Testing Embedded Cores Using Partial Isolation Rings

Intellectual property cores pose a significant test challenge. The core supplier may not give any information about the internal logic of the core, but simply provide a set of test vectors for the core which guarantees a particular fault coverage. If the core is embedded within a larger design, then the problem is how to apply the specified test vectors to the core and how to test the userdefined logic around the core. A simple and fast solution is to place a full isolation ring (i.e., boundary scan) around the core, however, the area and performance overhead for this may not be acceptable in many applications. This paper presents a systematic method for designing a partial isolation ring that provides the same fault coverage as a full isolation ring, but avoids adding MUXes on critical timing paths and reduces area overhead. Efficient ATPG techniques are used to analyze the user-defined logic surrounding the core and identify a maximal set of core inputs and outputs (that includes the critical timing paths) that do not need to be included in the partial isolation ring. Several different partial isolation ring selection strategies that vary in computational complexity are described. Experimental results are shown comparing the different strategies.