一、时钟树
STM32有4个时钟源：
1）HSE（高速外部时钟源） 外部晶振作为时钟源，范围为4~16MHz，常取为8MHz
2）HSI（高速内部时钟源） 由内部RC振荡器产生，频率为8MHz，但不稳定
3）LSE（低速外部时钟） 以外部晶振作为时钟源，主要供给实时时钟模块，一般用32.768KHz。
4）LSI（低速内部时钟）
由内部RC振荡器产生，也是提供给实时时钟模块，频率约为40KHz。
二、系统启动过程中时钟设置
以使用STM32库函数SystemInit为例进行说明：
上电后：默认使用HSI
SystemInit: 倘若调用了函数SetSysClockTo72，将启用外部晶振HSE，并将系统时钟设置到72MHz。
附SetSysClockTo72函数代码：
/**
* @brief Sets System clock frequency to 72MHz and configure HCLK, PCLK2
* and PCLK1 prescalers.
* @note This function should be used only after reset.
* @param None
* @retval None
*/
static void SetSysClockTo72(void)
{
__IO uint32_t StartUpCounter = 0, HSEStatus = 0;/* SYSCLK, HCLK, PCLK2 and PCLK1 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 */
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;#ifdef STM32F10X_CL
/* Configure PLLs ------------------------------------------------------*/
/* PLL2 configuration: PLL2CLK = (HSE / 5) * 8 = 40 MHz */
/* PREDIV1 configuration: PREDIV1CLK = PLL2 / 5 = 8 MHz */RCC->CFGR2 &= (uint32_t)~(RCC_CFGR2_PREDIV2 | RCC_CFGR2_PLL2MUL |
RCC_CFGR2_PREDIV1 | RCC_CFGR2_PREDIV1SRC);
RCC->CFGR2 |= (uint32_t)(RCC_CFGR2_PREDIV2_DIV5 | RCC_CFGR2_PLL2MUL8 |
RCC_CFGR2_PREDIV1SRC_PLL2 | RCC_CFGR2_PREDIV1_DIV5);/* Enable PLL2 */
RCC->CR |= RCC_CR_PLL2ON;
/* Wait till PLL2 is ready */
while((RCC->CR & RCC_CR_PLL2RDY) == 0)
{
}/* PLL configuration: PLLCLK = PREDIV1 * 9 = 72 MHz */
RCC->CFGR &= (uint32_t)~(RCC_CFGR_PLLXTPRE | RCC_CFGR_PLLSRC | RCC_CFGR_PLLMULL);
RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLXTPRE_PREDIV1 | RCC_CFGR_PLLSRC_PREDIV1 |
RCC_CFGR_PLLMULL9);
#else
/* PLL configuration: PLLCLK = HSE * 9 = 72 MHz */
RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_PLLSRC | RCC_CFGR_PLLXTPRE |
RCC_CFGR_PLLMULL));
RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLSRC_HSE | RCC_CFGR_PLLMULL9);
#endif /* STM32F10X_CL *//* 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 */
}
}