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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> 7.15 Gate and net delays </TITLE></HEAD><BODY BGCOLOR="#ffffff"><DIV><HR><P><A HREF="ch07.htm">Chapter start</A> <A HREF="ch07.e.htm">Previous page</A></P></DIV><H1 CLASS="Section"><A NAME="pgfId=1653"> </A>7.15 <A NAME="56851"> </A><A NAME="33083"> </A>Gate and<A NAME="marker=580"> </A> <A NAME="marker=581"> </A><A NAME="marker=582"> </A><A NAME="marker=583"> </A>net d<A NAME="marker=584"> </A>elays </H1><P CLASS="Body"><A NAME="pgfId=1654"> </A>Gate and net delays provide a means of more accurately describing delays through a circuit. The <I CLASS="Emphasis">gate delays</I> specify the signal propagation delay from any gate input to the gate output. Up to three values per output representing rise, fall, and turn-off delays can be specified (see <A HREF="ch07.2.htm#56809" CLASS="XRef">See And, nand, nor, or, xor, and xnor gates</A> - <A HREF="ch07.8.htm#gate descriptions end" CLASS="XRef">See Pullup and pulldown sources</A>).</P><P CLASS="Body"><A NAME="pgfId=1655"> </A><I CLASS="Emphasis">Net delays</I> refer to the time it takes from any driver on the net changing value to the time when the net value is updated and propagated further. Up to three delay values per net can be specified.</P><P CLASS="Body"><A NAME="pgfId=1656"> </A>For both gates and nets, the <I CLASS="Emphasis">default delay</I> shall be zero when no delay specification is given. When <A NAME="marker=593"> </A>one delay value is given, then this value shall be used for all <A NAME="marker=594"> </A><A NAME="marker=595"> </A>propagation delays associated with the gate or the net. When <A NAME="marker=591"> </A>two delays are given, the first shall specify the <A NAME="marker=592"> </A><A NAME="marker=596"> </A>rise delay and the second delay shall specify the <A NAME="marker=597"> </A><A NAME="marker=598"> </A>fall delay. The delay when the signal changes to <A NAME="marker=599"> </A>high impedance or to <A NAME="marker=600"> </A>unknown shall be the lesser of the two delay values.</P><P CLASS="Body"><A NAME="pgfId=1050"> </A>For a <A NAME="marker=604"> </A>three delay specification:</P><UL><LI CLASS="DashedList"><A NAME="pgfId=1657"> </A>the first delay refers to the transition to the 1 value (rise delay)</LI><LI CLASS="DashedList"><A NAME="pgfId=1658"> </A>the second delay refers to the transition to the 0 value (fall delay)</LI><LI CLASS="DashedList"><A NAME="pgfId=1659"> </A>the third delay refers to the transition to the high impedance value</LI></UL><P CLASS="Body"><A NAME="pgfId=1660"> </A>When a value changes to the <A NAME="marker=605"> </A>unknown (<CODE CLASS="code">x</CODE>) value, the delay is the smallest of the three delays. The strength of the input signal shall not affect the propagation delay from an input to an output.</P><P CLASS="Body"><A NAME="pgfId=1661"> </A><A HREF="ch07.f.htm#44752" CLASS="XRef">See : Rules for propagation delays</A> summarizes the from-to propagation delay choice for the two and three delay specifications.</P><TABLE><CAPTION><P CLASS="TableTitle"><A NAME="pgfId=1254"> </A>Table 7-10<A NAME="44752"> </A>: Rules for propagation delays</P></CAPTION><TR><TH ROWSPAN="1" COLSPAN="1"><P CLASS="CellHeading"><A NAME="pgfId=1258"> </A></P></TH><TH ROWSPAN="1" COLSPAN="1"><P CLASS="CellHeading"><A NAME="pgfId=1249"> </A></P></TH><TH ROWSPAN="1" COLSPAN="2"><P CLASS="CellHeading"><A NAME="pgfId=1255"> </A><EM CLASS="-">delay used if there are:</EM></P></TH></TR><TR><TH ROWSPAN="1" COLSPAN="1"><P CLASS="CellHeading"><A NAME="pgfId=1262"> </A><EM CLASS="-">from value</EM>:</P></TH><TH ROWSPAN="1" COLSPAN="1"><P CLASS="CellHeading"><A NAME="pgfId=1257"> </A><EM CLASS="-">to value</EM>: </P></TH><TH ROWSPAN="1" COLSPAN="1"><P CLASS="CellHeading"><A NAME="pgfId=1259"> </A><EM CLASS="-">2 delays</EM></P></TH><TH ROWSPAN="1" COLSPAN="1"><P CLASS="CellHeading"><A NAME="pgfId=1260"> </A><EM CLASS="-">3 delays</EM></P></TH></TR><TR><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1266"> </A>0</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1261"> </A>1</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1263"> </A>d1</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1264"> </A>d1</P></TD></TR><TR><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1270"> </A>0</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1265"> </A>x</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1267"> </A>min(d1, d2)</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1268"> </A>min(d1, d2, d3)</P></TD></TR><TR><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1274"> </A>0</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1269"> </A>z</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1271"> </A>min(d1, d2)</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1272"> </A>d3</P></TD></TR><TR><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1278"> </A>1</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1273"> </A>0</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1275"> </A>d2</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1276"> </A>d2</P></TD></TR><TR><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1282"> </A>1</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1277"> </A>x</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1279"> </A>min(d1, d2)</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1280"> </A>min(d1, d2, d3)</P></TD></TR><TR><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1286"> </A>1</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1281"> </A>z</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1283"> </A>min(d1, d2)</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1284"> </A>d3</P></TD></TR><TR><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1290"> </A>x</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1285"> </A>0</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1287"> </A>d2</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1288"> </A>d2</P></TD></TR><TR><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1294"> </A>x</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1289"> </A>1</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1291"> </A>d1</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1292"> </A>d1</P></TD></TR><TR><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1298"> </A>x</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody"><A NAME="pgfId=1293"> </A>z</P></TD><TD ROWSPAN="1" COLSPAN="1"><P CLASS="CellBody">?? 快捷鍵說明
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