|Testing is one of the final stages in the production cycle
and is of fundamental importance to assure that customers will get working
units. No product can be manufactured without an efficient test strategy,
guaranteeing that the percentage of defective products passing undetected
is so low as to be acceptable (this threshold level is not the same for
all products / manufacturers / customers, but its range is normally
measured in parts per million). The definition of the test strategy has
indeed to start early in the design stage, since any decisions capable of
simplifying the production test phase will normally lead to lower cost and
more reliable products.
Being such an important stage in terms of influencing customer satisfaction, test has over the years taken a very prominent role in the overall design and test cycle. We will therefore start by considering the basic test concepts, which have to be clearly understood in order to perceive the importance of the test infrastructures defined in the IEEE 1149.1 and 1149.4 standards. Our roadmap in this route starts with the concepts of fault models and controllability / observability issues, in order to prepare the reader for the elementary procedures leading to automatic test vector generation, and finally to testability improvement alternatives made possible by ad hoc and structured test methods.