STM32F103 使用HSI配置系统时钟为64MHZ

/**
  * @brief  Sets System clock frequency to 64MHz and configure HCLK, PCLK2 
  *         and PCLK1 prescalers. 
  * @note   OCS is HSI. This function should be used only after reset.
  * @param  None
  * @retval None
  */
void SetSysClockTo64Mhz(void)
{
  __IO uint32_t StartUpCounter = 0, HSEStatus = 0;
    RCC_DeInit();
  
  /* SYSCLK, HCLK, PCLK2 and PCLK1 configuration ---------------------------*/    
  /* Enable HSI */    
  RCC->CR |= ((uint32_t)RCC_CR_HSION);
 
  /* Wait till HSI is ready and if Time out is reached exit */
  do
  {
    HSEStatus = RCC->CR & RCC_CR_HSIRDY;
    StartUpCounter++;  
  } while((HSEStatus == 0) && (StartUpCounter != HSE_STARTUP_TIMEOUT));
  if ((RCC->CR & RCC_CR_HSIRDY) != RESET)
  {
    HSEStatus = (uint32_t)0x01;
  }
  else
  {
    HSEStatus = (uint32_t)0x00;
  }  
  if (HSEStatus == (uint32_t)0x01)
  {
    /* Enable Prefetch Buffer */
    FLASH->ACR |= FLASH_ACR_PRFTBE;
    /* Flash 2 wait state */
    FLASH->ACR &= (uint32_t)((uint32_t)~FLASH_ACR_LATENCY);
    FLASH->ACR |= (uint32_t)FLASH_ACR_LATENCY_2;    
 
    /* HCLK = SYSCLK */
    RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_DIV1;
      
    /* PCLK2 = HCLK */
    RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE2_DIV1;
    
    /* PCLK1 = HCLK */
    RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE1_DIV2;
  
    /*  PLL configuration: PLLCLK = HSI/2 * 16 = 64 MHz */
    RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_PLLSRC | RCC_CFGR_PLLXTPRE |
                                        RCC_CFGR_PLLMULL));
    RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLSRC_HSI_Div2| RCC_CFGR_PLLMULL16);

    /* Enable PLL */
    RCC->CR |= RCC_CR_PLLON;
    /* Wait till PLL is ready */
    while((RCC->CR & RCC_CR_PLLRDY) == 0)
    {
    }
    
    /* Select PLL as system clock source */
    RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW));
    RCC->CFGR |= (uint32_t)RCC_CFGR_SW_PLL;    
    /* Wait till PLL is used as system clock source */
    while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS) != (uint32_t)0x08)
    {
    }
  }
  else
  { /* If HSE fails to start-up, the application will have wrong clock 
         configuration. User can add here some code to deal with this error */
  }
}