STM32F4 external interrupt EXTI

It is also a half-baked thing, and a simple interrupt processing is performed.
Software environment: MDK470a
Hardware environment: STM32F4-Discovery. Button B1 is connected to the PA0 pin of the F4 chip. When the button is pressed, the pin level is pulled down.
Function description: When button B1 is pressed, an external interrupt is triggered. After entering the interrupt processing function, a message is sent through the serial port.
Implementation steps:
1. Turn on the PA clock and set the PA0 pin as input.

void EXTI_GPIO_Congig(void)
{
  GPIO_InitTypeDef GPIO_InitStructure;
  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA,ENABLE);
  
  RCC_APB2PeriphClockCmd(RCC_APB2Periph_SYSCFG,ENABLE);
  
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN;
  _InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
  GPIO_InitStructure.GPIO_Pin = GPIO_PinSource0;
  GPIO_InitStructure.GPIO_Speed ​​=GPIO_Speed_100MHz;
  GPIO_Init(GPIOA ,&GPIO_InitStructure);
  
}

2. Turn on the system configuration controller clock.
        RCC_APB2PeriphClockCmd(RCC_APB2Periph_SYSCFG,ENABLE);
The function of the system control register (SYSCFG) is newly added in the F4 series. The functions are as follows:
The system configuration controller is mainly used to remap the memory accessible in the code area, select the Ethernet PHY interface and manage the external interrupt line 
connection to the GPIOs.
SYSCFG is mainly used to map the memory access to the CODE area, select the Ethernet interface, and manage the connection of the external interrupt line to the GPIO.
To set an external interrupt, you must also set the external interrupt configuration register of SYSCFG.
3. Configure the working mode of the external interrupt EXTI.
    Map it to PA0, that is, line 0, and use the interrupt mode falling edge trigger.
   Setting the working mode of the EXTI register is handed over to the library function.

void EXTI_Config(void)
{
  EXTI_GPIO_Congig();
  RCC_APB2PeriphClockCmd(RCC_APB2Periph_SYSCFG,ENABLE);
  
  EXTI_InitStructure.EXTI_Line = EXTI_Line0; EXTI_InitStructure.EXTI_LineCmd
  = ENABLE;
  EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Inter rupt;
  EXTI_InitStructure.EXTI_Trigger = EXTI_Trigger_Falling;
  EXTI_Init(&EXTI_InitStructure);
  
}

4. Write an interrupt handling function to print information to the serial port.
     Fixed function name: void EXTI0_IRQHandler(void).
     After entering the interrupt handling function, first check whether it is an interrupt on line 0. If so, clear the interrupt flag. Then you can send a message.
     After the message is sent, clear the external interrupt that arrived during the external interrupt processing. Use EXTI_ClearITPendingBit() to complete

void EXTI0_IRQHandler(void)
{
  if(SET == EXTI_GetITStatus(EXTI_Line0))
  {    
    EXTI_ClearFlag(EXTI_Line0);
    printf("i am in exti irqhandler\r\n");
    printf("and the extiflag is cleared\r\n" );
    EXTI_ClearITPendingBit(EXTI_Line0);
  }  
}