Overview

This chapter deals with how to design digital integrated circuits so they can be tested. As digital integrated circuits continually get larger, it is essential that IC designers seriously consider how an IC is to be tested, both during the prototype design and debugging phase, and during production. These considerations must start from the time of the first conception of the overall system design. Designing an IC so that it is testable is a nontrivial task, but it is one that the designer must succeed at or all other efforts are for naught. It is this author's opinion that almost every digital IC should include circuitry whose sole function is to help with the on-chip testing. Readers should not assume that because this chapter is last in the book the importance of testing is minimal; rather, for digital ICs intended for realizing real systems, testing may be one of the most important considerations.

A reason why modern ICs, and even more so complete systems, are difficult and sometimes impossible to test is twofold: first, it is difficult to access internal nodes during testing; second, as circuits become larger and more complicated, the number of test vectors required to exercise all aspects of a digital circuit with a high probability of not missing any faults is growing exponentially. This results in large testing times, incomplete testing, and ultimately loss of profit. The only solution is the typical engineering solution to every problem resulting from complexity: "divide and conquer."

Through...