Partial scan paths improve the testability of digital circuits, and incur minimal costs
in the area overhead and test application time. Design constraints may require that
a partial scan path include only those flip-flops that provide the greatest
improvements in circuit testability. STAFFS, a tool that identifies such flip-flops,
has been developed. It uses a statistical testability measure to acquire quantitative
data for the controllabilities and observabilities of the nodes of a circuit. It predicts
the changes that would occur in the data due to the scanning of specific flip-flops,
and uses those predictions to select flip-flops. STAFFS weights the observability
data versus the controllability data when selecting flip-flops, and it can efficiently
select alternative scan designs for different weights. Experimental results for
thirteen sequential benchmark circuits reveal that STAFFS consistently selects scan
designs with fault coverages that are significantly higher than those of arbitrarily
selected scan designs.
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