void RCC_init(void){
      // Enable external oscillator
    RCC->CR |= RCC_CR_HSEON;
      while(!(RCC->CR & RCC_CR_HSERDY));

      // Config FLASH (?) source - youtube: Nordic Energy
      FLASH->ACR = FLASH_ACR_PRFTEN | FLASH_ACR_LATENCY;

      // Manual disable PLL before config, just in case
      RCC->CR &= ~RCC_CR_PLLON;
      while((RCC->CR & RCC_CR_PLLRDY));

      // Set all CFGR bits to 0
      RCC->PLLCFGR &= RCC_PLLCFGR_PLLM;
      RCC->PLLCFGR &= RCC_PLLCFGR_PLLN;
      RCC->PLLCFGR &= RCC_PLLCFGR_PLLP;

      // Config PLL
      RCC->PLLCFGR |= RCC_PLLCFGR_PLLSRC_HSE; // source - HSE
      RCC->PLLCFGR |= RCC_PLLCFGR_PLLM_2;     // divide for 4 (second bit to 1)000100 = 4 8MHz/4=2MHz
      RCC->PLLCFGR |= RCC_PLLCFGR_PLLN_3 | RCC_PLLCFGR_PLLN_6 | RCC_PLLCFGR_PLLN_7; // multiply for 200 011001000 2MHz*200=400MHz
      RCC->PLLCFGR |= RCC_PLLCFGR_PLLP_0; // set PLLP to 4 400MHz/4 = 100MHz

      RCC->CFGR = RCC_CFGR_HPRE_DIV1;  // AHB = SYSCLK/1 =100MHz
      RCC->CFGR = RCC_CFGR_PPRE1_DIV4; // APB1 = HCLK/4 =25MHz
      RCC->CFGR = RCC_CFGR_PPRE2_DIV2; // APB2 = HCLK/2 =50MHz

      // Enable PLL
      RCC->CR |= RCC_CR_PLLON;
      while(!(RCC->CR & RCC_CR_PLLRDY));


        // Set PLL as system clock
        RCC->CFGR &= ~RCC_CFGR_SW;
        RCC->CFGR |= RCC_CFGR_SW_PLL;
        while((RCC->CFGR & RCC_CFGR_SWS) != RCC_CFGR_SWS_1);
}