STM32 system clock monitoring and switching

In the company's project some time ago, it was required to use the STM32 processor to automatically switch to the internal crystal oscillator when the external crystal oscillator is abnormal. After searching for a lot of information on the Internet, I finally found an official seminar PPT on the Internet that briefly introduced it. So I followed this idea to try to write code. I didn't expect that the idea provided by the official was quite reliable!
———————————————————I am a gorgeous dividing line——————————————————— —————————————————I am a gorgeous dividing line——————————————————— Function 1, void INIT_CLOCK(void) {    #if 1     ErrorStatus HSEStartUpStatus;     RCC_DeInit(); //Reset RCC register     RCC_HSEConfig(RCC_HSE_ON); //Set external high-speed crystal oscillator (HSE)     HSEStartUpStatus = RCC_WaitForHSEStartUp(); //Wait for HSE to start #if 1     //Judge whether the external crystal oscillator is OK. If OK, use the external crystal oscillator. If not, use the internal crystal oscillator, giving priority to the external crystal oscillator.     if(HSEStartUpStatus == SUCCESS)     {         RCC_ClockSecuritySystemCmd(ENABLE); //Start the clock security system CSS         //External crystal configuration         RCC_HCLKConfig(RCC_SYSCLK_Div1); //Set ABH clock (HCLK): HCLK=SYSCLK         //Set PLL clock source and multiplication factor, PLLCLK = HSE*PLLMul = 8*8 = 64MHz Actually: 8*8=64MHz         RCC_PLLConfig(RCC_PLLSource_PREDIV1, RCC_PLLMul_2);         RCC_PLLCmd(ENABLE); //Enable PLL         //Check whether the specified RCC flag is set or not         while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY) == RESET); // Wait till PLL is ready         RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK); //Set system clock (SYSCLK)         //Return the clock source used as system clock         // Wait till PLL is used as system clock source         while(RCC_GetSYSCLKSource() != 0x08);     }     else #endif     {         //Internal crystal configuration         RCC_DeInit(); //Reset RCC register         RCC_HSEConfig(RCC_HSE_OFF); //Turn off external crystal (HSE)         RCC_HCLKConfig(RCC_SYSCLK_Div1); //Set ABH clock (HCLK): HCLK=SYSCLK         //Set PLL clock source and multiplication factor, PLLCLK = HSI/2*PLLMul = 8/2*12 = 48MHz Actually: 48MHz         RCC_PLLConfig(RCC_PLLSource_HSI_Div2, RCC_PLLMul_12);         RCC_PLLCmd(ENABLE); //Enable PLL RCC_SYSCLKConfig         (RCC_SYSCLKSource_PLLCLK         ); //Set system clock (SYSCLK)         //Return the clock source of the system clock         // Wait till PLL is used as system clock source         while(RCC_GetSYSCLKSource() != 0x08);     }     #endif } —————————————————I am a gorgeous dividing line———————————————————         In which of the above functions is the clock safety system started only during the external crystal oscillator startup initialization (PS: the red bold part) because only when the external crystal oscillator fails initially will it switch to the internal crystal oscillator. In this function, the external crystal oscillator is started first and the internal crystal oscillator is started only when the external crystal oscillator fails to start. Function 2, void NMIException(void) {     if(RCC_GetITStatus(RCC_IT_CSS) != RESET)     { #if 0 //Internal crystal configuration         RCC_DeInit(); //Reset RCC register         RCC_HSEConfig(RCC_HSE_OFF); //Turn off external crystal (HSE)         RCC_HCLKConfig(RCC_SYSCLK_Div1); //Set ABH clock (HCLK): HCLK=SYSCLK






































































       //Set PLL clock source and multiplication factor, PLLCLK = HSI/2*PLLMul = 8/2*12 = 48MHz Actually: 48MHz
        RCC_PLLConfig(RCC_PLLSource_HSI_Div2, RCC_PLLMul_12);
        RCC_PLLCmd(ENABLE); //Enable PLL

        //Check whether the specified RCC flag is set
        //while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY) == RESET);

        RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK); //Set system clock (SYSCLK)

        //Return the clock source used as system clock
        // Wait till PLL is used as system clock source
        while(RCC_GetSYSCLKSource() != 0x08);
        CLKOUT_8M_INIT(); //PA8 port clock output provides 8MHz clock for 24025RCC_ITConfig

        (RCC_IT_HSIRDY,ENABLE); //Enable HSI ready interruptRCC_ITConfig
        (RCC_IT_PLLRDY,ENABLE); //Enable PLL ready interrupt
#else
        NVIC_SystemReset(); //RestartRCC_ITConfig
        (RCC_IT_HSERDY,ENABLE); //Enable HSE ready
        interruptRCC_ITConfig(RCC_IT_PLLRDY,ENABLE); //Enable PLL ready interrupt       
#endif
        RCC_ClearITPendingBit(RCC_IT_CSS); //Clear the pending bit of the clock security system interrupt
    }
}
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The NMIException() function is the crystal oscillator exception function processing function. This function is called in the exception interrupt function. The red part is the internal crystal oscillator initialization. So the red part of the code can achieve the "seamless" switch of the external crystal oscillator to the internal crystal oscillator (if it is used in specific occasions such as: medical equipment can use this code), but because our product does not have very high requirements for real-time performance, I decided to restart the device and initialize the crystal oscillator.
Function three,

void NMI_Handler(void)
{
    NMIException();
}
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The NMI_Handler() function is in the "stm32f0xx_it.c" in the STM32 library file. We put the crystal oscillator exception processing function in this non-shieldable exception interrupt function to achieve the equivalent of handling when the crystal oscillator is abnormal.

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int main(void)
{
    SystemInit();
    INIT_CLOCK(); //Initialize the system clock using the HSI clock
    for(;;)
    {
     ......
    }
}