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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="ch09.css"><TITLE> 9.3 Procedural continuous assignments </TITLE></HEAD><BODY BGCOLOR="#ffffff"><DIV><HR><P><A HREF="ch09.htm">Chapter start</A> <A HREF="ch09.2.htm">Previous page</A> <A HREF="ch09.4.htm">Next page</A></P></DIV><H1 CLASS="Section"><A NAME="pgfId=500"> </A>9.3 <A NAME="43159"> </A>Procedural <A NAME="marker=340"> </A>continuous assignmen<A NAME="marker=341"> </A>ts </H1><P CLASS="Body"><A NAME="pgfId=501"> </A>The <I CLASS="Emphasis">procedural continuous assignments</I><A NAME="marker=342"> </A> (using keywords <B CLASS="Keyword">assign</B> and <B CLASS="Keyword">force</B>) are procedural statements that allow expressions to be driven continuously onto registers or nets. The <A NAME="marker=343"> </A><A NAME="marker=344"> </A><A NAME="marker=346"> </A><A NAME="marker=347"> </A><A NAME="marker=348"> </A><A NAME="marker=349"> </A>syntax for these statements follows:</P><P CLASS="Body"><A NAME="pgfId=693"> </A></P><DIV><IMG SRC="ch09-11.gif"></DIV><P CLASS="BNFCapBody"><A NAME="pgfId=695"> </A>Syntax 9-3: Syntax for procedural continuous assignments</P><P CLASS="Body"><A NAME="pgfId=701"> </A>The left-hand side of the assignment in the <I CLASS="Emphasis">assign statement</I> shall be a register reference or a concatenation of registers. It shall not be a memory element (array reference) or a bit-select or a part-select of a register.</P><P CLASS="Body"> </A>In contrast, the left-hand side of the assignment in the <I CLASS="Emphasis">force</I> <I CLASS="Emphasis">statement</I> can be a register reference or a net reference. It can be a concatenation of any of the above. Bit-selects and part-selects of vector registers or vector nets are not allowed.</P><P CLASS="SubSection"><A NAME="pgfId=713"> </A>The assign and deassign procedural statements</P><P CLASS="Body"><A NAME="pgfId=714"> </A><A NAME="marker=352"> </A><A NAME="marker=353"> </A><A NAME="marker=354"> </A><A NAME="marker=355"> </A>The <I CLASS="Emphasis">assign</I> procedural continuous assignment statement shall override all procedural assignments to a register. The <I CLASS="Emphasis">deassign</I> procedural statement shall end a procedural continuous assignment to a register. The value of the register shall remain the same until the register is assigned a new value through a procedural assignment or a procedural continuous assignment. The assign and deassign procedural statements allow, for example, <A NAME="marker=356"> </A>modeling of asynchronous clear/preset on a D-type edge-triggered flip-flop, where the clock is inhibited when the clear or preset is active. </P><P CLASS="Body"><A NAME="pgfId=483"> </A>If the keyword <B CLASS="Keyword">assign</B> is applied to a register for which there is already a procedural continuous assignment, then this new procedural continuous assignment shall deassign the register before making the new procedural continuous assignment.</P><DIV><H2 CLASS="Example"><A NAME="pgfId=716"> </A></H2><P CLASS="Body"><A NAME="pgfId=482"> </A>The example below shows a use of the assign and deassign procedural statements in a behavioral description of a D-type flip-flop with preset and clear inputs.</P><P CLASS="Body"><A NAME="pgfId=720"> </A></P><DIV><IMG SRC="ch09-12.gif"></DIV><P CLASS="Body"><A NAME="pgfId=738"> </A>If either <CODE CLASS="code">clear</CODE> or <CODE CLASS="code">preset</CODE> is low, then the output <CODE CLASS="code">q</CODE> will be held continuously to the appropriate constant value and a positive edge on the <CODE CLASS="code">clock</CODE> will not affect <CODE CLASS="code">q</CODE>. When both the <CODE CLASS="code">clear</CODE> and <CODE CLASS="code">preset</CODE> are high, then <CODE CLASS="code">q</CODE> is deassigned. <A NAME="marker=359"> </A><A NAME="marker=360"> </A><A NAME="marker=361"> </A><A NAME="marker=362"> </A></P><P CLASS="SubSection"><A NAME="pgfId=679"> </A><A NAME="22168"> </A><A NAME="marker=333"> </A>The force<A NAME="marker=334"> </A> and release<A NAME="marker=335"> </A> procedural statements</P><P CLASS="Body"><A NAME="pgfId=684"> </A>Another form of procedural continuous assignment is provided by the <I CLASS="Emphasis">force</I> and <I CLASS="Emphasis">release</I> procedural statements. These statements have a similar effect to the assign-deassign pair, but a force can be applied to nets as well as to registers. The left-hand side of the assignment can be a register, a net, a constant bit select of a vector net, a part select of a vector net, or a concatenation. It cannot be a memory element (array reference) or a bit-select or a part-select of a vector register.</P><P CLASS="Body"><A NAME="pgfId=762"> </A>A <I CLASS="Emphasis">force</I><A NAME="marker=364"> </A><A NAME="marker=365"> </A> statement to a register shall override a procedural assignment or procedural continuous assignment that takes place on the register until a release procedural statement is executed on the register. After the <I CLASS="Emphasis">release</I> procedural statement is executed, the register shall not immediately change value (as would a net that is forced). The value specified in the force statement shall be maintained in the register until the next procedural assignment takes place, except in the case where a procedural continuous assignment is active on the register.</P><P CLASS="Body"><A NAME="pgfId=773"> </A>A force procedural statement on a net overrides all drivers of the net--gate outputs, module outputs, and continuous assignments--until a release procedural statement is executed on the net.</P><P CLASS="Body"><A NAME="pgfId=780"> </A><A NAME="marker=363"> </A>Releasing a register that currently has an active procedural continuous assignment shall re-establish that assignment. </P></DIV><DIV><H2 CLASS="Example"><A NAME="pgfId=789"> </A></H2><P CLASS="Body"><A NAME="pgfId=798"> </A><EM CLASS="DocSetOnly"></EM></P><DIV><IMG SRC="ch09-13.gif"></DIV><P CLASS="Body"><A NAME="pgfId=807"> </A><EM CLASS="DocSetOnly">In the example above, an </EM><B CLASS="Keyword">and</B><EM CLASS="DocSetOnly"> gate instance </EM><CODE CLASS="code">and1</CODE><EM CLASS="DocSetOnly"> is "patched" as an </EM><B CLASS="Keyword">or</B><EM CLASS="DocSetOnly"> gate b</EM>y a force proce<EM CLASS="DocSetOnly">dural statement that forces its output to the value of its or-ed inputs, and an </EM>assign<EM CLASS="DocSetOnly"> procedural statement of and-ed values is "patc</EM>hed" as an assign procedural statement of or-ed values.</P><P CLASS="Body"><A NAME="pgfId=815"> </A>The right hand side of a procedural continuous assignment or a force statement can be an expression; this shall be treated just as a continuous assignment, that is if any variable on right hand side of the assignment changes, the assignment shall be re-evaluated while the assign or force is in effect. For example,</P><PRE CLASS="CodeIndent"><A NAME="pgfId=816"> </A><B CLASS="Keyword">force</B> a = b + f(c) ;</PRE><P CLASS="Body"><A NAME="pgfId=817"> </A>Here, if <CODE CLASS="code">b</CODE> changes or <CODE CLASS="code">c</CODE> changes, <CODE CLASS="code12">a</CODE> will be forced to the new value of the expression <CODE CLASS="code">b+f(c)</CODE>.</P></DIV><HR><P><A HREF="ch09.htm">Chapter start</A> <A HREF="ch09.2.htm">Previous page</A> <A HREF="ch09.4.htm">Next page</A></P></BODY></HTML>?? 快捷鍵說明
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