stm32F051 HSI 時鐘作為主時鐘

最近要用 stm32F051， 因為闆子比較小，故就沒有外部晶體，想用内部8M RC為作48M系統時鐘的來源。但是一下子，沒弄出來，上網找了找，其實想快弄出來。結果還是看了一下代碼，和手冊，既然花了時間，就分享給大家吧，免得再耽誤時間了。

兩種方法：

1. 在系統啟動時自動檢測，沒有外部晶體，就用内部HSI作為時鐘源，代碼如下：

/**
  * @brief  Configures the System clock frequency, AHB/APBx prescalers and Flash
  *         settings.
  * @note   This function should be called only once the RCC clock configuration
  *         is reset to the default reset state (done in SystemInit() function).
  * @param  None
  * @retval None
  */
static void SetSysClock(void)
{
  __IO uint32_t StartUpCounter = 0, HSEStatus = 0;

/******************************************************************************/
/*            PLL (clocked by HSE) used as System clock source                */
/******************************************************************************/
  
  /* SYSCLK, HCLK, PCLK configuration ----------------------------------------*/
  /* Enable HSE */    
  RCC->CR |= ((uint32_t)RCC_CR_HSEON);
 
  /* Wait till HSE is ready and if Time out is reached exit */
  do
  {
    HSEStatus = RCC->CR & RCC_CR_HSERDY;
    StartUpCounter++;  
  } while((HSEStatus == 0) && (StartUpCounter != HSE_STARTUP_TIMEOUT));

  if ((RCC->CR & RCC_CR_HSERDY) != RESET)
  {
    HSEStatus = (uint32_t)0x01;
  }
  else
  {
    HSEStatus = (uint32_t)0x00;
  }  

  if (HSEStatus == (uint32_t)0x01)
  {
    /* Enable Prefetch Buffer and set Flash Latency */
    FLASH->ACR = FLASH_ACR_PRFTBE | FLASH_ACR_LATENCY;
 
    /* HCLK = SYSCLK */
    RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_DIV1;
      
    /* PCLK = HCLK */
    RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE_DIV1;

    /* PLL configuration */
    RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_PLLSRC | RCC_CFGR_PLLXTPRE | RCC_CFGR_PLLMULL));
    RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLSRC_PREDIV1 | RCC_CFGR_PLLXTPRE_PREDIV1 | RCC_CFGR_PLLMULL6);
            
    /* 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)RCC_CFGR_SWS_PLL)
    {
    }
  }
  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 */
		 	
	/* enable HSI */
	RCC->CR &= ~RCC_CR_HSION;
	while(RCC_GetFlagStatus(RCC_FLAG_HSIRDY) == RESET)
	{}
	
    /* Enable Prefetch Buffer and set Flash Latency */
    FLASH->ACR = FLASH_ACR_PRFTBE | FLASH_ACR_LATENCY;

    /* HCLK = SYSCLK */
    RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_DIV1;
      
    /* PCLK = HCLK */
    RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE_DIV1;

    /* PLL configuration */
    RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_PLLSRC | RCC_CFGR_PLLMULL));
    RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLSRC_HSI_PREDIV | RCC_CFGR_PLLMULL12);

    /* 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)RCC_CFGR_SWS_PLL)
    {}
  }  
}
           

将system_stm32f0xx.c  中的 SetSysClock() 函數的 else 填充加粗部分代碼，這樣，HSE啟動不了的時候，自動轉HSI

2. 自己寫函數，可以通過這個函數，在有外部晶體的時候主動切換到HSI，代碼如下：

/******************************************************************************* 
* Function Name: system_clk_config
* Description:
*
* Input Params:
*
* Return:
********************************************************************************/
void system_clk_config_HSI(void)
{
    RCC_DeInit();
	/* enable HSI */
	RCC_HSICmd(ENABLE);
	while(RCC_GetFlagStatus(RCC_FLAG_HSIRDY) == RESET)
	{}
	
	/* Enable Prefetch Buffer */
	FLASH_PrefetchBufferCmd(ENABLE);
	/* set Flash Latency, 0-24M: FLASH_Latency_0, 24M < sysclk <48M: 
    FLASH_Latency_1*/
	FLASH_SetLatency(FLASH_Latency_1);
	
	/* Configures the AHB clock (HCLK)  */
	RCC_HCLKConfig(RCC_CFGR_HPRE_DIV1);
	
	/* Configures the APB clock (PCLK) */
	RCC_PCLKConfig(RCC_CFGR_PPRE_DIV1);
	
	/* Configures PLL source, (8M/2) * 12 = 48M */
	RCC_PLLConfig(RCC_CFGR_PLLSRC_HSI_DIV2, RCC_CFGR_PLLMULL12); 
	RCC_PLLCmd(ENABLE);
	while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY) == RESET)
	{}
	
	/* Configures system clock source */
	RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK);
    /* Wait till PLL is used as system clock source */
    while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS) != (uint32_t)RCC_CFGR_SWS_PLL)
    {}
}
           

注：下面要以看到STM32F051的時鐘樹，HSI供給PLL的時候是強制 2分頻的，是以到PLL就隻有 4M了，