第二课：初步认识51芯片 (上一课我们的第一个项目完成了，可能有懂c语言的朋友会说......).htm

单片机的c语言教程,属于初级教材,适合初学者,高手们就不要下了啊!!

vAlign=top width=284>
<P align=center class=MsoNormal 
style="TEXT-ALIGN: center"><SPAN 
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'"><font font>串行通讯输出（</font></SPAN><font font><SPAN 
lang=EN-US>TXD</SPAN><SPAN 
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">）</SPAN></font></P></TD></TR>
<TR>
<TD 
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: windowtext 0.5pt solid; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 213.2pt; mso-border-top-alt: solid windowtext .5pt" 
vAlign=top width=284>
<P align=center class=MsoNormal 
style="TEXT-ALIGN: center"><SPAN lang=EN-US><font font>P3.2</font></SPAN></P></TD>
<TD 
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: medium none; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 213.2pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt" 
vAlign=top width=284>
<P align=center class=MsoNormal 
style="TEXT-ALIGN: center"><SPAN 
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'"><font font>外部中断</font></SPAN><font font><SPAN 
lang=EN-US>0</SPAN><SPAN 
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">（</SPAN> 
<SPAN lang=EN-US>INT0</SPAN><SPAN   
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">）</SPAN></font></P></TD></TR>   
<TR>   
<TD   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: windowtext 0.5pt solid; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 213.2pt; mso-border-top-alt: solid windowtext .5pt"   
vAlign=top width=284>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN lang=EN-US><font font>P3.3</font></SPAN></P></TD>   
<TD   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: medium none; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 213.2pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt"   
vAlign=top width=284>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN   
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'"><font font>外部中断</font></SPAN><font font><SPAN   
lang=EN-US>1</SPAN><SPAN   
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">（</SPAN><SPAN   
lang=EN-US>INT1</SPAN><SPAN   
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">）</SPAN></font></P></TD></TR>   
<TR>   
<TD   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: windowtext 0.5pt solid; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 213.2pt; mso-border-top-alt: solid windowtext .5pt"   
vAlign=top width=284>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN lang=EN-US><font font>P3.4</font></SPAN></P></TD>   
<TD   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: medium none; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 213.2pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt"   
vAlign=top width=284>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN   
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'"><font font>定时器</font></SPAN><font font><SPAN   
lang=EN-US>0</SPAN><SPAN   
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">输入</SPAN><SPAN   
lang=EN-US>(T0)</SPAN></font></P></TD></TR>   
<TR>   
<TD height=15   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: windowtext 0.5pt solid; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 213.2pt; mso-border-top-alt: solid windowtext .5pt"   
vAlign=top width=284>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN lang=EN-US><font font>P3.5</font></SPAN></P></TD>   
<TD height=15   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: medium none; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 213.2pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt"   
vAlign=top width=284>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN   
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'"><font font>定时器</font></SPAN><font font><SPAN   
lang=EN-US>1</SPAN><SPAN   
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">输入</SPAN><SPAN   
lang=EN-US>(T1)</SPAN></font></P></TD></TR>   
<TR>   
<TD   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: windowtext 0.5pt solid; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 213.2pt; mso-border-top-alt: solid windowtext .5pt"   
vAlign=top width=284>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN lang=EN-US><font font>P3.6</font></SPAN></P></TD>   
<TD   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: medium none; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 213.2pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt"   
vAlign=top width=284>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN   
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'"><font font>外部数据存储器写选通</font></SPAN><font font><SPAN   
lang=EN-US>WR</SPAN></font></P></TD></TR>   
<TR>   
<TD   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: windowtext 0.5pt solid; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 213.2pt; mso-border-top-alt: solid windowtext .5pt"   
vAlign=top width=284>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN lang=EN-US><font font>P3.7</font></SPAN></P></TD>   
<TD   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: medium none; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 213.2pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt"   
vAlign=top width=284>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN   
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'"><font font>外部数据存储器写选通</font></SPAN><font font><SPAN   
lang=EN-US>RD</SPAN></font></P></TD></TR></TBODY></TABLE></TD></TR>   
<TR>   
<TD width="714">   
<DIV align=center><font font>表2－2　P3端口引脚兼用功能表<BR></font></DIV></TD></TR>
<TR>
<TD width="714"><font font>　　呼！一口气说了那么多，停一下吧。嗯，什么？什么叫上拉电阻？上拉电阻简单来说就是把电平拉高，通常用4.7－10K的电阻接到Vcc电源，下拉电阻则是把电平拉低，电阻接到GND地线上。具体说明也不是这里要讨论的，接下来还是接着看其它的引脚功能吧。<BR>　　5.其它的控制或复用引脚<BR>　　　　(1)   
ALE/PROG 30   
访问外部存储器时，ALE（地址锁存允许）的输出用于锁存地址的低位字节。即使不访问外部存储器，ALE端仍以不变的频率输出脉冲信号(此频率是振荡器频率的1/6)。在访问外部数据存储器时，出现一个ALE脉冲。对Flash存储器编程时，这个引脚用于输入编程脉冲PROG<BR>　　　　(2)   
PSEN 29   
该引是外部程序存储器的选通信号输出端。当AT89C51由外部程序存储器取指令或常数时，每个机器周期输出2个脉冲即两次有效。但访问外部数据存储器时，将不会有脉冲输出。<BR>　　　　(3)   
EA/Vpp 31   
外部访问允许端。当该引脚访问外部程序存储器时，应输入低电平。要使AT89C51只访问外部程序存储器（地址为0000H-FFFFH）,这时该引脚必须保持低电平。对Flash存储器编程时，用于施加Vpp编程电压。Vpp电压有两种，类似芯片最大频率值要根据附加的编号或芯片内的特征字决定。具体如表2－3所列。<BR></font></TD></TR>   
<TR>   
<TD width="714">   
<TABLE align=center border=1 cellPadding=0 cellSpacing=0   
style="BORDER-BOTTOM: medium none; BORDER-COLLAPSE: collapse; BORDER-LEFT: medium none; BORDER-RIGHT: medium none; BORDER-TOP: medium none; mso-border-alt: solid windowtext .5pt; mso-padding-alt: 0cm 5.4pt 0cm 5.4pt">   
<TBODY>   
<TR>   
<TD   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: windowtext 0.5pt solid; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: windowtext 0.5pt solid; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 142.1pt"   
vAlign=top width=189>   
<P align=center class=MsoNormal style="TEXT-ALIGN: center"><font font><![if !supportEmptyParas]><![endif]>&nbsp;   
<SPAN lang=EN-US   
style="FONT-FAMILY: 宋体"><O:P></O:P></SPAN></font></P></TD>   
<TD colSpan=2   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: medium none; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: windowtext 0.5pt solid; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 142.15pt; mso-border-left-alt: solid windowtext .5pt"   
vAlign=top width=190>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN lang=EN-US   
style="FONT-FAMILY: 宋体"><font font>Vpp = 12V<O:P></O:P></font></SPAN></P></TD>   
<TD colSpan=2   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: medium none; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: windowtext 0.5pt solid; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 142.15pt; mso-border-left-alt: solid windowtext .5pt"   
vAlign=top width=190>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN lang=EN-US   
style="FONT-FAMILY: 宋体"><font font>Vpp = 5V<O:P></O:P></font></SPAN></P></TD></TR>   
<TR style="HEIGHT: 48.25pt">   
<TD   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: windowtext 0.5pt solid; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; HEIGHT: 48.25pt; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 142.1pt; mso-border-top-alt: solid windowtext .5pt"   
vAlign=top width=189>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN lang=EN-US   
style="FONT-FAMILY: 宋体"><font font><![if !supportEmptyParas]>&nbsp;<![endif]>   
<O:P></O:P></font></SPAN></P>   
<P align=center class=MsoNormal   
style="TEXT-ALIGN: center"><SPAN   
style="FONT-FAMILY: 宋体"><font font>印刷在芯片面上的型号<SPAN   
lang=EN-US><O:P></O:P></SPAN></font></SPAN></P></TD>   
<TD   
style="BORDER-BOTTOM: windowtext 0.5pt solid; BORDER-LEFT: medium none; BORDER-RIGHT: windowtext 0.5pt solid; BORDER-TOP: medium none; HEIGHT: 48.25pt; PADDING-BOTTOM: 0cm; PADDING-LEFT: 5.4pt; PADDING-RIGHT: 5.4pt; PADDING-TOP: 0cm; WIDTH: 71.05pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt"   
vAlign=top width=95>