Scan Chain Routing with Minimal Test Application Time

Abstract

Macro Test requires access to every leaf-macro in the design. Access is required for both the data flow and the control signals enabling the data flow. In general, choices can be made in establishing this access. The choices can be grouped into two categories. Either, use is made of functionally existing paths and control signals, or design entities are added to create the required access. The latter has to be used when the first possibility is not available. Which choice to take depends mainly on the complexity of the initial test plan, the design architecture, the availability of transfer information, and the constraints on number of pins, use of silicon area, performance, and test data volume; see Chapter 3. The set of design entities and device pins involved in creating the access for leaf-macro test data and control is called the leaf-macro access path. The influence of the choice of the initial test plan can best be seen when comparing a leaf-macro tested with Built-in Self-Test or with partial scan. A Built-In Self-Test function only needs to be initialized, executed, and observed. Only a few leaf-macro terminals have to be accessible; see Chapter 4. On the other hand, for a partial scannable leaf-macro, all terminals have to be accessible with a complicated timing protocol added to it. In this comparison, the price of an easy access is a set of local design adaptions for data and control generation for the Built-In Self-Test function.