详解经典51单片机P0口

shuszhao

<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">学习单片机最基础、最重要的就是掌握I/O口的使用。会使用I/O，也就具备了进一步深入学习的基础和条件。51单片机P0口比较特殊，本文就带你死磕P0口，扫清对它的一切困惑。快来看看下面几个小问题你都搞清楚没有？<ul style='margin: 0px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="margin: 0px; padding: 0px; font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; list-.: none; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);"><li style="list-style: none; margin: 0px; padding: 0px; word-wrap: break-word; box-sizing: border-box;" .="padding: 0px; margin: 0px; list-.: none;"><div style="margin: 0px 0px 10px; padding: 0px; word-wrap: break-word; box-sizing: border-box;" .="font-family: inherit;"&gt0口为什么要接上拉电阻？<li style="list-style: none; margin: 0px; padding: 0px; word-wrap: break-word; box-sizing: border-box;" .="padding: 0px; margin: 0px; list-.: none;"><div style="margin: 0px 0px 10px; padding: 0px; word-wrap: break-word; box-sizing: border-box;" .="font-family: inherit;">吸电流和拉电流有什么区别？<li style="list-style: none; margin: 0px; padding: 0px; word-wrap: break-word; box-sizing: border-box;" .="padding: 0px; margin: 0px; list-.: none;"><div style="margin: 0px 0px 10px; padding: 0px; word-wrap: break-word; box-sizing: border-box;" .="font-family: inherit;">什么是准双向I/O？什么是漏极开路输出？<li style="list-style: none; margin: 0px; padding: 0px; word-wrap: break-word; box-sizing: border-box;" .="padding: 0px; margin: 0px; list-.: none;"><div style="margin: 0px 0px 10px; padding: 0px; word-wrap: break-word; box-sizing: border-box;" .="font-family: inherit;">什么是推挽式输出？<li style="list-style: none; margin: 0px; padding: 0px; word-wrap: break-word; box-sizing: border-box;" .="padding: 0px; margin: 0px; list-.: none;"><div style="margin: 0px 0px 10px; padding: 0px; word-wrap: break-word; box-sizing: border-box;" .="font-family: inherit;"&gt0到底怎样做总线复用？<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">P0口的结构<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">1、P0的8位皆为漏极开路输出（Open Drain，OD），千万不要误解为图腾式输出，每个引脚可驱动8个LS型TTL负载；<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">2、P0内部无上拉电阻，执行输出功能时，外部必须接上拉电阻（10kΩ即可）；<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">3、若有执行输入功能，必须先输出高电平（1）才能读取该口所连接的外部数据；<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">4、若连接外部存储器，则P0可作为地址总线（A0~A7）及数据总线（D0~D7）的复用引脚，此时内部具有上拉电阻，不用外接。

                               
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<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);"&gt0口结构图<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">由上图可见，P0口由输入缓冲器、锁存器、切换开关、一个与非门、一个与门及场效应管驱动电路构成，下面逐个分析。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">输入缓冲器<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">在P0口，有两个三态的缓冲器。三态门有三个状态，即在高电平、低电平，高阻状态(或称为禁止状态)，大家看上图，上面一个是读锁存器的缓冲器，也就是说，要读取D锁存器输出端Q的数据，那就得使读锁存器的这个缓冲器的三态控制端有效。下面一个是读引脚的缓冲器，要读取P0.X引脚上的数据，也要使标号为‘读引脚’的这个三态缓冲器的控制端有效，引脚上的数据才会传输到我们单片机的内部数据总线上。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">D锁存器<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">51单片机的32根I/O口线中都是用一个D触发器来构成锁存器的。上图中的D锁存器，D端是数据输入端，控制端CP是时序控制信号输入端，Q是输出端，Q非是反向输出端。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">对于D触发器来讲，当D输入端有一个输入信号，如果这时控制端CP没有信号(也就是时序脉冲没有到来)，这时输入端D的数据是无法传输到输出端Q及反向输出端Q非的。如果时序控制端CP的时序脉冲一旦到了，这时D端输入的数据就会传输到Q及Q非端。数据传送过来后，即使CP时序控制端的时序信号消失，输出端还会保持着上次输入端D的数据(即把上次的数据锁存起来了)。如果下一个时序控制脉冲信号来了，这时D端的数据才再次传送到Q端，从而改变Q端的状态。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">多路开关<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">多路开关：在51单片机中，如果需要外扩存储器时，P0就可以复用为‘地址/数据’总线。多路选择开关就是用于选择是做为普通I/O口使用还是作为‘数据/地址’总线使用的选择开关。大家看上图，当多路开关与下面接通时，P0口是作为普通的I/O口使用的，当多路开关是与上面接通时，P0口是作为‘地址/数据’总线使用的。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">输出驱动部份<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">输出驱动部份：从上图中我们已看出，P0口的输出是由两个MOS管组成的推拉式结构，也就是说，这两个MOS管一次只能导通一个，当V1导通时，V2就截止，当V2导通时，V1截止。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">漏极开路输出<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">如下图，单片机P1，P2，P3口都可以理解成左图，注意vcc下面有个电阻（约30k），因此可以理解成：引脚拉出电流能力弱。地那边没有电阻，可以理解成引脚吸入电流能力强。而P0口，可以理解成右图。这就是集电极开路输出，也叫集电极开路OC输出，（注意单片机引脚内部是MOS管，应该是漏极开路OD输出）。可以看出，当CTR=1时，三极管导通，引脚被接地;当ctr=0时，三极管截止，引脚浮空，也叫三态。这个端口这么做的目的是考虑P0口肩负读写数据和地址复用，因此，P0口做出输出时要加合适的上拉电阻。

                               
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<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">
<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">
<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">吸电流or拉电流<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">上面我们已经知道单片机P1，P2，P3口有上拉电阻（约30k），因此引脚拉出电流能力弱。对地那边没有电阻，可以理解成引脚吸入电流能力强。所以51单片机I/O口通常设计成吸电流的形式。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">以驱动LED发光二极管为例，正向偏压时LED两端约有1.7V左右管压降，驱动LED的电流以10~20mA为宜。P1、P2、P3内部具有30kΩ上拉电阻，若要拉出10~20mA电流是不可能的（5V/30k=0.1667mA）。但采用吸电流的方式，电流就大多了。

                               
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<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">从P1口吸电流和拉电流驱动LED实验效果，可以看出LED发光强度有明显区别：<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);"><div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);"&gt1口吸电流<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);"><div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);"&gt1口拉电流<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">准双向I/O<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">“准”就是“基本上”“勉强算”的意思，“准双向口”不是正真的双向口。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">51单片机P1、P2、P3有固定的内部上拉电阻，当用做输入时被拉高，低则要靠外部电路拉低，当这三个准双向I/O口作输入口使用时，要向该口先写1，使下面的MOS管截止，以防止被拉低。这样的I/O口无高阻的“浮空”状态，不是真正的双向I/O。

                               
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<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);"&gt0口的真双向指的是它被用作地址／数据端口时，只有在这个时候，P0口才处于两个开关管推挽状态；当两个开关管都关闭时，会出现高阻状态。双向与准双向，根本原则是双向包含了高阻这个状态，而不在于是否需要先写１或者不写，P1~P3口因为有内部上拉电阻，因此无论如何不是双向；P０口内部无上拉电阻，在处于数据／地址功能时，自动完成三态的转换，是双向，处于一般I/O口时，如果不接外部上拉，而且先向端口写了１，那么就处于高阻状态，此时，它也是一个人为的双向口，这与它处于地址／数据功能时的自动双向有区别，以及与P1~P3处于输入时输出锁存器为1是有区别的。跟I2C总线上那种漏极开路或者集电极开路结构差不多，通过上拉电阻(或者下拉电阻)来提供一种电平的驱动.<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">准双向口在做为输入使用时，实际上还是一种输出状态。而真正的双向IO口，有方向控制寄存器，作为输入使用时输出部分被断开。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">双向口与准双向口的区别为双向口有高阻态，输入为真正的外部信号，准双向口内部有上拉，故高电平为内部给出不是真正的外部信号，软件做处理时都要先向口写“1”以断开开关连接。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">图腾式输出<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">图腾式输出（Totem Pole 的音译），此结构电路有点像印第安人的图腾柱，所以叫图腾式输出。输出级分别采用一个NPN型晶体管和一个PNP型晶体管，NPN管集电极接正电源（或接地），发射极接下面PNP管的发射极，同时输出；PNP管的集电极接地（或负电源）。两管的基极同时接前级的控制端。上下两管输出管，从直流的角度看是串联，从交流的角度看是并联，两管发射极连接处为输出端，实际是一对射极跟随器。追随输入电平，上管导通，下管截止，输出高电平；上管截止，下管导通，输出低电平，形成类似推挽式的输出方式。

                               
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<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);"&gt0口的各单元部件已了解清楚了，下面我们就来研究一下P0口做为I/O口及地址/数据总线使用时的具体工作过程。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">
<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">作为I/O端口使用时<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">当多路开关的控制信号为0（低电平）时，多路开关是与锁存器的‘Q非’端相接的，此时P0口作为I/O口线使用。多路开关的控制信号同时还与与门的一个输入端是相接的，我们知道与门的逻辑是“全1出1，有0出0”，那么当控制信号是0的话，这时与门输出的也是一个0（低电平），V1管就截止。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);"&gt0口用作I/O口线时输出状态的工作过程：当写锁存器信号CP有效，数据总线的信号→锁存器的输入端D→锁存器的反向输出Q非端→多路开关→V2管的栅极→V2的漏极到输出端P0.X。由于V1管是截止的，所以作为输出口时，P0是漏极开路输出，当驱动上接电流负载时，需要外接上拉电阻。

                               
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<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">读引脚<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">读引脚数据时，读引脚缓冲器打开（即三态缓冲器的控制端要有效），通过内部数据总线输入，请看下图（红色箭头）。但应注意先向口线写1，以截止V2管，以免引脚电平被内部拉低。

                               
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<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">读锁存器<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">通过打开读锁存器三态缓冲器读取锁存器输出端Q的状态。在输入状态下，从锁存器和从引脚上读来的信号一般是一致的，但也有例外。例如，当从内部总线输出低电平后，锁存器Q＝0，Q非＝1，场效应管T2开通，端口线呈低电平状态。此时无论端口线上外接的信号是低电乎还是高电平，从引脚读入单片机的信号都是低电平，因而不能正确地读入端口引脚上的信号。又如，当从内部总线输出高电平后，锁存器Q＝1，Q非＝0，场效应管T2截止。如外接引脚信号为低电平，从引脚上读入的信号就与从锁存器读入的信号不同。为此，8051单片机在对端口P0~P3的输入操作上，有如下约定：凡属于读-修改-写方式的指令，从锁存器读入信号，其它指令则从端口引脚线上读入信号。

                               
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<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">读-修改-写指令的特点是，从端口输入(读)信号，在单片机内加以运算(修改)后，再输出(写)到该端口上。下面是几条读-修改-写指令的例子。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">ANL P0,#立即数 0&立即数→P0<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">ORL P0,A 0|A→P0<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">INC P1 ;P1+1→P1<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">DEC P3 ;P3-1→P3<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">CPL P2 ;~P2→P2<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">这样安排的原因在于读-修改-写指令需要得到端口原输出的状态，修改后再输出，读锁存器而不是读引脚，可以避免因外部电路的原因而使原端口的状态被读错。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">作为地址/数据复用口<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">在访问外部存储器时P0口作为地址/数据复用口使用。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">这时多路开关‘控制’信号为‘1’，‘与门’解锁，‘与门’输出信号电平由“地址/数据”线信号决定；多路开关与反相器的输出端相连，地址信号经“地址/数据”线→反相器→V2场效应管栅极→V2漏极输出。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">例如：控制信号为1，地址信号为“0”时，与门输出低电平，V1管截止；反相器输出高电平，V2管导通，输出引脚的地址信号为低电平。

                               
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<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">反之，控制信号为“1”、地址信号为“1”，“与门”输出为高电平，V1管导通；反相器输出低电平，V2管截止，输出引脚的地址信号为高电平。请看下图：

                               
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<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">可见，在输出“地址/数据”信息时，V1、V2管是交替导通的，负载能力很强，可以直接与外设存储器相连，无须增加总线驱动器。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">P0口又作为数据总线使用。在访问外部程序存储器时，P0口输出低8位地址信息后，将变为数据总线，以便读指令码（输入）。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">在取指令期间，“控制”信号为“0”，V1管截止，多路开关也跟着转向锁存器反相输出端Q非；CPU自动将0FFH（11111111，即向D锁存器写入一个高电平‘1’）写入P0口锁存器，使V2管截止，在读引脚信号控制下，通过读引脚三态门电路将指令码读到内部总线。请看下图

                               
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<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">如果该指令是输出数据，如MOVX @DPTR，A（将累加器的内容通过P0口数据总线传送到外部RAM中），则多路开关“控制”信号为‘1’，“与门”解锁，与输出地址信号的工作流程类似，数据据由“地址/数据”线→反相器→V2场效应管栅极→V2漏极输出。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">如果该指令是输入数据（读外部数据存储器或程序存储器），如MOVX A，@DPTR（将外部RAM某一存储单元内容通过P0口数据总线输入到累加器A中），则输入的数据仍通过读引脚三态缓冲器到内部总线，其过程类似于上图中的读取指令码流程图。<div style='margin: 0px 0px 10px; padding: 0px; text-align: justify; color: rgb(100, 100, 100); text-transform: none; text-indent: 0px; letter-spacing: normal; font-family: myFont, "Microsoft YaHei"; font-size: 16px; font-style: normal; font-weight: normal; word-spacing: 0px; white-space: normal; word-wrap: break-word; box-sizing: border-box; orphans: 2; widows: 2; background-color: rgb(255, 255, 255); font-variant-ligatures: normal; font-variant-caps: normal; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;' .="font-family: 微软雅黑, 宋体, 黑体, Arial, Helvetica, sans-serif; color: rgb(47, 47, 47); font-size: 16px; background-color: rgb(204, 232, 207);">通过以上的分析可以看出，当P0作为地址/数据总线使用时，在读指令码或输入数据前，CPU自动向P0口锁存器写入0FFH，破坏了P0口原来的状态。因此，不能再作为通用的I/O端口。大家以后在系统设计时务必注意，即程序中不能再含有以P0口作为操作数（包含源操作数和目的操作数）的指令

sun5304

感谢楼主提供！学习学习。

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