/* Configure the System clock frequency, HCLK, PCLK2 and PCLK1 prescalers */
/* Configure the Flash Latency cycles and enable prefetch buffer */
SetSysClock();//最终调用时钟设置函数，下面分析
/* Wait till HSE is ready and if Time out is reached exit */
do
{
HSEStatus = RCC->CR & RCC_CR_HSERDY;//RCC_CR_HSERDY((uint32_t)0x00020000) 测试HSE状态
StartUpCounter++; //计数等待
} while((HSEStatus == 0) && (StartUpCounter != HSEStartUp_TimeOut)); //当计数 > HSEStartUp_TimeOut(0x500)时且HSE可用则跳出
/* Wait till PLL is ready */
while((RCC->CR & RCC_CR_PLLRDY) == 0) //等待PLL准备好
{
}
/* Select PLL as system clock source */ //设置系统时钟为PLL时钟源
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 */ //等待系统时钟准备好
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 */
/* Go to infinite loop */
while (1) //如果HSE不能用，则跳入死循环
{
}
}
}
