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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML EXPERIMENTAL 970324//EN"><HTML><HEAD><META NAME="GENERATOR" CONTENT="Adobe FrameMaker 5.5/HTML Export Filter"><LINK REL="STYLESHEET" HREF="ch13.css"><TITLE> 13.6 Driving wired logic</TITLE></HEAD><BODY BGCOLOR="#ffffff"><DIV><HR><P><A HREF="ch13.htm">Chapter start</A> <A HREF="ch13.5.htm">Previous page</A> <A HREF="ch13.7.htm">Next page</A></P></DIV><H1 CLASS="Section"><A NAME="pgfId=628"> </A>13.6 <A NAME="37376"> </A><A NAME="marker=251"> </A><A NAME="marker=252"> </A>Driving wired logic<A NAME="driving wired logic"> </A></H1><P CLASS="Body"><A NAME="pgfId=630"> </A>Module path output nets shall not have more than one driver within the module. Therefore, wired logic is not allowed at module path outputs.</P><P CLASS="Body"><A NAME="pgfId=367"> </A><A HREF="ch13.6.htm# Illegal module paths" CLASS="XRef">See : Illegal module paths</A> illustrates a violation of this wired-output rule, and a method of avoiding the rule violation.</P><P CLASS="Body"><A NAME="pgfId=520"> </A> </P><DIV><IMG SRC="ch13-27.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=624"> </A>Figure 13-5:<A NAME=" Illegal module paths"> </A> Legal and illegal module paths </P><P CLASS="Body"><A NAME="pgfId=625"> </A>In <A HREF="ch13.6.htm# Illegal module paths" CLASS="XRef">See : Legal and illegal module paths</A> (a), any module path to <CODE CLASS="code">S</CODE> is illegal because the path destination has two drivers.</P><P CLASS="Body"><A NAME="pgfId=626"> </A>Assuming signal <CODE CLASS="code">S</CODE> in <A HREF="ch13.6.htm# Illegal module paths" CLASS="XRef">See : Legal and illegal module paths</A> (a) is a <I CLASS="Emphasis">wired-and</I>, this limitation can be circumvented by replacing wired logic with gated logic to create a single driver to the output. <A HREF="ch13.6.htm# Illegal module paths" CLASS="XRef">See : Legal and illegal module paths</A> (b) shows how adding a third <B CLASS="Keyword">and</B> gate--the shaded one--solves the problem for the module in <A HREF="ch13.6.htm# Illegal module paths" CLASS="XRef">See : Legal and illegal module paths</A> (a). </P><P CLASS="Body"><A NAME="pgfId=633"> </A>The example in <A HREF="ch13.6.htm# Illegal module paths" CLASS="XRef">See : Illegal module paths</A> is also illegal. In this example, when the outputs <CODE CLASS="code">Q</CODE> and <CODE CLASS="code">R</CODE> are wired together, it creates a condition where both paths have multiple drivers from within the same module.</P><P CLASS="Body"><A NAME="pgfId=631"> </A> </P><DIV><IMG SRC="ch13-28.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=632"> </A>Figure 13-6:<A NAME=" Illegal module paths"> </A> Illegal module paths</P><P CLASS="Body"><A NAME="pgfId=640"> </A><BR>Although multiple output drivers to a path destination are prohibited inside the same module, they are allowed <A NAME="marker=259"> </A>outside the module. The example in <A HREF="ch13.6.htm#Legal module paths" CLASS="XRef">See : Legal module paths</A> is legal since <CODE CLASS="code">Q</CODE> and <CODE CLASS="code">R</CODE> each have only one driver within the module in which the module paths are specified.</P><P CLASS="Body"><A NAME="pgfId=641"> </A></P><DIV><IMG SRC="ch13-29.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=642"> </A>Figure 13-7: <A NAME="Legal module paths"> </A>Legal module paths</P><HR><P><A HREF="ch13.htm">Chapter start</A> <A HREF="ch13.5.htm">Previous page</A> <A HREF="ch13.7.htm">Next page</A></P></BODY></HTML>?? 快捷鍵說明
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