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Design for Testability
45
To observe an input of the AND gate at
d
requires setting the other input to 1;
therefore, the combinational and sequential observability measures for both inputs
a
and
b
are:
CO
±a²
=
CO
±d²
+
CC1
±b²
+
1
SO
±a²
=
SO
±d²
+
SC1
±b²
CO
±b²
=
CO
±d²
+
CC1
±a²
+
1
SO
±b²
=
SO
±d²
+
SC1
±a²
It is important to note that controllability and observability measures calculated
using SCOAP are heuristics and only approximate the actual testability of a logic
circuit. When scan design is used, testability analysis can assume that all scan
cells are directly controllable and observable. It was also shown in [Agrawal 1982]
that SCOAP may overestimate testability measures for circuits containing many
reconvergent fanouts; however, by being able to perform testability analysis in
O
(
n
) computational complexity for
n
signals in a circuit, SCOAP provides a quick
estimate of the circuit’s testability that can be used to guide testability enhancement
and test generation.
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This note was uploaded on 05/16/2011 for the course ENGINEERIN mp108 taught by Professor Elbarki during the Spring '08 term at Alexandria University.
 Spring '08
 elbarki
 Gate

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