main.c

void delay1(volatile uint32_t delay) {
    while (delay--);
}

int main(void) {
    rcc_init();
    // LED - PD13

    RCC->AHB1ENR |=RCC_AHB1ENR_GPIODEN;

    // set 0b01 (General output)
    GPIOD->MODER |= GPIO_MODER_MODE13_0;

    GPIOD->OTYPER &= ~GPIO_OTYPER_OT13;

    GPIOD->OSPEEDR |= (GPIO_OSPEEDR_OSPEED13_0 | GPIO_OSPEEDR_OSPEED13_1);

    while(1) {
        GPIOD->ODR |= GPIO_ODR_ODR_13;
        delay1(168000000);
        // OFF LED
        GPIOD->ODR &= ~GPIO_ODR_ODR_13;
        delay1(168000000);
    }

    return 0;
}

rcc.c

void rcc_init() {
  // HSE is enabled
  RCC->CR |= RCC_CR_HSEON;
  while( !(RCC->CR & RCC_CR_HSERDY) ) {};

  // HSE is PLL source
  RCC->PLLCFGR |= RCC_PLLCFGR_PLLSRC_HSE;

  RCC->PLLCFGR &= ~RCC_PLLCFGR_PLLP; 
  // set PLL P prescaler (0b00 = 2)

  // clear PLL N 
  RCC->PLLCFGR &= ~(RCC_PLLCFGR_PLLN); 	
  // set PLL N (0b101010000 = 336)
  RCC->PLLCFGR |= (RCC_PLLCFGR_PLLN_4 | RCC_PLLCFGR_PLLN_6 | RCC_PLLCFGR_PLLN_8);

  // clear PLL M
  RCC->PLLCFGR &= ~(RCC_PLLCFGR_PLLM); 
  // set PLL M prescaler (0b001000 = 8)
  RCC->PLLCFGR |= RCC_PLLCFGR_PLLM_3;


  // AHB Prescaler (not divided)
  RCC->CFGR &= ~(RCC_CFGR_HPRE); 				
  RCC->CFGR |= RCC_CFGR_HPRE_DIV1; 

  //APB1 Prescaler (divide by 4)
  RCC->CFGR &= ~(RCC_CFGR_PPRE1);
  RCC->CFGR |= RCC_CFGR_PPRE1_DIV4;

  //APB2 Prescaler (divide by 2)
  RCC->CFGR &= ~(RCC_CFGR_PPRE2);
  RCC->CFGR |= RCC_CFGR_PPRE2_DIV2;

  //PLL enable
  RCC->CR |= RCC_CR_PLLON; 					
  // Wait PLL is ready			
  while((RCC->CR & RCC_CR_PLLRDY) == 0) {} ;

  FLASH->ACR = FLASH_ACR_PRFTEN | FLASH_ACR_ICEN |FLASH_ACR_DCEN |FLASH_ACR_LATENCY_5WS;
  //PLL System
  RCC->CFGR &= ~RCC_CFGR_SW;
  RCC->CFGR |= RCC_CFGR_SW_PLL; 					
  while((RCC->CFGR & RCC_CFGR_SWS) != RCC_CFGR_SWS_PLL) {};
}