void SystemInit(void){
if ((RCC->CFGR & RCC_CFGR_SWS) == RCC_CFGR_SWS_PLL)  /* (1) */
         {
          RCC->CFGR &= (uint32_t) (~RCC_CFGR_SW); /* (2) */
          while ((RCC->CFGR & RCC_CFGR_SWS) != RCC_CFGR_SWS_HSI) /* (3) */
          {
          /* For robust implementation, add here time-out management */
          }
         }

         RCC->CR &= (uint32_t)(~RCC_CR_PLLON); /* (4) */
         while((RCC->CR & RCC_CR_PLLRDY) != 0) /* (5) */
         {
          /* For robust implementation, add here time-out management */
         }
         FLASH->ACR = FLASH_ACR_PRFTBE | FLASH_ACR_LATENCY;
         RCC->CFGR|=RCC_CFGR_PLLSRC_0;
         RCC->CFGR = (RCC->CFGR & (~RCC_CFGR_PLLMUL)) | (RCC_CFGR_PLLMUL16); /* (6) */
         RCC->CR |= RCC_CR_PLLON; /* (7) */
         while((RCC->CR & RCC_CR_PLLRDY) == 0) /* (8) */
         {
          /* For robust implementation, add here time-out management */
         }
         RCC->CFGR |= (uint32_t) (RCC_CFGR_SW_PLL); /* (9) */
         while ((RCC->CFGR & RCC_CFGR_SWS) != RCC_CFGR_SWS_PLL)  /* (10) */
         {
          /* For robust implementation, add here time-out management */
         }
}