https://blog.csdn.net/ForeverIT/article/details/81612334
//dma 接收
https://blog.csdn.net/ForeverIT/article/details/81750696
//不定长接收
https://blog.csdn.net/ForeverIT/article/details/82055917
HAL_UART_Transmit_IT不能连续发送数据，如果相邻的HAL_UART_Transmit_IT函数间隔太短的话就会出错。因为 HAL_UART_Transmit_IT的实现第一行代码为 if(husart->State == HAL_USART_STATE_READY) . 可以使用 HAL_UART_TxCpltCallback 来异步等待是否空闲.