【单选题】【消耗次数：1】
8051单片机串行接口在工作方式0时，其引脚TXD和RXD的作用是。
①
发送数据和接收数据
②
接收数据和发送数据
③
时钟输出和收发数据
④
收发数据和时钟输出
参考答案：
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