0; row<NUM_ROWS; row++) {
            if (!gpio_get(rowPins[row])) {
                // Key pressed at [row][col]!
                while(!gpio_get(rowPins[row])){} // wait release
                gpio_put(colPins[col], 1);
                return keyMap[row][col];
            }
        }
        gpio_put(colPins[col], 1); // deactivate col
    }
    return 0;
}

int main() {
    stdio_init_all();
    initializeKeypad();
    while(1) {
        char key = readKeypad();
        if (key != 0) {
            printf("Key pressed: %c\n", key);
            sleep_ms(200); // debounce
        }
    }
}