ch14.7.htm

介绍asci设计的一本书

 <SPAN CLASS="Symbol">
&#8853;</SPAN>
 Q2<SUB CLASS="Subscript">
t</SUB>
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Q1<SUB CLASS="Subscript">
t+1</SUB>
 = Q0<SUB CLASS="Subscript">
t</SUB>
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Q2<SUB CLASS="Subscript">
t+1</SUB>
 = Q1<SUB CLASS="Subscript">
t</SUB>
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<P CLASS="TableFirst">
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Z = </P>
<P CLASS="TableFirst">
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Q0'.Q1 + Q1.Q2</P>
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R0<SUB CLASS="Subscript">
t+1</SUB>
 = </P>
<P CLASS="TableFirst">
<A NAME="pgfId=83642">
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Z<SUB CLASS="Subscript">
t</SUB>
 <SPAN CLASS="Symbol">
&#8853;</SPAN>
 R0<SUB CLASS="Subscript">
t</SUB>
 <SPAN CLASS="Symbol">
&#8853;</SPAN>
 R2<SUB CLASS="Subscript">
t</SUB>
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R1<SUB CLASS="Subscript">
t+1</SUB>
 = R0<SUB CLASS="Subscript">
t</SUB>
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R2<SUB CLASS="Subscript">
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1</P>
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0</P>
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0</P>
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1</P>
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0</P>
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1</P>
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1</P>
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1</P>
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1</P>
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1</P>
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1</P>
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1</P>
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0</P>
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0</P>
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1</P>
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1</P>
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1</P>
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&nbsp;</P>
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&nbsp;</P>
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&nbsp;</P>
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&nbsp;</P>
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&nbsp;</P>
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<TD ROWSPAN="1" COLSPAN="7">
<P CLASS="TableFigTitleSide">
<A NAME="pgfId=118236">
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FIGURE&nbsp;14.25&nbsp;<A NAME="11447">
 </A>
BIST example. (a)&nbsp;A simple BIST structure showing bit sequences for both good and bad circuits. (b)&nbsp;Bit sequence calculations for the good circuit. The signature appears on the eighth clock cycle (after seven positive clock edges) and is R0 = '0', R1 = '1', R2 = '1'; with R2 as the MSB this is '011' or hex 3.</P>
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<P CLASS="TableFigure">
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(a)</P>
<DIV>
<IMG SRC="CH14-28.gif">
</DIV>
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<P CLASS="TableFigure">
<A NAME="pgfId=83773">
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(b)</P>
<DIV>
<IMG SRC="CH14-29.gif">
</DIV>
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<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFigure">
<A NAME="pgfId=83779">
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(c)</P>
<DIV>
<IMG SRC="CH14-30.gif">
</DIV>
</TD>
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<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFigureTitle">
<A NAME="pgfId=83785">
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FIGURE&nbsp;14.26&nbsp;<A NAME="13228">
 </A>
The waveforms of the BIST circuit of <A HREF="CH14.7.htm#11447" CLASS="XRef">
Figure&nbsp;14.25</A>
. (a)&nbsp;The good-circuit response. The waveforms Q1 and Q2, as well as R1 and R2, are delayed by one clock cycle as they move through each stage of the shift registers. (b)&nbsp;The same good-circuit response with the register outputs Q0&#8211;Q2 and R0&#8211;R2 grouped and their values displayed in hexadecimal (Q0 and R0 are the MSBs). The signature hex 3 or '011' (R0 = 0, R1 = 1, R2 = 1) in R appears seven positive clock edges after the reset signal is taken high. This is one clock cycle after the generator completes its first sequence (hex pattern 4, 2, 5, 6, 7, 3, 1). (b)&nbsp;The response of the bad circuit with fault F1 and fault signature hex 0 (circled).</P>
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</DIV>
<DIV>
<H2 CLASS="Heading2">
<A NAME="pgfId=15178">
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14.7.4&nbsp;<A NAME="16912">
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Aliasing</H2>
<P CLASS="BodyAfterHead">
<A NAME="pgfId=83563">
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In <A HREF="CH14.7.htm#13228" CLASS="XRef">
Figure&nbsp;14.26</A>
 the good and bad circuits produced different signatures. There is a small probability that the signature of a bad circuit will be the same as a good circuit. This problem is known as <A NAME="marker=83562">
 </A>
<SPAN CLASS="Definition">
aliasing</SPAN>
 or <SPAN CLASS="Definition">
error masking</SPAN>
<A NAME="marker=83822">
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. For the example in <A HREF="CH14.7.htm#11447" CLASS="XRef">
Figure&nbsp;14.25</A>
, the bit stream input to the signature analysis register is 7 bits long. There are 2<SUP CLASS="Superscript">
7</SUP>
 or 128 possible 7-bit-long bit-stream patterns. We assume that each of these 128 bit-stream patterns is equally likely to produce any of the eight (all-zeros is an allowed pattern in a signature register) possible 3-bit signatures. It turns out that this is a good assumption. Thus there are 128 / 8 or 16 bit-streams that produce the good signature, one of these belongs to the good circuit, the remaining 15 cause aliasing. Since there are a total of 128 &#8211; 1 = 127 bit-streams due to bad circuits, the fraction of bad-circuit bit-streams that cause aliasing is 15 / 127, or 0.118. If all bad circuit bit-streams are equally likely (and this is a poor assumption) then 0.118 is also the probability of aliasing.</P>
<P CLASS="Body">
<A NAME="pgfId=87022">
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In general, if the length of the test sequence is <SPAN CLASS="EquationVariables">
L</SPAN>
 and the length of the signature register is <SPAN CLASS="EquationVariables">
R</SPAN>
 the probability <SPAN CLASS="EquationVariables">
p</SPAN>
 of aliasing (not detecting an error) is  </P>
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