void setup() {
  GPIOB->MODER &= ~(0xFFFF);
  GPIOB->MODER = 0x5555;

  GPIOD->MODER &= ~(0xFFFF);
  GPIOD->MODER = 0xAAAA;

  // GPIOB->ODR = 0x01;
}

void loop() {
  // GPIOB->ODR <<= 1;
  // if (GPIOB->ODR >= 0xFF) GPIOB->ODR = 0x01;

  GPIOB->ODR = GPIOD->IDR;

  delay(250);
}