如题~
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这是由于51的IO口的硬件电路决定的····http://image.baidu.com/i?ct=503316480&z=&tn=baiduimagedetail&word=51%B5%A5%C6%AC%BB%FA%B5%C4IO%CA%E4%C8%EB&in=14480&cl=2&lm=-1&st=&pn=2&rn=1&di=84600506490&ln=1588&fr=&fm=&fmq=1332139973656_R&ic=&s=&se=&sme=0&tab=&width=&height=&face=&is=&istype=#pn6&-1&di78826147095&objURLhttp%3A%2F%2Fwww.dzsj.net%2Fmcs51%2F2006810_164918.gif&fromURLhttp%3A%2F%2Fwww.dzsj.net%2Fmcs51%2F51%25B5%25A5%25C6%25AC%25BB%25FAio%25BF%25DA%25D3%25A6%25D3%25C3%25CF%25EA%25BD%25E2.htm&W320&H224&T7824&S16&TPgif