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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> 8.7 Mixing level and edge-sensitive descriptions </TITLE></HEAD><BODY BGCOLOR="#ffffff"><DIV><HR><P><A HREF="ch08.htm">Chapter start</A> <A HREF="ch08.6.htm">Previous page</A> <A HREF="ch08.8.htm">Next page</A></P></DIV><H1 CLASS="Section"><A NAME="pgfId=378"> </A>8.7 M<A NAME="marker=152"> </A><A NAME="marker=153"> </A>ixing level and edge-sensitive descriptions </H1><P CLASS="Body"><A NAME="pgfId=379"> </A><EM CLASS="-">UDP definitions allow a mixing of the level-sensitive and the edge-sensitive constructs in the same table. When the input changes, the edge-sensitive cases are processed first, followed by level-sensitive cases. Thus, when level-sensitive and edge-sensitive cases specify different output values, the result is specified by the level-sensitive case.</EM> </P><P CLASS="Body"><A NAME="pgfId=393"> </A><EM CLASS="-">Example:</EM></P><P CLASS="Body"><A NAME="pgfId=380"> </A></P><DIV><IMG SRC="ch08-11.gif"></DIV><P CLASS="Body"><A NAME="pgfId=381"> </A><EM CLASS="-">In this example, the preset and clear logic is level-sensitive. Whenever the preset and clear combination is </EM><CODE CLASS="code">01</CODE><EM CLASS="-">, the output has value </EM><CODE CLASS="code">1</CODE><EM CLASS="-">. Similarly, whenever the preset and clear combination has value </EM><CODE CLASS="code">10</CODE><EM CLASS="-">, the output has value</EM> <CODE CLASS="code">0</CODE><EM CLASS="-">.</EM></P><P CLASS="Body"><A NAME="pgfId=382"> </A><EM CLASS="-">The remaining logic is sensitive to edges of the clock. In the normal clocking cases, the flip-flop is sensitive to the rising clock edge as indicated by an</EM><B CLASS="codebold"> </B><CODE CLASS="code">r</CODE><EM CLASS="-"> in the clock field in those entries. The insensitivity to the falling edge of clock is indicated by a hyphen (-</EM>)<EM CLASS="-"> in the output field (see <A HREF="ch08.1.htm#62691" CLASS="XRef">See : UDP table symbols</A>) for the entry with an </EM><CODE CLASS="code">f</CODE><EM CLASS="-"> as the value of clock. Remember that the desired output for this input transition must be specified to avoid unwanted </EM><CODE CLASS="code">x</CODE><EM CLASS="-"> values at the output. The last two entries show that the transitions in</EM> <CODE CLASS="code">j</CODE><EM CLASS="-"> and </EM><CODE CLASS="code">k</CODE><EM CLASS="-"> inputs do not change the output on a steady low or high </EM><CODE CLASS="code">clock</CODE><EM CLASS="-">.</EM><A NAME="marker=155"> </A><A NAME="marker=156"> </A></P><HR><P><A HREF="ch08.htm">Chapter start</A> <A HREF="ch08.6.htm">Previous page</A> <A HREF="ch08.8.htm">Next page</A></P></BODY></HTML>?? 快捷鍵說明
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