ch13.6.htm

介绍asci设计的一本书

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Propagation delay</SPAN>
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Area</SPAN>
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Performance</SPAN>
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From input<A HREF="#pgfId=119045" CLASS="footnote">
2</A>
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To output</SPAN>
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Extrinsic</SPAN>
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nspF  <SUP CLASS="Superscript">
&#8211;1</SUP>
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Intrinsic </SPAN>
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ns</P>
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Extrinsic </SPAN>
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ns</P>
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Intrinsic </SPAN>
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ns</P>
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D0\</P>
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Z\</P>
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2.10</P>
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1.42</P>
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0.5</P>
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0.8</P>
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D0/</P>
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Z/</P>
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3.66</P>
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1.23</P>
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0.68</P>
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0.70</P>
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D1\</P>
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Z\</P>
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2.10</P>
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1.42</P>
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0.50</P>
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0.80</P>
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D1/</P>
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Z/</P>
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3.66</P>
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1.23</P>
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0.68</P>
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0.70</P>
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SD\</P>
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Z\</P>
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2.10</P>
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1.42</P>
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0.50</P>
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0.80</P>
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SD\</P>
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Z/</P>
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3.66</P>
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1.09</P>
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0.70</P>
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0.73</P>
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SD/</P>
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Z\</P>
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2.10</P>
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2.09</P>
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0.5</P>
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1.09</P>
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SD/</P>
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Z/</P>
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3.66</P>
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1.23</P>
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0.68</P>
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0.70</P>
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Both intrinsic delay and extrinsic delay values are given in <A HREF="CH13.6.htm#11038" CLASS="XRef">
Table&nbsp;13.8</A>
. For example, the delay t<SUB CLASS="SubscriptVariable">
PD</SUB>
 (from <SPAN CLASS="BodyComputer">
DO\ </SPAN>
to <SPAN CLASS="BodyComputer">
Z</SPAN>
\) of a 2:1 MUX from the small library is  </P>
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<SPAN CLASS="EquationVariables">
t</SPAN>
<SUB CLASS="SubscriptVariable">
PD</SUB>
 = 1.42 ns + (2.10 ns/pF) <SPAN CLASS="Symbol">
&#165;</SPAN>
 <SPAN CLASS="EquationVariables">
C</SPAN>
<SUB CLASS="SubscriptVariable">
L</SUB>
 (pF) <SPAN CLASS="Symbol">
.</SPAN>
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(13.9)</P>
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ASIC cell libraries may be characterized and the delay information presented in several ways in a data book. Some manufacturers simulate under worst-case slow conditions (4.5 V, 100 &#176;C, and slow process conditions, for example) and then derate each delay value to convert delays to nominal conditions (5.0 V, 25 &#176;C, and nominal process). This allows nominal delays to be used in the data book while maintaining accurate predictions for worst-case behavior. Other manufacturers characterize using nominal conditions and include worst-case values in the data book. In either case, we always design with worst-case values. Data books normally include process, voltage, and temperature derating factors as tables or graphs such as those shown in Tables <A HREF="CH13.6.htm#16609" CLASS="XRef">
13.9</A>
 and <A HREF="CH13.6.htm#31997" CLASS="XRef">
13.10</A>
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For example, suppose we are measuring the performance of an ASIC on the bench and the lab temperature (25  &#176;C) and the power supply voltage (5  V) correspond to nominal operating conditions. We shall assume, in the absence of other information, that we have an ASIC from a nominal process lot. We have data book values given as worst case (worst-case temperature, 100  &#176;C; worst-case voltage, 4.5  V; slow process) and we wish to find nominal values for delay to compare them with our measured results. From <A HREF="CH13.6.htm#16609" CLASS="XRef">
Table&nbsp;13.9</A>
 the derating factor from nominal process to slow process is 1.31. From <A HREF="CH13.6.htm#31997" CLASS="XRef">
Table&nbsp;13.10</A>
 the derating factor from 100  &#176;C and 4.5  V to nominal (25  &#176;C and 5  V) is 1.60. The derating factor from nominal to worst-case (data book values) is thus:  </P>
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worst-case = nominal <SPAN CLASS="Symbol">
&#165;</SPAN>
 1.31 (slow process) <SPAN CLASS="Symbol">
&#165;</SPAN>
 1.60 (4.5  V, 100  &#176;C).</P>
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(13.10)</P>
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worst-case = 	nominal <SPAN CLASS="Symbol">
&#165;</SPAN>
 1.31 (slow process) <SPAN CLASS="Symbol">
&#165;</SPAN>
 1.60 (4.5  V, 100  &#176;C).(13.8)</P>
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To get from the data book values to nominal operating conditions we use the following equation:  </P>
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nominal =  worst-case/(1.31 <SPAN CLASS="Symbol">
&#165;  </SPAN>
1.60)  =  0.477 <SPAN CLASS="Symbol">
&#165;</SPAN>
 worst-case.</P>
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(13.11)</P>
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	nominal =  worst-case/(1.31 <SPAN CLASS="Symbol">
&#165;  </SPAN>
1.60)  =  0.477 <SPAN CLASS="Symbol">
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 worst-case. (13.9)</P>
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TABLE&nbsp;13.9&nbsp;<A NAME="16609">
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Process derating factors.</P>
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TABLE&nbsp;13.10&nbsp;<A NAME="31997">
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Temperature and voltage derating factors.</P>
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Process </SPAN>
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Derating factor</SPAN>
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Supply voltage</SPAN>
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Slow</P>
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