Chapter 7 focused on methods for integrating design and test activities by capturing verification suites written by logic designers and converting them to test programs. For some ICs, especially those with reasonably high yield, test programs derived from a thorough design verification suite, combined with an IDDQ test (cf. Chapter 11), may produce quality levels that meet or exceed corporate requirements.
When it is not possible, or practical, to achieve fault coverage that satisfies acceptable quality levels (AQL) through the use of design verification suites, an alternative is to use an automatic test pattern generator (ATPG). Ideally, one would like to reach fault coverage goals merely by pushing a button. That, however, is not consistent with existing state of the art. It was pointed out in Chapter 4 that several ATPG algorithms can, in theory at least, create a test for any fault in combinational logic for which a test exists. In practice, even when a test exists for a large block of combinational logic, such as an array multiplier, the ATPG may fail to generate a test because of the sheer volume of data that must be manipulated.
However, the real stumbling block for ATPG has been sequential logic. Because of the inability of ATPGs to successfully deal with sequential logic, a growing number of digital designs are being designed ...