Built In Test for VLSI: Pseudorandom Techniques

8

Built-in Test Structures

There is no one best built-in test structure. Built-in test is a collection of possibilities, the choice of which depends upon the application. Factors to consider include fault coverage required, the system overhead which is tolerable, the system performance and the performance impact of the built-in test technique, and the socket or test time which is allowable.

This chapter presents several different implementation approaches which have been suggested or used in the past, and discusses the hazards and rewards of each one. It concentrates almost exclusively upon random pattern or probabilistic built-in test structures. The chapter is divided into three parts: The first describes structures which build upon the concept of scan path; the second covers special topics in structures; and the third discusses structures which are constructed from off-the-shelf components.

8.1 SCAN-PATH STRUCTURES

Scan path refers to a disciplined design standard for all storage elements (other than memory arrays) which has the express purpose of making the stored values easy to control and easy to observe. With this facility the storage element becomes in effect both a primary input and primary output. Test input signals can be introduced or test results observed wherever one ...