Things don’t always work as intended. Some devices are manufactured incorrectly,
others break or wear out after extensive use. In order to determine if a device was
manufactured correctly, or if it continues to function as intended, it must be tested.
The test is an evaluation based on a set of requirements. Depending on the complexity of the product, the test may be a mere perusal of the product to determine whether it suits one’s personal whims, or it could be a long, exhaustive checkout of a complex system to ensure compliance with many performance and safety criteria.
Emphasis may be on speed of performance, accuracy, or reliability.
Consider the automobile. One purchaser may be concerned simply with color and
styling, another may be concerned with how fast the automobile accelerates, yet
another may be concerned solely with reliability records. The automobile manufacturer must be concerned with two kinds of test. First, the design itself must be tested for factors such as performance, reliability, and serviceability. Second, individual units must be tested to ensure that they comply with design specifications.
Testing will be considered within the context of digital logic. The focus will be on technical issues, but it is important not to lose sight of the economic aspects of the problem. Both the cost of developing tests and the cost of applying tests to individual units will be considered. In some cases it becomes necessary to make trade-offs. For example, some algorithms for testing memories are easy to create; a computer program to generate test vectors can be written in less than 12 hours. However, the set of test vectors thus created may require several millenia to apply to an actual device.
Such a test is of no practical value. It becomes necessary to invest more effort into initially creating a test in order to reduce the cost of applying it to individual units.
This chapter begins with a discussion of quality. Once we reach an agreement on
the meaning of quality, as it relates to digital products, we shift our attention to the subject of testing. The test will first be defined in a broad, generic sense. Then we put the subject of digital logic testing into perspective by briefly examining the overall design process. Problems related to the testing of digital components and assemblies can be better appreciated when viewed within the context of the overall design process. Within this process we note design stages where testing is required.
We then look at design aids that have evolved over the years for designing and
testing digital devices. Finally, we examine the economics of testing.