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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="ch08.css"><TITLE> 8.3 Level-sensitive sequential UDPs </TITLE></HEAD><BODY BGCOLOR="#ffffff"><DIV><HR><P><A HREF="ch08.htm">Chapter start</A> <A HREF="ch08.2.htm">Previous page</A> <A HREF="ch08.4.htm">Next page</A></P></DIV><H1 CLASS="Section"><A NAME="pgfId=348"> </A>8.3 <A NAME="sequential primitives"> </A>L<A NAME="marker=90"> </A><A NAME="marker=91"> </A><A NAME="marker=92"> </A>evel-sensitive sequential UDPs </H1><P CLASS="Body"><A NAME="pgfId=349"> </A>Level-sensitive sequential behavior is represented the same way as combinational behavior, except that the output is declared to be of type <B CLASS="Keyword">reg</B><A NAME="marker=93"> </A>, and there is an additional field in each table entry. This new field represents the current state of the UDP. The output field in a sequential UDP represents the next state.</P><P CLASS="Body"><A NAME="pgfId=351"> </A>Example:</P><P CLASS="Body"><A NAME="pgfId=340"> </A>Consider the example of a latch:</P><P CLASS="Body"><A NAME="pgfId=323"> </A></P><DIV><IMG SRC="ch08-4.gif"></DIV><P CLASS="Body"><A NAME="pgfId=352"> </A>This description <A NAME="marker=100"> </A><A NAME="marker=101"> </A>differs from a combinational UDP model in two ways. First, the output identifier <CODE CLASS="code">q</CODE> has an additional <B CLASS="Keyword">reg</B> declaration to indicate that there is an internal state <CODE CLASS="code">q</CODE>. The output value of the UDP is always the same as the internal state. Second, a field for the current state, which is separated by colons from the inputs and the output, has been added.<A NAME="marker=102"> </A><A NAME="marker=103"> </A><A NAME="marker=104"> </A></P><HR><P><A HREF="ch08.htm">Chapter start</A> <A HREF="ch08.2.htm">Previous page</A> <A HREF="ch08.4.htm">Next page</A></P></BODY></HTML>?? 快捷鍵說明
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