• MCU: ATMEGA32U4-AU
• Framework: Arduino AVR core, Leonardo-compatible pin numbering
• Build system: PlatformIO or Arduino IDE
• USB mode: Native USB HID keyboard/macropad

Mapping note: this schematic maps D18/D19 using the common ATmega32U4 Leonardo mapping: D18 = PF7, D19 = PF6. The pasted requirement labels these as A4/A5; verify the exact firmware variant before committing production firmware.

Ini
[env:leonardo]
platform = atmelavr
board = leonardo
framework = arduino
monitor_speed = 115200
lib_deps =
  nicohood/HID-Project@^2.8.4

Cpp
#include <Arduino.h>
#include <HID-Project.h>

// Key inputs: active-low, using internal pull-ups.
static const uint8_t KEY_PINS[] = {
  2, 3, 4, 5, 6, 7, 8, 9, 10, 14, 15, 16
};
static const uint8_t NUM_KEYS = sizeof(KEY_PINS) / sizeof(KEY_PINS[0]);

static const uint8_t ENC_A_PIN = 18;
static const uint8_t ENC_B_PIN = 19;

static const uint16_t DEBOUNCE_MS = 5;
static bool keyState[NUM_KEYS];
static bool lastRawState[NUM_KEYS];
static uint32_t lastChangeMs[NUM_KEYS];

static int8_t lastEncState = 0;
static int32_t encoderPosition = 0;

// Example keymap. Replace with QMK-style actions or custom HID shortcuts as needed.
static const KeyboardKeycode KEYMAP[NUM_KEYS] = {
  KEY_F13, KEY_F14, KEY_F15, KEY_F16,
  KEY_F17, KEY_F18, KEY_F19, KEY_F20,
  KEY_F21, KEY_F22, KEY_F23, KEY_F24
};

static int8_t readEncoderState() {
  uint8_t a = digitalRead(ENC_A_PIN) == LOW ? 1 : 0;
  uint8_t b = digitalRead(ENC_B_PIN) == LOW ? 1 : 0;
  return (a << 1) | b;
}

static void handleEncoder() {
  static const int8_t transitionTable[16] = {
     0, -1,  1,  0,
     1,  0,  0, -1,
    -1,  0,  0,  1,
     0,  1, -1,  0
  };

  int8_t state = readEncoderState();
  int8_t transition = (lastEncState << 2) | state;
  int8_t delta = transitionTable[transition & 0x0F];
  lastEncState = state;

  if (delta != 0) {
    encoderPosition += delta;
    // Two quadrature transitions per detent is common; adjust for your encoder.
    if (encoderPosition >= 2) {
      Consumer.write(MEDIA_VOLUME_UP);
      encoderPosition = 0;
    } else if (encoderPosition <= -2) {
      Consumer.write(MEDIA_VOLUME_DOWN);
      encoderPosition = 0;
    }
  }
}

static void scanKeys() {
  uint32_t now = millis();

  for (uint8_t i = 0; i < NUM_KEYS; i++) {
    bool rawPressed = (digitalRead(KEY_PINS[i]) == LOW);

    if (rawPressed != lastRawState[i]) {
      lastRawState[i] = rawPressed;
      lastChangeMs[i] = now;
    }

    if ((now - lastChangeMs[i]) >= DEBOUNCE_MS && rawPressed != keyState[i]) {
      keyState[i] = rawPressed;
      if (keyState[i]) {
        BootKeyboard.press(KEYMAP[i]);
      } else {
        BootKeyboard.release(KEYMAP[i]);
      }
    }
  }
}

void setup() {
  for (uint8_t i = 0; i < NUM_KEYS; i++) {
    pinMode(KEY_PINS[i], INPUT_PULLUP);
    keyState[i] = false;
    lastRawState[i] = false;
    lastChangeMs[i] = 0;
  }

  pinMode(ENC_A_PIN, INPUT_PULLUP);
  pinMode(ENC_B_PIN, INPUT_PULLUP);
  lastEncState = readEncoderState();

  BootKeyboard.begin();
  Consumer.begin();
}

void loop() {
  scanKeys();
  handleEncoder();
  delay(1);
}

Bash
pio run
pio run -t upload
pio device monitor