ch13.6.htm

介绍asci设计的一本书

  +  0.65  <SPAN CLASS="Symbol">
&#165; </SPAN>
4.6642)  <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
L</SUB>
</P>
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&nbsp;</P>
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<A NAME="pgfId=132511">
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=  0.053  +  2.749<SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
L</SUB>
 .</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableEqnNumber">
<A NAME="pgfId=132513">
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<A NAME="22987">
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(13.21)</P>
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</TABLE>
<P CLASS="Body">
<A NAME="pgfId=69818">
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	We can now compare Eq.&nbsp;<A HREF="CH13.6.htm#22987" CLASS="XRef">
13.21</A>
 with the prop&#8211;ramp model. The prop&#8211;ramp parameters for this logic cell (from the primitive model in <A HREF="CH13.5.htm#40162" CLASS="XRef">
Section&nbsp;13.5.1</A>
) are:</P>
<P CLASS="ComputerOneLine">
<A NAME="pgfId=69676">
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tA1D_fr = |( Rec prop = 0.078;  ramp = 2.749;  End);</P>
<P CLASS="Body">
<A NAME="pgfId=69668">
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These parameters predict the following prop&#8211;ramp delay (0.35/0.65 trip points):  </P>
<TABLE>
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<P CLASS="EquationAlign">
<A NAME="pgfId=132554">
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<SPAN CLASS="EquationVariables">
D</SPAN>
<SUB CLASS="SubscriptVariable">
PR</SUB>
    (65  %)  =  0.078  +  2.749<SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
L</SUB>
 .</P>
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<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableEqnNumber">
<A NAME="pgfId=132556">
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(13.22)</P>
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</TABLE>
<P CLASS="BodyAfterHead">
<A NAME="pgfId=90700">
 </A>
The input-slope delay model and the prop&#8211;ramp delay model predict similar delays in the fast-ramp region, but for slower inputs the differences can become significant.</P>
</DIV>
<DIV>
<H2 CLASS="Heading2">
<A NAME="pgfId=90702">
 </A>
13.6.3&nbsp;<A NAME="12306">
 </A>
Limitations of Logic Simulation</H2>
<P CLASS="BodyAfterHead">
<A NAME="pgfId=13512">
 </A>
<A HREF="CH13.6.htm#11892" CLASS="XRef">
Table&nbsp;13.11</A>
 shows the switching characteristics of a two-input NAND gate (1X drive) from a commercial 1  <SPAN CLASS="Symbol">
m</SPAN>
m gate-array family. The difference in propagation delay (with FO  =  0) between the inputs A and B is </P>
<P CLASS="Equation">
<A NAME="pgfId=13514">
 </A>
(0.25  &#8211;  0.17)  <SPAN CLASS="Symbol">
&#165;</SPAN>
  2  /  (0.25  +  0.17)  =  38  %.</P>
<P CLASS="BodyAfterHead">
<A NAME="pgfId=13516">
 </A>
This difference is taken into account only by a pin-to-pin delay model.</P>
<P CLASS="Body">
<A NAME="pgfId=119703">
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&nbsp;</P>
<TABLE>
<TR>
<TD ROWSPAN="1" COLSPAN="8">
<P CLASS="TableTitle">
<A NAME="pgfId=119708">
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TABLE&nbsp;13.11&nbsp;<A NAME="11892">
 </A>
Switching characteristics of a two-input NAND gate.</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=119724">
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&nbsp;</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=119726">
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&nbsp;</P>
</TD>
<TD ROWSPAN="1" COLSPAN="5">
<P CLASS="TableFirst">
<A NAME="pgfId=119731">
 </A>
Fanout<A HREF="#pgfId=119730" CLASS="footnote">
3</A>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=119741">
 </A>
&nbsp;</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=119743">
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<SPAN CLASS="TableHeads">
Symbol</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=119745">
 </A>
<SPAN CLASS="TableHeads">
Parameter</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=119747">
 </A>
<SPAN CLASS="TableHeads">
FO = 0</SPAN>
</P>
<P CLASS="TableFirst">
<A NAME="pgfId=119748">
 </A>
/ns</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=119750">
 </A>
<SPAN CLASS="TableHeads">
FO = 1</SPAN>
</P>
<P CLASS="TableFirst">
<A NAME="pgfId=119751">
 </A>
/ns</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=119753">
 </A>
<SPAN CLASS="TableHeads">
FO = 2</SPAN>
</P>
<P CLASS="TableFirst">
<A NAME="pgfId=119754">
 </A>
/ns</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=119756">
 </A>
<SPAN CLASS="TableHeads">
FO = 4</SPAN>
</P>
<P CLASS="TableFirst">
<A NAME="pgfId=119757">
 </A>
/ns</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=119759">
 </A>
<SPAN CLASS="TableHeads">
FO = 8</SPAN>
</P>
<P CLASS="TableFirst">
<A NAME="pgfId=119760">
 </A>
/ns</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=119762">
 </A>
<SPAN CLASS="TableHeads">
K</SPAN>
</P>
<P CLASS="TableFirst">
<A NAME="pgfId=119763">
 </A>
/nspF<SUP CLASS="Superscript">
&#8211;1</SUP>
</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119765">
 </A>
<SPAN CLASS="EquationVariables">
t</SPAN>
<SUB CLASS="Subscript">
PLH</SUB>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableLeft">
<A NAME="pgfId=119767">
 </A>
Propagation delay, A to X</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119769">
 </A>
0.25</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119771">
 </A>
0.35</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119773">
 </A>
0.45</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119775">
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0.65</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119777">
 </A>
1.05</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119779">
 </A>
1.25</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119781">
 </A>
<SPAN CLASS="EquationVariables">
t</SPAN>
<SUB CLASS="Subscript">
PHL</SUB>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableLeft">
<A NAME="pgfId=119783">
 </A>
Propagation delay, B to X</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119785">
 </A>
0.17</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119787">
 </A>
0.24</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119789">
 </A>
0.30</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119791">
 </A>
0.42</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119793">
 </A>
0.68</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119795">
 </A>
0.79</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119797">
 </A>
<SPAN CLASS="EquationVariables">
t</SPAN>
<SUB CLASS="Subscript">
r</SUB>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableLeft">
<A NAME="pgfId=119799">
 </A>
Output rise time, X</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119801">
 </A>
1.01</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119803">
 </A>
1.28</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119805">
 </A>
1.56</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119807">
 </A>
2.10</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119809">
 </A>
3.19</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119811">
 </A>
3.40</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119813">
 </A>
<SPAN CLASS="EquationVariables">
t</SPAN>
<SUB CLASS="Subscript">
f</SUB>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableLeft">
<A NAME="pgfId=119815">
 </A>
Output fall time, X</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119817">
 </A>
0.54</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119819">
 </A>
0.69</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119821">
 </A>
0.84</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119823">
 </A>
1.13</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119825">
 </A>
1.71</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=119827">
 </A>
1.83</P>
</TD>
</TR>
</TABLE>
<P CLASS="Body">
<A NAME="pgfId=69914">
 </A>
Timing information for most gate-level simulators is calculated once, before simulation, using a delay calculator. This works as long as the logic cell delays and signal ramps do not change. There are some cases in which this is not true. <A HREF="CH13.6.htm#35261" CLASS="XRef">
Table&nbsp;13.12</A>
 shows the switching characteristics of a half adder. In addition to pin-to-pin timing differences there is a timing difference depending on state. For example, the pin-to-pin timing from input pin A to the output pin S depends on the state of the input pin B. Depending on whether B = '0' or B = '1' the difference in propagation delay (at FO  =  0) is </P>
<P CLASS="Equation">
<A NAME="pgfId=13533">
 </A>
(0.93  &#8211;  0.58)  <SPAN CLASS="Symbol">
&#165;</SPAN>
  2  /  (0.93  +  0.58)  =  46  %.</P>
<P CLASS="BodyAfterHead">
<A NAME="pgfId=13535">
 </A>
This <SPAN CLASS="Definition">
state-dependent timing</SPAN>
<A NAME="marker=81689">
 </A>
 is not taken into account by simple pin-to-pin delay models and is not accounted for by most gate-level simulators.</P>
<TABLE>
<TR>
<TD ROWSPAN="1" COLSPAN="8">
<P CLASS="TableTitle">
<A NAME="pgfId=30262">
 </A>
TABLE&nbsp;13.12&nbsp;<A NAME="35261">
 </A>
Switching characteristics of a half adder. </P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=32781">
 </A>
&nbsp;</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=32783">
 </A>
&nbsp;</P>
</TD>
<TD ROWSPAN="1" COLSPAN="5">
<P CLASS="TableFirst">
<A NAME="pgfId=32785">
 </A>
Fanout<SPAN CLASS="TableHeads">
<A HREF="#pgfId=32801" CLASS="footnote">
4</A>
</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=32795">
 </A>
&nbsp;</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=30279">
 </A>
<SPAN CLASS="TableHeads">
Symbol</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=30281">
 </A>
<SPAN CLASS="TableHeads">
Parameter</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=30286">
 </A>
<SPAN CLASS="TableHeads">
FO = 0</SPAN>
</P>
<P CLASS="TableFirst">
<A NAME="pgfId=30427">
 </A>
/ns</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=30288">
 </A>
<SPAN CLASS="TableHeads">
FO = 1</SPAN>
</P>
<P CLASS="TableFirst">
<A NAME="pgfId=30426">
 </A>
/ns</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=30290">
 </A>
<SPAN CLASS="TableHeads">
FO = 2</SPAN>
</P>
<P CLASS="TableFirst">
<A NAME="pgfId=30432">
 </A>
/ns</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=30292">
 </A>
<SPAN CLASS="TableHeads">
FO = 4</SPAN>
</P>
<P CLASS="TableFirst">
<A NAME="pgfId=30437">
 </A>
/ns</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=30294">
 </A>
<SPAN CLASS="TableHeads">
FO = 8</SPAN>
</P>
<P CLASS="TableFirst">
<A NAME="pgfId=30442">
 </A>
/ns</P>
</TD>
<TD ROWSPAN="1" COLSPAN="