接收异步串口数据，将数据写到接收fifo中，可设置超时来接收多字节数据，当设置超时时间内未出现数据，ready信号有效，表示接收完整数据包，可从fifo中读取数据。-Receive asynchronous serial data, the data is written to the receiving fifo, you can set the timeout to receive multi-byte data, set the timeout period when the data does not appear, ready signal is valid, that receives complete packet, the data can be read from the fifo.