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<HTML><HEAD> <META NAME="GENERATOR" CONTENT="Adobe PageMill 2.0 Mac"> <LINK REL="STYLESHEET" HREF="ch03.css"> <TITLE> 3.7 Net types </TITLE></HEAD><BODY BGCOLOR="#ffffff"><P><A NAME="pgfId=366"></A><HR ALIGN=LEFT></P><P><A HREF="ch03.htm">Chapter start</A> <A HREF="ch03.6.htm">Previous page</A> <A HREF="ch03.8.htm">Next page</A></P><H1>3.7 Net types</H1><P><P CLASS="Body"><A NAME="pgfId=577"></A>There are several distinct typesof nets as shown in Table 3-1.</P><P><TABLE BORDER="1" CELLSPACING="2" CELLPADDING="0"><CAPTION ALIGN="TOP"><P CLASS="TableTitle"><A NAME="pgfId=417"></A>Table 3-1: Net Types</CAPTION><TR><TD><P CLASS="CellBody"><A NAME="pgfId=465"></A><B>wire</B></TD><TD><P CLASS="CellBody"><A NAME="pgfId=466"></A><B>tri</B></TD><TD><P CLASS="CellBody"><A NAME="pgfId=481"></A><B>tri0</B></TD><TD><P CLASS="CellBody"><A NAME="pgfId=482"></A><B>supply0</B></TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=484"></A><B>wand</B></TD><TD><P CLASS="CellBody"><A NAME="pgfId=539"></A><B>triand</B></TD><TD><P CLASS="CellBody"><A NAME="pgfId=540"></A><B>tri1</B></TD><TD><P CLASS="CellBody"><A NAME="pgfId=541"></A><B>supply1</B></TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=542"></A><B>wor</B></TD><TD><P CLASS="CellBody"><A NAME="pgfId=557"></A><B>trior</B></TD><TD><P CLASS="CellBody"><A NAME="pgfId=558"></A><B>trireg</B></TD><TD><P CLASS="CellBody"><A NAME="pgfId=562"></A> </TD></TR></TABLE><P CLASS="SubSection"><A NAME="pgfId=600"></A>Wire and tri nets</P><P><P CLASS="Body"><A NAME="pgfId=485"></A>The <I>wire</I> and <I>tri</I>nets connect elements. The net types wire and tri shall be identical intheir syntax and functions; two names are provided so that the name of anet can indicate the purpose of the net in that model. A wire net can beused for nets that are driven by a single gate or continuous assignment.The tri net type can be used where multiple drivers drive a net.</P><P><P CLASS="Body"><A NAME="pgfId=486"></A>Logical conflicts from multiplesources on a wire or a tri net result in unknown values unless the net iscontrolled by logic strength.</P><P><P CLASS="Body"><A NAME="pgfId=394"></A>Table 3-2 is a truth tablefor wire and tri nets. Note that it assumes equal strengths for both drivers.Please refer to 7.10 for a discussion of logic strength modeling.</P><P><TABLE BORDER="1" CELLSPACING="2" CELLPADDING="0"><CAPTION ALIGN="TOP"><P CLASS="TableTitle"><A NAME="pgfId=395"></A>Table 3-2: Truth tablefor wire and tri nets</CAPTION><TR><TH><P CLASS="CellHeading"><A NAME="pgfId=459"></A><B>wire/<BR>tri</B></TH><TH><P CLASS="CellHeading"><A NAME="pgfId=467"></A>0</TH><TH><P CLASS="CellHeading"><A NAME="pgfId=468"></A>1</TH><TH><P CLASS="CellHeading"><A NAME="pgfId=469"></A>x</TH><TH><P CLASS="CellHeading"><A NAME="pgfId=470"></A>z</TH></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=477"></A>0</TD><TD><P CLASS="CellBody"><A NAME="pgfId=480"></A>0</TD><TD><P CLASS="CellBody"><A NAME="pgfId=483"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=487"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=488"></A>0</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=489"></A>1</TD><TD><P CLASS="CellBody"><A NAME="pgfId=490"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=491"></A>1</TD><TD><P CLASS="CellBody"><A NAME="pgfId=494"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=495"></A>1</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=497"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=528"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=530"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=533"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=537"></A>x</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=548"></A>z</TD><TD><P CLASS="CellBody"><A NAME="pgfId=553"></A>0</TD><TD><P CLASS="CellBody"><A NAME="pgfId=572"></A>1</TD><TD><P CLASS="CellBody"><A NAME="pgfId=581"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=587"></A>z</TD></TR></TABLE><P CLASS="SubSection"><A NAME="pgfId=589"></A>Wired nets</P><P><P CLASS="Body"><A NAME="pgfId=492"></A>Wired nets are of type <I>wor</I>, <I>wand</I> , <I>trior</I> , and <I>triand</I> , and are used to modelwired logic configurations. Wired nets resolve the conflicts that resultwhen multiple drivers drive the same net. The wor and trior nets shall create<I>wired or</I> configurations, such that when any of the drivers is <CODE>1</CODE>, the resulting value of the net is <CODE>1</CODE> . The wand and triandnets shall create <I>wired and</I> configurations, such that if any driveris <CODE>0</CODE> , the value of the net is <CODE>0</CODE> .</P><P><P CLASS="Body"><A NAME="pgfId=493"></A>The net types wor and trior shallbe identical in their syntax and functionality. The net types wand and triandshall be identical in their syntax and functionality. Table 3-3 andTable 3-4 gives the truth tables for wired nets. Note that it assumesequal strengths for both drivers. See 7.10 for a discussion of logic strengthmodeling.</P><P><TABLE BORDER="1" CELLSPACING="2" CELLPADDING="0"><CAPTION ALIGN="TOP"><P CLASS="TableTitle"><A NAME="pgfId=599"></A>Table 3-3: Truth tablesfor wand and triand nets</CAPTION><TR><TH><P CLASS="CellHeading"><A NAME="pgfId=368"></A><B>wand/<BR>triand</B></TH><TH><P CLASS="CellHeading"><A NAME="pgfId=389"></A>0</TH><TH><P CLASS="CellHeading"><A NAME="pgfId=390"></A>1</TH><TH><P CLASS="CellHeading"><A NAME="pgfId=391"></A>x</TH><TH><P CLASS="CellHeading"><A NAME="pgfId=392"></A>z</TH></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=393"></A>0</TD><TD><P CLASS="CellBody"><A NAME="pgfId=654"></A>0</TD><TD><P CLASS="CellBody"><A NAME="pgfId=655"></A>0</TD><TD><P CLASS="CellBody"><A NAME="pgfId=656"></A>0</TD><TD><P CLASS="CellBody"><A NAME="pgfId=657"></A>0</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=658"></A>1</TD><TD><P CLASS="CellBody"><A NAME="pgfId=659"></A>0</TD><TD><P CLASS="CellBody"><A NAME="pgfId=660"></A>1</TD><TD><P CLASS="CellBody"><A NAME="pgfId=661"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=662"></A>1</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=663"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=664"></A>0</TD><TD><P CLASS="CellBody"><A NAME="pgfId=665"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=666"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=667"></A>x</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=668"></A>z</TD><TD><P CLASS="CellBody"><A NAME="pgfId=669"></A>0</TD><TD><P CLASS="CellBody"><A NAME="pgfId=670"></A>1</TD><TD><P CLASS="CellBody"><A NAME="pgfId=671"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=672"></A>z</TD></TR></TABLE> <TABLE BORDER="1" CELLSPACING="2" CELLPADDING="0"><CAPTION ALIGN="TOP"><P CLASS="TableTitle"><A NAME="pgfId=605"></A>Table 3-4: Truth tablesfor wor and trior nets</CAPTION><TR><TH><P CLASS="CellHeading"><A NAME="pgfId=673"></A><B>wor/<BR>trior</B></TH><TH><P CLASS="CellHeading"><A NAME="pgfId=674"></A>0</TH><TH><P CLASS="CellHeading"><A NAME="pgfId=675"></A>1</TH><TH><P CLASS="CellHeading"><A NAME="pgfId=676"></A>x</TH><TH><P CLASS="CellHeading"><A NAME="pgfId=677"></A>z</TH></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=678"></A>0</TD><TD><P CLASS="CellBody"><A NAME="pgfId=679"></A>0</TD><TD><P CLASS="CellBody"><A NAME="pgfId=680"></A>1</TD><TD><P CLASS="CellBody"><A NAME="pgfId=681"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=682"></A>0</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=683"></A>1</TD><TD><P CLASS="CellBody"><A NAME="pgfId=684"></A>1</TD><TD><P CLASS="CellBody"><A NAME="pgfId=685"></A>1</TD><TD><P CLASS="CellBody"><A NAME="pgfId=686"></A>1</TD><TD><P CLASS="CellBody"><A NAME="pgfId=687"></A>1</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=688"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=689"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=690"></A>1</TD><TD><P CLASS="CellBody"><A NAME="pgfId=691"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=692"></A>x</TD></TR><TR><TD><P CLASS="CellBody"><A NAME="pgfId=693"></A>z</TD><TD><P CLASS="CellBody"><A NAME="pgfId=694"></A>0</TD><TD><P CLASS="CellBody"><A NAME="pgfId=695"></A>1</TD><TD><P CLASS="CellBody"><A NAME="pgfId=696"></A>x</TD><TD><P CLASS="CellBody"><A NAME="pgfId=697"></A>z</TD></TR></TABLE><P CLASS="SubSection"><A NAME="pgfId=496"></A>Trireg net</P><P><P CLASS="Body"><A NAME="pgfId=498"></A>The <I>trireg</I> net storesa value and is used to model charge storage nodes. A trireg net can be inone of two states:</P><P><P CLASS="Type"><A NAME="pgfId=499"></A><I>driven state</I> Whenat least one driver of a trireg net has a value of <CODE>1</CODE> , <CODE>0</CODE>, or <CODE>x</CODE> , the resolved value propagates into the trireg netand is the trireg net's driven value.</P><P><P CLASS="Type"><A NAME="pgfId=500"></A><I>capacitive state</I> Whenall the drivers of a trireg net are at the high impedance value (<CODE>z</CODE> ), the trireg net retains its last driven value; the high impedancevalue does not propagate from the driver to the trireg.</P><P><P CLASS="Body"><A NAME="pgfId=501"></A>The strength of the value onthe trireg net in the capacitive state can be <B>small</B>, <B>medium</B>,or <B>large</B>, depending on the size specified in the declaration of thetrireg net. The strength of a trireg net in the driven state can be <B>supply</B><CODE>,</CODE><B>strong</B>,<CODE> </CODE><B>pull</B>, or <B>weak</B> dependingon the strength of the driver.</P><P><P CLASS="Body"><A NAME="pgfId=502"></A>Example:</P><P><P CLASS="Body"><A NAME="pgfId=432"></A>Figure 3-1 shows a schematicthat includes a trireg net whose size is <B>medium</B>, its driver, andthe simulation results.</P><P><P CLASS="Body"><A NAME="pgfId=503"></A> </P><P><IMG SRC="ch03-1.gif" WIDTH="376" HEIGHT="160" NATURALSIZEFLAG="3" ALIGN="BOTTOM"> <P CLASS="FigCapBody"><A NAME="pgfId=433"></A>Figure 3-1:Simulation values of a trireg and its driver</P><OL> <P><P CLASS="NumberedLista"><A NAME="pgfId=474"></A>a) At simulation time 0, wire <CODE>a</CODE> and wire <CODE>b</CODE> have a value of <CODE>1</CODE> . A value of <CODE>1</CODE> with a <CODE>strong</CODE> strength propagates from the <B>and</B> gate through the <B>nmos</B> switches connected to each other by wire <CODE>c</CODE> , into trireg net <CODE>d</CODE> . <P><P CLASS="NumberedListb"><A NAME="pgfId=507"></A>b) At simulation time 10, wire <CODE>a</CODE> changes value to <CODE>0</CODE>?? 快捷鍵說明
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