volatile int led1_level; // set this variable to control the duty cycle in 2% steps
void dim_leds() { // call this function from SysTick_Handler()
  static int cnt;
  cnt = (cnt + 1) % 50;
  if(cnt < led1_level)
    GPIOG->BSRR = 1 << 13;
  else
    GPIOG->BSRR = 1 << (13 + 16);
}