最近要用 stm32F051， 因为板子比较小，故就没有外部晶体，想用内部8M RC为作48M系统时钟的来源。但是一下子，没弄出来，上网找了找，其实想快弄出来。结果还是看了一下代码，和手册，既然花了时间，就分享给大家吧，免得再耽误时间了。
两种方法：
1. 在系统启动时自动检测，没有外部晶体，就用内部HSI作为时钟源，代码如下：
/**
* @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() 函数的 else 填充加粗部分代码，这样，HSE启动不了的时候，自动转HSI
2. 自己写函数，可以通过这个函数，在有外部晶体的时候主动切换到HSI，代码如下：
/*******************************************************************************
* Function Name: system_clk_config
* Description:
*
* Input Params:
*
* Return:
********************************************************************************/
void system_clk_config_HSI(void)
{
<span style="white-space:pre"> </span>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)
{}
}