传输模式的选择:同步还是异步,默认为异步模式,可通过选择USART控制和状态寄存器UCSR0C中的UMSEL位来选择,UMSEL为0,是异步模式.-The choice of transfer mode: synchronous or asynchronous, defaults to asynchronous mode can by choose USART Control and the state register UCSR0C in UMSEL bit to select, UMSEL