📄 数字电路的基本概念.htm
字号:
<I>t<SUB>PLH</SUB></I>=2<I>t<SUB>pd</SUB></I><SUB> </SUB></SPAN><BR><SPAN
class=text><FONT size=2> <A
href="http://210.41.4.20/course/46/46/ÍøÂç¿Î³Ì/800X600/web/text_web/07/07080000.htm#null"><IMG
onclick="openPictureWindow_Fever('../../../images/07/070806.jpg','487','270','self','0','0')"
height=30 src="数字电路的基本概念.files/but_01.gif" width=30
border=0></A></FONT></SPAN> <SPAN class=text_blue>图7.8.4
维持阻塞<I>D</I>触发器的电路和动态波形</SPAN> <BR><SPAN
class=css02> </SPAN><SPAN
class=text>4.最高时钟频率：为保证由门G<SUB>1</SUB><FONT face=宋体
size=3>～</FONT>G<SUB>4</SUB>组成的同步RS触发器能可靠地翻转，<I>CP</I>高电平的持续时间应大于
<I>t</I><SUB><I>PHL</I>,</SUB>所以时钟信号高电平的宽度t<SUB>WH</SUB>应大于t<SUB>PHL</SUB>。而为了在下一个<I>CP</I>上升沿到达之前确保门G<SUB>5</SUB>和G<SUB>6</SUB>新的输出
电平得以稳定地建立，<I>CP</I>低电平的持续时间不应小于门G<SUB>4</SUB>的传输延迟时间和<I>t</I><SUB><I>set</I></SUB>之和，即时钟信号低电平的宽度<I>t</I><SUB><I>WL</I></SUB>≥<I>t</I><SUB><I>set</I></SUB>+<I>t</I><SUB><I>pd</I></SUB>，因此得到:
</SPAN><SPAN
class=css02><BR> <IMG
height=48 src="数字电路的基本概念.files/070805.gif"
width=280><BR> </SPAN><SPAN
class=text>最后说明一点，在实际集成触发器中，每个门传输时间是不同的，并且作了不同形式的简化，因此上面讨论的结果只是一些定性的物理概念。其真实参数由实验测定。
</SPAN><SPAN
class=css02><B><BR> <BR> </B></SPAN><B><SPAN
class=text>7.8.5 集成触发器</SPAN></B><SPAN
class=text><BR> 集成<I>D</I>触发器的定型产品种类比较多，这里介绍双D触发器74HC74,实际上，74型号的产品种类较多，比如还有7474、74H74等。
</SPAN><BR>
<TABLE width="100%" border=0>
<TBODY>
<TR>
<TD height=115>
<DIV align=center><IMG height=178 src="数字电路的基本概念.files/070803.jpg"
width=300></DIV></TD>
<TD rowSpan=2>
<DIV align=center><SPAN class=text><BR>表7.8.2 <I>D</I>
触发器74HC74的逻辑功能表 </SPAN><BR></DIV>
<TABLE style="BORDER-LEFT-WIDTH: 0px; BORDER-RIGHT-WIDTH: 0px"
height=159 cellSpacing=0 cellPadding=0 width=438 align=center
border=1>
<TBODY>
<TR>
<TD style="BORDER-LEFT: medium none" colSpan=4>
<P class=text align=center>输 入</P></TD>
<TD style="BORDER-RIGHT: medium none" colSpan=2>
<P class=text align=center>输 出</P></TD></TR>
<TR>
<TD style="BORDER-LEFT: medium none" width=68>
<DIV align=center><FONT
size=2><I>S<SUB>D</SUB></I></FONT></DIV></TD>
<TD width=61>
<DIV align=center><FONT
size=2><I>R<SUB>D</SUB></I></FONT></DIV></TD>
<TD width=60>
<DIV align=center><FONT size=2><I>CP</I></FONT></DIV></TD>
<TD width=70>
<DIV align=center><FONT size=2><I>D</I></FONT></DIV></TD>
<TD width=63>
<DIV align=center><FONT size=2><I>Q</I></FONT></DIV></TD>
<TD style="BORDER-RIGHT: medium none" width=76>
<DIV align=center><FONT size=2><SPAN
style="TEXT-DECORATION: overline"><I>Q</I></SPAN></FONT></DIV></TD></TR>
<TR>
<TD style="BORDER-LEFT: medium none; BORDER-BOTTOM: medium none"
width=68>
<DIV align=center><FONT size=2>L</FONT></DIV></TD>
<TD style="BORDER-BOTTOM: medium none" width=61>
<DIV align=center><FONT size=2>H</FONT></DIV></TD>
<TD style="BORDER-BOTTOM: medium none" width=60>
<DIV align=center><FONT size=2>×</FONT></DIV></TD>
<TD style="BORDER-BOTTOM: medium none" width=70>
<DIV align=center><FONT size=2>×</FONT></DIV></TD>
<TD style="BORDER-BOTTOM: medium none" width=63>
<DIV align=center><FONT size=2>H</FONT></DIV></TD>
<TD
style="BORDER-RIGHT: medium none; BORDER-BOTTOM: medium none"
width=76>
<DIV align=center><FONT size=2>L</FONT></DIV></TD></TR>
<TR>
<TD
style="BORDER-TOP: medium none; BORDER-LEFT: medium none; BORDER-BOTTOM: medium none"
width=68>
<DIV align=center><FONT size=2>H</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=61>
<DIV align=center><FONT size=2>L</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=60>
<DIV align=center><FONT size=2>×</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=70>
<DIV align=center><FONT size=2>×</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=63>
<DIV align=center><FONT size=2>L</FONT></DIV></TD>
<TD
style="BORDER-RIGHT: medium none; BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=76>
<DIV align=center><FONT size=2>H</FONT></DIV></TD></TR>
<TR>
<TD
style="BORDER-TOP: medium none; BORDER-LEFT: medium none; BORDER-BOTTOM: medium none"
width=68>
<DIV align=center><FONT size=2>H</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=61>
<DIV align=center><FONT size=2>H</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=60>
<DIV align=center><FONT size=2>↑</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=70>
<DIV align=center><FONT size=2>H</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=63>
<DIV align=center><FONT size=2>H</FONT></DIV></TD>
<TD
style="BORDER-RIGHT: medium none; BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=76>
<DIV align=center><FONT size=2>L</FONT></DIV></TD></TR>
<TR>
<TD
style="BORDER-TOP: medium none; BORDER-LEFT: medium none; BORDER-BOTTOM: medium none"
width=68>
<DIV align=center><FONT size=2>H</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=61>
<DIV align=center><FONT size=2>H</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=60>
<DIV align=center><FONT size=2>↑</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=70>
<DIV align=center><FONT size=2>L</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=63>
<DIV align=center><FONT size=2>L</FONT></DIV></TD>
<TD
style="BORDER-RIGHT: medium none; BORDER-TOP: medium none; BORDER-BOTTOM: medium none"
width=76>
<DIV align=center><FONT size=2>H</FONT></DIV></TD></TR>
<TR>
<TD style="BORDER-TOP: medium none; BORDER-LEFT: medium none"
width=68 height=22>
<DIV align=center><FONT size=2>H</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none" width=61 height=22>
<DIV align=center><FONT size=2>H</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none" width=60 height=22>
<DIV align=center><FONT size=2>L</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none" width=70 height=22>
<DIV align=center><FONT size=2>×</FONT></DIV></TD>
<TD style="BORDER-TOP: medium none" width=63 height=22>
<DIV align=center><FONT
size=2><I>Q</I><SUB>0</SUB></FONT></DIV></TD>
<TD style="BORDER-RIGHT: medium none; BORDER-TOP: medium none"
width=76 height=22>
<DIV align=center><FONT size=2><SPAN
style="TEXT-DECORATION: overline"><I>Q</I></SPAN><SUB>0</SUB></FONT></DIV></TD></TR></TBODY></TABLE></TD></TR>
<TR>
<TD height=2>
<DIV align=center><FONT class=text size=2>(a)
逻辑符号 (b)
引脚分布<BR>图7.8.5<I> D</I>触发器74HC74的逻辑符号和引脚分布</FONT></DIV></TD></TR></TBODY></TABLE><SPAN
class=css02> </SPAN><SPAN
class=text>通过图7.8.5中的逻辑符号和<I>D</I>触发器74HC74的逻辑功能表我们可以看出，HC74是带有预置、清零输入，上跳沿触发的边沿触发器。</SPAN><SPAN
class=css02><BR><BR> </SPAN><SPAN
class=text>综上所述，对边沿<I>D</I>触发器归纳为以下几点：</SPAN><SPAN
class=text><BR> 1.边沿<I>D</I>触发器具有接收并记忆信号的功能，又称为锁存器；
<BR> 2.边沿<I>D</I>触发器属于脉冲触发方式；
<BR> 3.边沿<I>D</I>触发器不存在约束条件和一次变化现象，抗干扰性能好，工作速度快。</SPAN></TD></TR>
<TR>
<TD height=2><BR>
<TABLE cellSpacing=0 cellPadding=0 width=320 align=right>
<TBODY>
<TR>
<TD class=text_blue width=79><A class=css02
href="http://210.41.4.20/course/46/46/ÍøÂç¿Î³Ì/800X600/web/text_web/07/07080000.htm#"
target=_self></A><A class=css02
href="http://210.41.4.20/course/46/46/ÍøÂç¿Î³Ì/800X600/web/text_web/07/07070000.htm"
target=_self><FONT class=text_blue color=#333333>上一节</FONT></A></TD>
<TD class=text_blue width=79><A class=css02
href="http://210.41.4.20/course/46/46/ÍøÂç¿Î³Ì/800X600/web/text_web/07/07090000.htm"
target=_self><FONT class=text_blue color=#333333>下一节</FONT></A></TD>
<TD class=text_blue width=78><A class=css02
href="http://210.41.4.20/course/46/46/ÍøÂç¿Î³Ì/800X600/web/text_web/07/07000000.htm"
target=_self><FONT class=text_blue color=#333333>回章首</FONT></A></TD>
<TD class=text_blue width=82><A class=css02
href="http://210.41.4.20/course/46/46/ÍøÂç¿Î³Ì/800X600/index.htm"
target=_parent><FONT class=text_blue
color=#333333>回首页</FONT></A></TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE>
<SCRIPT language=JavaScript>
<!--
function play1()
{
window.open("../../../../mov/r070801.htm", "rm", "Scrollbars=0,toolbar=0,width=350,height=300");
}
-->
</SCRIPT>
</BODY></HTML>
⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -