STM32F030F4P6单片机由外部时钟改为内部时钟的步骤

一.看程序运行的时候初始化时钟部分:
 

二.查看systeminit定义如下:
 


这就是打开的函数内容这就是使用外部时钟的配置,也就是库函数的默认配置):static 

void SetSysClock(void)
{
  __IO uint32_t StartUpCounter = 0, HSEStatus = 0;
  
  /* SYSCLK, HCLK, PCLK configuration ----------------------------------------*/
/* Enable HSE */ //打开外部时钟开关
  RCC->CR |= ((uint32_t)RCC_CR_HSEON);

  /* Wait till HSE is ready and if Time out is reached exit *///判断外部时钟是否起振
  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总线时钟使能
    FLASH->ACR = FLASH_ACR_PRFTBE | FLASH_ACR_LATENCY;

    /* HCLK = SYSCLK *///外设AHB总线时钟等于系统时钟
    RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_DIV1;

    /* PCLK = HCLK *///外设APB总线时钟等于系统时钟
    RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE_DIV1;

    /* PLL configuration = HSE * 6 = 48 MHz *///外部时钟6倍频 =系统时钟
    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 *///使能锁相环倍频开关
    RCC->CR |= RCC_CR_PLLON;

    /* Wait till PLL is ready *///等待锁相环就绪
    while((RCC->CR & RCC_CR_PLLRDY) == 0)
    {
    }

/* Select PLL as system clock source *///选择锁相环输出时钟作为系统时钟
    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 *///等待锁相环输出时钟已经成为系统时钟
    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 */
  }  
}

三.接下来我们想将系统时钟改为内部时钟,修改办法依旧是修改  SetSysClock()这个函数中的配置.首先我们要知道单片机内部的时钟到底是怎样一种结构,可以通过芯片文档来查找,另外比较方便的方法就是利用cubemx工具来查看,时钟结构如下:


四.那么我们就按照上图的结构配置来修改SetSysClock()中的配置即可采用内部RC时钟作为时钟源,蓝色字体为修改关键处):

static void SetSysClock(void)
{
  __IO uint32_t StartUpCounter = 0, HSIStatus = 0;
  
  /* SYSCLK, HCLK, PCLK configuration ----------------------------------------*/
/* Enable HSI*/ //使能内部时钟
  RCC->CR |= ((uint32_t)RCC_CR_HSION;

  /* Wait till HSI is ready and if Time out is reached exit */ //等待内部时钟起振
  do
  {
   HSIStatus = RCC->CR & RCC_CR_HSIRDY;
    StartUpCounter++;  
  } while((HSIStatus== 0) && (StartUpCounter != HSI_STARTUP_TIMEOUT));

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

  if (HSIStatus == (uint32_t)0x01)
  {
/* Enable Prefetch Buffer and set Flash Latency */ //flash总线时钟使能
    FLASH->ACR = FLASH_ACR_PRFTBE | FLASH_ACR_LATENCY;

/* HCLK = SYSCLK *///外设AHB总线时钟等于系统时钟
    RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_DIV1;
      
/* PCLK = HCLK *///外设APB总线时钟等于系统时钟
    RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE_DIV1;

   /* PLL configuration = HSI/2 * 12= 48 MHz */
    RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_PLLSRC | RCC_CFGR_PLLSRC | RCC_CFGR_PLLMULL));
    RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLSRC_HSI_Div2 | RCC_CFGR_PLLMULL12); //RC时钟2分频后 进行12倍频
            
/* Enable PLL *///使能锁相环倍频开关 
    RCC->CR |= RCC_CR_PLLON;

/* Wait till PLL is ready *///等待锁相环就绪 
    while((RCC->CR & RCC_CR_PLLRDY) == 0)
    {
    }

/* Select PLL as system clock source *///选择锁相环输出时钟作为系统时钟 
    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 *///等待锁相环输出时钟已经成为系统时钟 
    while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS) != (uint32_t)RCC_CFGR_SWS_PLL)
    {
    }
  }
  else
  { /* If HSI fails to start-up, the application will have wrong clock 
         configuration. User can add here some code to deal with this error */
                
  }  
}

好了,以上就是外部时钟源改为内部时钟源的总结.