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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="ch06.css"><TITLE> 6.1 Continuous assignments </TITLE></HEAD><BODY BGCOLOR="#ffffff"><DIV><HR><P><A HREF="ch06.htm">Chapter start</A> <A HREF="ch06.htm">Previous page</A> <A HREF="ch06.2.htm">Next page</A></P></DIV><H1 CLASS="Section"><A NAME="pgfId=116"> </A>6.1 <A NAME="marker=28"> </A>C<A NAME="marker=29"> </A>ontinuous assignments </H1><P CLASS="Body"><A NAME="pgfId=117"> </A>Continuous assignments shall drive values onto nets, both vector and scalar. This assignment shall occur whenever the value of the right-hand side changes. Continuous assignments provide a way to model combinational logic without specifying an interconnection of gates. Instead, the model specifies the logical expression that drives the net. </P><P CLASS="Body"><A NAME="pgfId=118"> </A>The syntax for continuous assignments is as follows:</P><P CLASS="Body"><A NAME="pgfId=119"> </A></P><DIV><IMG SRC="ch06-1.gif"></DIV><DIV><H2 CLASS="BNFCapPage"><A NAME="pgfId=120"> </A>: Syntax for continuous assignment</H2><P CLASS="SubSection"><A NAME="pgfId=122"> </A>The net declaration assignment</P><P CLASS="Body"><A NAME="pgfId=123"> </A>The first two alternatives in the net declaration are discussed in <A HREF="/Humuhumu/Files/Prof_Smith/Academic/ASICs/Web/ASICs/HTML/Verilog/LRM/HTML/06/ch03.htm#86941" CLASS="XRef"></A> (see <A HREF="/Humuhumu/Files/Prof_Smith/Academic/ASICs/Web/ASICs/HTML/Verilog/LRM/HTML/06/ch03.htm#76390" CLASS="XRef"></A>). The third alternative, the net declaration assignment, allows a continuous assignment to be placed on a net in the same statement that declares the net. </P><P CLASS="Body"><A NAME="pgfId=124"> </A>Example:</P><P CLASS="Body"><A NAME="pgfId=125"> </A>The following is an example of the net declaration form of a continuous assignment:</P><PRE CLASS="CodeIndent"><A NAME="pgfId=99"> </A><B CLASS="Keyword">wire</B> (<B CLASS="Keyword">strong1</B>, <B CLASS="Keyword">pull0</B>) mynet = enable ;</PRE><P CLASS="Note"><A NAME="pgfId=100"> </A>NOTE--Because a net can be declared only once, only one net declaration assignment can be made for a particular net. This contrasts with the continuous assignment statement; one net can receive multiple assignments of the continuous assignment form.</P><P CLASS="SubSection"><A NAME="pgfId=127"> </A>The continuous assignment statement</P><P CLASS="Body"><A NAME="pgfId=128"> </A>The continuous assignment statement shall place a <A NAME="marker=43"> </A>continuous assignment on a net that has been previously declared, either explicitly by declaration or <A NAME="marker=44"> </A>implicitly by using its name in the terminal list of a gate, user-defined primitive, or module instance (see <A HREF="/Humuhumu/Files/Prof_Smith/Academic/ASICs/Web/ASICs/HTML/Verilog/LRM/HTML/06/ch03.htm#48381" CLASS="XRef"></A>). </P><P CLASS="Body"><A NAME="pgfId=130"> </A>Assignments on nets shall be continuous and automatic. This means that whenever an operand in the right-hand side expression changes value, the whole right-hand side shall be evaluated and assigned to the left-hand side. </P><P CLASS="Body"><A NAME="pgfId=157"> </A>Examples:</P><P CLASS="Body"><A NAME="pgfId=163"> </A>1. The following is an example of a continuous assignment to a net that has been previously declared:</P><PRE CLASS="CodeIndent"><A NAME="pgfId=164"> </A><B CLASS="Keyword">wire</B><CODE CLASS="code12"> mynet ;</CODE></PRE><PRE CLASS="CodeIndent"><A NAME="pgfId=129"> </A><B CLASS="Keyword">assign</B><CODE CLASS="code12"> (</CODE><B CLASS="Keyword">strong1</B><CODE CLASS="code12">, </CODE><B CLASS="Keyword">pull0</B><CODE CLASS="code12">) mynet = enable ;</CODE></PRE><P CLASS="Body"><A NAME="pgfId=92"> </A>2. The following is an example of the use of a continuous assignment to model a four bit adder with carry. The assignment could not be specified directly in the declaration of the nets because it requires a concatenation on the left-hand side.</P><P CLASS="Body"><A NAME="pgfId=105"> </A></P><DIV><IMG SRC="ch06-2.gif"></DIV><P CLASS="Body"><A NAME="pgfId=134"> </A>3. The following example describes a module with one 16-bit output bus. It selects between one of four input busses and connects the selected bus to the output bus.</P><P CLASS="Body"><A NAME="pgfId=135"> </A></P><DIV><IMG SRC="ch06-3.gif"></DIV><P CLASS="Body"><A NAME="pgfId=137"> </A>The following sequence of events is experienced during simulation of the above example:</P><OL><P CLASS="NumberedLista"><A NAME="pgfId=138"> </A>a) The value of <CODE CLASS="code">s</CODE>, a bus selector input variable, is checked in the assign statement; based on the value of <CODE CLASS="code">s</CODE>, the net <CODE CLASS="code">data</CODE> receives the data from one of the four input busses.</P><P CLASS="NumberedListb"><A NAME="pgfId=139"> </A>b) The setting of <CODE CLASS="code">data</CODE> net triggers the continuous assignment in the net declaration for <CODE CLASS="code">busout</CODE>; if <CODE CLASS="code">enable</CODE> is set, the contents of <CODE CLASS="code">data</CODE> are assigned to <CODE CLASS="code">busout</CODE>; if <CODE CLASS="code">enable</CODE> is <CODE CLASS="code">0</CODE>, the contents of <CODE CLASS="code">Zee</CODE> are assigned to <CODE CLASS="code">busout</CODE>.</P></OL><P CLASS="SubSection"><A NAME="pgfId=141"> </A><A NAME="marker=53"> </A>Delays </P><P CLASS="Body"><A NAME="pgfId=142"> </A>A delay given to a continuous assignment shall specify the time duration between a right-hand side operand value change and the assignment made to the left-hand side. If the left-hand side references a scalar net, then the delay shall be treated in the same way as for gate delays--that is, different delays can be given for the output rising, falling and changing to high impedance (see <A HREF="/Humuhumu/Files/Prof_Smith/Academic/ASICs/Web/ASICs/HTML/Verilog/LRM/HTML/06/ch07.htm#36723" CLASS="XRef"></A>).</P><P CLASS="Body"><A NAME="pgfId=143"> </A>If the left-hand side references a vector net, then up to three delays can be applied. The following rules determine which delay controls the assignment:</P><UL><LI CLASS="DashedList"><A NAME="pgfId=144"> </A>If the right-hand side makes a transition from non-zero to zero, then the falling delay shall be used.</LI><LI CLASS="DashedList"><A NAME="pgfId=145"> </A>If the right-hand side makes a transition to <CODE CLASS="code">z</CODE>, then the turn-off delay shall be used.</LI><LI CLASS="DashedList"><A NAME="pgfId=146"> </A>For all other cases, the rising delay shall be used.</LI></UL><P CLASS="Body"><A NAME="pgfId=147"> </A>Specifying the delay in a continuous assignment that is part of the net declaration shall be treated different from specifying a net delay and then making a continuous assignment to the net. A delay value can be applied to a net in a net declaration, as in the following example:</P><PRE CLASS="CodeIndent"><A NAME="pgfId=148"> </A><B CLASS="Keyword">wire</B> #10 wireA;</PRE><P CLASS="Body"><A NAME="pgfId=149"> </A>This syntax, called a net delay, means that any value change that is to be applied to <CODE CLASS="code">wireA</CODE> by some other statement shall be delayed for ten time units before it takes effect. When there is a continuous assignment in a declaration, the delay is part of the continuous assignment and is not a net delay. Thus, it shall not be added to the delay of other drivers on the net. Furthermore, if the assignment is to a vector net, then the rising and falling delays shall not be applied to the individual bits if the assignment is included in the declaration. </P><P CLASS="Body"><A NAME="pgfId=150"> </A><EM CLASS="-">In situations where a right-hand side operand changes before a previous change has had time to propagate to the left-hand side, then the latest value change</EM> shall be the only one to be applied. That is, only one assignment shall occur<EM CLASS="-">.</EM></P><P CLASS="SubSection"><A NAME="pgfId=153"> </A>S<A NAME="marker=70"> </A>trength </P><P CLASS="Body"><A NAME="pgfId=154"> </A>The driving strength of a continuous assignment can be specified by the user. This applies only to assignments to <A NAME="marker=71"> </A>scalar nets of the types listed below:</P><P CLASS="Body"><A NAME="pgfId=155"> </A><B CLASS="Keyword"> wire tri trireg<BR> wand triand tri0<BR> wor trior tri1</B></P><P CLASS="Body"><A NAME="pgfId=156"> </A>Continuous assignments driving strengths can be specified in either a net declaration or in a stand-alone assignment, using the <B CLASS="Keyword">assign</B> keyword. The strength specification, if provided, shall immediately follow the keyword (either the<EM CLASS="-"> keyword for the</EM> net type or<EM CLASS="-"> </EM><B CLASS="Keyword">assign</B>) and precede any delay specified. Whenever the continuous assignment drives the net, the strength of the value shall be simulated as specified.</P><P CLASS="Body"><A NAME="pgfId=91"> </A>A drive strength specification shall contain one strength value that applies when the value being assigned to the net is <CODE CLASS="code">1</CODE> and a second strength value that applies when the assigned value is <CODE CLASS="code">0</CODE>. The following keywords shall specify the strength value for an assignment of <CODE CLASS="code">1</CODE>:</P><P CLASS="Body"><A NAME="pgfId=158"> </A><B CLASS="Keyword"> supply1 strong1 pull1 weak1 highz1</B><A NAME="marker=72"> </A></P><P CLASS="Body"><A NAME="pgfId=159"> </A>The following keywords shall specify the strength value for an assignment of <CODE CLASS="code">0</CODE>:</P><P CLASS="Body"><A NAME="pgfId=160"> </A><B CLASS="Keyword"> supply0 strong0 pull0 weak0 highz0</B></P><P CLASS="Body"><A NAME="pgfId=161"> </A>The order of the two strength specifications shall be arbitrary. The following two rules shall constrain the use of drive strength specifications:</P><UL><LI CLASS="DashedList"><A NAME="pgfId=162"> </A><EM CLASS="-">The strength specifications (</EM><B CLASS="Keyword">highz1</B><EM CLASS="-">, </EM><B CLASS="Keyword">highz0</B><EM CLASS="-">) and (</EM><B CLASS="Keyword">highz0</B><EM CLASS="-">, </EM><B CLASS="Keyword">highz1</B><EM CLASS="-">) shall be treated as illegal constructs.</EM></LI><LI CLASS="DashedList"><A NAME="pgfId=182"> </A>If drive strength is not specified, it shall default to <CODE CLASS="code">(</CODE><B CLASS="Keyword">strong1</B>, <B CLASS="Keyword">strong0</B><CODE CLASS="code">)</CODE>.</LI></UL></DIV><HR><P><A HREF="ch06.htm">Chapter start</A> <A HREF="ch06.htm">Previous page</A> <A HREF="ch06.2.htm">Next page</A></P></BODY></HTML>?? 快捷鍵說明
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