COCO: Configuration-Based Compaction of a Compressed Topped-Off Test Set

Comprehensive defect coverage requires test sets that detect faults from several fault models. A test set is typically topped-off to detect faults from an additional fault model that are not already detected. This creates large test sets whose last tests detect small numbers of additional faults. Reducing the storage requirements of topped-off test sets (or test sets for fault models with large numbers of faults) is the topic of this article. Instead of storing the last tests in their entirety, it was shown previously that it is possible to produce the last tests of the test set from tests that appear earlier by complementing single bits. The storage requirements are reduced when only complemented bits are stored; however, the number of applied tests is increased. This article observes that changing the configuration by which decompressed test data are shifted into scan chains produces new tests that are effective in replacing tests at the end of a topped-off test set without increasing the number of applied tests. This approach is developed in this article in an academic environment and implemented using academic software tools. It is applied to benchmark circuits to demonstrate its effectiveness.