USART接收中断

1. 发送数据不需要中断，但是接收数据需要利用中断来处理，这样效率比较高。

2. 本实验设计为：上位机串口发送字符，单片机收到这些字符后，又向上位机转发过去。

  CLK_PeripheralClockConfig(CLK_Peripheral_USART1, ENABLE);
  USART_Init(USART1, 9600, USART_WordLength_8b, USART_StopBits_1, USART_Parity_No, USART_Mode_Rx | USART_Mode_Tx);
  USART_ITConfig (USART1,USART_IT_RXNE,ENABLE);//开启串口中断
  USART_Cmd(USART1, ENABLE);//使能串口
  enableInterrupts();//使能中断
  
  while(1);

INTERRUPT_HANDLER(USART1_RX_TIM5_CC_IRQHandler,28)
{
    /* In order to detect unexpected events during development,
       it is recommended to set a breakpoint on the following instruction.
    */
  uint8_t chr;
  //while(!USART_GetFlagStatus (USART1,USART_FLAG_RXNE));
  chr = USART_ReceiveData8(USART1);
  
  //while(!USART_GetFlagStatus (USART1,USART_FLAG_TXE));
  USART_SendData8(USART1, chr);
  
  USART_ClearITPendingBit(USART1, USART_IT_RXNE);//清除中断标志
}