// STM32 Nucleo C031C6
// 4-Bit Binary LED Counter
// Sequence: 0000 to 1111

const int LED1 = D2;  // LSB
const int LED2 = D3;
const int LED3 = D4;
const int LED4 = D5;  // MSB

const int TIME_DELAY = 1000;  // 1 second

void setup() {
  pinMode(LED1, OUTPUT);
  pinMode(LED2, OUTPUT);
  pinMode(LED3, OUTPUT);
  pinMode(LED4, OUTPUT);

  // Start at 0000
  digitalWrite(LED1, LOW);
  digitalWrite(LED2, LOW);
  digitalWrite(LED3, LOW);
  digitalWrite(LED4, LOW);
}

void loop() {

  // Count from 0000 to 1111
  for (int number = 0; number <= 15; number++) {

    // Bit 0 -> LED1 (D2)
    digitalWrite(LED1, (number & 0b0001) ? HIGH : LOW);

    // Bit 1 -> LED2 (D3)
    digitalWrite(LED2, (number & 0b0010) ? HIGH : LOW);

    // Bit 2 -> LED3 (D4)
    digitalWrite(LED3, (number & 0b0100) ? HIGH : LOW);

    // Bit 3 -> LED4 (D5)
    digitalWrite(LED4, (number & 0b1000) ? HIGH : LOW);

    // Wait 1 second
    delay(TIME_DELAY);
  }
}