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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="ch07.css"><TITLE> 7.11 Strengths and values of combined signals</TITLE></HEAD><BODY BGCOLOR="#ffffff"><DIV><HR><P><A HREF="ch07.htm">Chapter start</A> <A HREF="ch07.a.htm">Previous page</A> <A HREF="ch07.c.htm">Next page</A></P></DIV><H1 CLASS="Section"><A NAME="pgfId=1550"> </A>7.11 <A NAME="marker=361"> </A><A NAME="marker=362"> </A>S<A NAME="marker=363"> </A><A NAME="marker=364"> </A>trengths and values of combined signals</H1><P CLASS="Body"><A NAME="pgfId=1551"> </A>In addition to a signal value, a net shall have either a single unambiguous strength level or an ambiguous strength, consisting of more than one level. When signals combine, their strengths and values shall determine the strength and value of the resulting signal in accordance with the principles in the four subsections that follow.</P><P CLASS="SubSection"><A NAME="pgfId=1552"> </A>Combined signals of unambiguous strength</P><P CLASS="Body"><A NAME="pgfId=1553"> </A>This subsection deals with combinations of signals in which each signal has a known value and a single strength level.</P><P CLASS="Body"><A NAME="pgfId=1036"> </A>If two or more signals of unequal strength combine in a wired net configuration, the stronger signal shall dominate all the weaker drivers and determine the result. The combination of two or more signals of like value shall result in the same value with the greater of all the strengths. The combination of signals identical in strength and value shall result in the same signal.</P><P CLASS="Body"><A NAME="pgfId=1037"> </A>The combination of signals with unlike values and the same strength can have three possible results. Two of the results occur in the presence of wired logic and the third occurs in its absence. Section <A HREF="ch07.b.htm#12436" CLASS="XRef">See Wired logic net types</A> discusses wired logic. The result in the absence of wired logic is the subject of the first figure in the next subsection.</P><DIV><H2 CLASS="Example"><A NAME="pgfId=1035"> </A></H2><P CLASS="Body"><A NAME="pgfId=1554"> </A> </P><DIV><IMG SRC="ch07-12.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=1555"> </A>Figure 7-3<A NAME="65091"> </A>: Combining unequal strengths</P><P CLASS="Body"><A NAME="pgfId=1556"> </A>In <A HREF="ch07.b.htm#65091" CLASS="XRef">See : Combining unequal strengths</A>, the numbers in parentheses indicate the relative strengths of the signals. The combination of a <B CLASS="Keyword">pull </B><CODE CLASS="code">1</CODE> and a <B CLASS="Keyword">strong</B> <CODE CLASS="code">0</CODE> results in a <B CLASS="Keyword">strong</B> <CODE CLASS="code">0</CODE>, which is the stronger of the two signals. </P><P CLASS="SubSection"><A NAME="pgfId=1557"> </A><A NAME="48463"> </A>A<A NAME="marker=374"> </A>mbiguous s<A NAME="marker=375"> </A>trengths: sources and combinations</P><P CLASS="Body"><A NAME="pgfId=1558"> </A>There are several classifications of signals possessing ambiguous strengths:</P><UL><LI CLASS="DashedList"><A NAME="pgfId=1559"> </A>signals with known values and multiple strength levels</LI><LI CLASS="DashedList"><A NAME="pgfId=1560"> </A>signals with a value <CODE CLASS="code">x</CODE>, which have strength levels consisting of subdivisions of both the strength1 and the strength0 parts of the scale of strengths in <A HREF="ch07.a.htm#75920" CLASS="XRef">See : Scale of strengths</A></LI><LI CLASS="DashedList"><A NAME="pgfId=1561"> </A>signals with a value <CODE CLASS="code">L</CODE>, which have strength levels that consist of high impedance joined with strength levels in the strength0 part of the scale of strengths in <A HREF="ch07.a.htm#75920" CLASS="XRef">See : Scale of strengths</A></LI><LI CLASS="DashedList"><A NAME="pgfId=1562"> </A>signals with a value <CODE CLASS="code">H</CODE>, which have strength levels that consist of high impedance joined with strength levels in the strength1 part of the scale of strengths in <A HREF="ch07.a.htm#75920" CLASS="XRef">See : Scale of strengths</A></LI></UL><P CLASS="Body"><A NAME="pgfId=1563"> </A>Many configurations can produce signals of ambiguous strength. When two signals of equal strength and opposite value combine, the result shall be a value <CODE CLASS="code">x</CODE> and the strength levels of both signals and all the smaller strength levels. </P></DIV><DIV><H2 CLASS="Example"><A NAME="pgfId=1038"> </A></H2><P CLASS="Body"><A NAME="pgfId=1039"> </A><A HREF="ch07.b.htm#49828" CLASS="XRef">See : Combination of signals of equal strength and opposite values</A> shows the combination of a <B CLASS="Keyword">weak</B> signal with a value <CODE CLASS="code">1</CODE> and a <B CLASS="Keyword">weak</B> signal with a value <CODE CLASS="code">0</CODE> yielding a signal with <B CLASS="Keyword">weak</B> strength and a value <CODE CLASS="code">x</CODE>. </P><P CLASS="Body"><A NAME="pgfId=1564"> </A></P><DIV><IMG SRC="ch07-13.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=1565"> </A>Figure 7-4: Combination of signals of equal strength and opposite values<A NAME="49828"> </A></P><P CLASS="Body"><A NAME="pgfId=1566"> </A>This signal is described in <A HREF="ch07.b.htm#19451" CLASS="XRef">See : Weak x signal strength</A>.</P><P CLASS="Body"><A NAME="pgfId=1567"> </A></P><DIV><IMG SRC="ch07-14.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=1568"> </A>Figure 7-5<A NAME="19451"> </A>: Weak x signal strength</P><P CLASS="Body"><A NAME="pgfId=1569"> </A>An ambiguous signal strength can be a range of possible values<A NAME="marker=395"> </A>. An example is the strength of the output from the tristate drivers with unknown control inputs as shown in <A HREF="ch07.b.htm#26201" CLASS="XRef">See : Bufifs with control inputs of x</A>. </P><P CLASS="Body"><A NAME="pgfId=1570"> </A></P><DIV><IMG SRC="ch07-15.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=1571"> </A>Figure 7-6<A NAME="26201"> </A>: Bufifs with control inputs of x</P><P CLASS="Body"><A NAME="pgfId=1572"> </A>The output of the <B CLASS="Keyword">bufif1</B> in <A HREF="ch07.b.htm#26201" CLASS="XRef">See : Bufifs with control inputs of x</A> is a <B CLASS="Keyword">strong</B> <CODE CLASS="code">H</CODE>, composed of the range of values described in <A HREF="ch07.b.htm#76790" CLASS="XRef">See : Strong H range of values</A>.</P><P CLASS="Body"><A NAME="pgfId=1573"> </A></P><DIV><IMG SRC="ch07-16.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=1574"> </A>Figure 7-7<A NAME="76790"> </A>: Strong H range of values</P><P CLASS="Body"><A NAME="pgfId=1575"> </A>The output of the <B CLASS="Keyword">bufif0</B> in <A HREF="ch07.b.htm#26201" CLASS="XRef">See : Bufifs with control inputs of x</A> is a <B CLASS="Keyword">weak</B> <CODE CLASS="code">L</CODE>, composed of the range of values described in <A HREF="ch07.b.htm#90949" CLASS="XRef">See : Weak L range of values</A>.</P><P CLASS="Body"><A NAME="pgfId=1576"> </A></P><DIV><IMG SRC="ch07-17.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=1577"> </A>Figure 7-8<A NAME="90949"> </A>: Weak L range of values</P><P CLASS="Body"><A NAME="pgfId=1578"> </A>The combination of two signals of ambiguous strength shall result in a signal of ambiguous strength. The resulting signal shall have a range of strength levels that includes the strength levels in its component signals. The combination of outputs from two tristate drivers with unknown control inputs, shown in <A HREF="ch07.b.htm#86500" CLASS="XRef">See : Combined signals of ambiguous strength</A>, is an example.</P><P CLASS="Body"><A NAME="pgfId=1579"> </A><EM CLASS="-"></EM></P><DIV><IMG SRC="ch07-18.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=1580"> </A>Figure 7-9<EM CLASS="-">: Combined signals of ambiguous strength</EM><A NAME="86500"> </A></P><P CLASS="Body"><A NAME="pgfId=1581"> </A><EM CLASS="-">In </EM><A HREF="ch07.b.htm#86500" CLASS="XRef">See : Combined signals of ambiguous strength</A><EM CLASS="-">, the combination of signals of ambiguous strengths produces a range which includes the extremes of the signals and all the strengths between them, as described in <A HREF="ch07.b.htm#72440" CLASS="XRef">See : An unknown signal's range of strengths</A>.</EM></P><P CLASS="Body"><A NAME="pgfId=1582"> </A><EM CLASS="-"></EM></P><DIV><IMG SRC="ch07-19.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=1583"> </A>Figure 7-10<A NAME="72440"> </A>: An unknown signal's range of strengths</P><P CLASS="Body"><A NAME="pgfId=1584"> </A><EM CLASS="-">The result is a value </EM><CODE CLASS="code">x</CODE><EM CLASS="-"> because its range includes the values </EM><CODE CLASS="code">1</CODE><EM CLASS="-"> and </EM><CODE CLASS="code">0</CODE><EM CLASS="-">. The number </EM><CODE CLASS="code">35</CODE><EM CLASS="-">, which precedes the </EM><CODE CLASS="code">x</CODE><EM CLASS="-">, is a concatenation of two digits. The first is the digit 3, which corresponds to the highest strength0 level for the result. The second digit, 5, corresponds to the highest strength1 level for the result. </EM></P><P CLASS="Body"><A NAME="pgfId=1585"> </A>Switch networks can produce a ranges of strengths of the same value, such as the signals from the upper and lower configurations in <A HREF="ch07.b.htm#15003" CLASS="XRef">See : Ambiguous strengths from switch networks</A>.</P><P CLASS="Body"><A NAME="pgfId=1586"> </A></P><DIV><IMG SRC="ch07-20.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=1587"> </A>Figure 7-11<A NAME="15003"> </A>: Ambiguous strengths from switch networks</P><P CLASS="Body"><A NAME="pgfId=1588"> </A>In <A HREF="ch07.b.htm#15003" CLASS="XRef">See : Ambiguous strengths from switch networks</A>, the upper combination of a register, a gate controlled by a register of unspecified value, and a pullup produces a signal with a value of 1 and a range of strengths (651) described in <A HREF="ch07.b.htm#32362" CLASS="XRef">See : Range of two strengths of a defined value</A>.</P><P CLASS="Body"><A NAME="pgfId=1589"> </A></P><DIV><IMG SRC="ch07-21.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=1590"> </A>Figure 7-12<A NAME="32362"> </A>: Range of two strengths of a defined value</P><P CLASS="Body"><A NAME="pgfId=1591"> </A>In <A HREF="ch07.b.htm#15003" CLASS="XRef">See : Ambiguous strengths from switch networks</A> the lower combination of a <B CLASS="Keyword">pulldown</B>, a gate controlled by a register of unspecified value, and an <B CLASS="Keyword">and</B> gate produces a signal with a value <CODE CLASS="code">0</CODE> and a range of strengths (<CODE CLASS="code">53</CODE> <CODE CLASS="code">0</CODE>) described in <A HREF="ch07.b.htm#78053" CLASS="XRef">See : Range of three strengths of a defined value</A>.</P><P CLASS="Body"><A NAME="pgfId=1592"> </A></P><DIV><IMG SRC="ch07-22.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=1593"> </A>Figure 7-13<A NAME="78053"> </A>: Range of three strengths of a defined value</P><P CLASS="Body"><A NAME="pgfId=1594"> </A>When the signals from the upper and lower configurations in <A HREF="ch07.b.htm#15003" CLASS="XRef">See : Ambiguous strengths from switch networks</A> combine, the result is an unknown with a range (<CODE CLASS="code">56x</CODE>) determined by the extremes of the two signals shown in <A HREF="ch07.b.htm#89362" CLASS="XRef">See : Unknown value with a range of strengths</A><A HREF="ch07.b.htm#98318" CLASS="XRef">See </A>.</P><P CLASS="Body"><A NAME="pgfId=1595"> </A></P><DIV><IMG SRC="ch07-23.gif"></DIV><P CLASS="FigCapBody"><A NAME="pgfId=1596"> </A>Figure 7-14<A NAME="89362"> </A>: Unknown value with a range of strengths</P><P CLASS="Body"><A NAME="pgfId=1597"> </A>In <A HREF="ch07.b.htm#15003" CLASS="XRef">See : Ambiguous strengths from switch networks</A>, replacing the<CODE CLASS="code"> </CODE><B CLASS="Keyword">pulldown</B> in the lower configuration with a <B CLASS="Keyword">supply0</B> would change the range of the result to the range (<CODE CLASS="code">StX</CODE>?? 快捷鍵說明
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