说明：  8051 串行接口是一个可编程的全双工串行通讯接口。它可用作异步通讯方式（UART），与串行传送信息的外 部设备相连接，或用于通过标准异步通讯协议进行全双工的8051 多机系统也可以通过同步方式，使用TTL 或CMOS 移位寄存器来扩充I/O 口。
(8051 serial interface is a programmable full-duplex serial communication interface. It can be used as asynchronous communication (UART), serial transmission of information with external devices connected, or used through a standard full-duplex asynchronous communication protocol for the more than 8051 systems can also be synchronized using a TTL or CMOS shift register to expand the I/O port.)