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
a
=
CO
d
+
CC1
b
+
1
SO
a
=
SO
d
+
SC1
b
CO
b
=
CO
d
+
CC1
a
+
1
SO
b
=
SO
d
+
SC1
a
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.
This is the end of the preview.
Sign up
to
access the rest of the document.
 Spring '08
 elbarki
 Gate, Automatic test pattern generation, SCOAP, testability analysis, testability measures

Click to edit the document details