Fault isolation using tests for non-isolated blocks

Summary form only given. Design methodologies for large designs produce circuits that consist of interconnections of functional blocks. If the blocks are large, as in core-based designs, they may be isolated for testing purposes (e.g., by test wrappers) such that different blocks can be tested independently. However, even if a test wrapper exists, it is advantageous to test functional paths that go through two or more blocks by using test vectors that propagate fault effects through several blocks. This contributes to testing of defects that cannot be detected if each block is tested separately. One of the issues that arises when several blocks are tested by the same test is that of fault isolation. If a test that propagates fault effects through blocks C₁ and C₂ produces a faulty response on the outputs of C₂, the goal of fault isolation is to identify which one of C₁ and C₂ is faulty. Fault isolation is perfect if every faulty response on the outputs of the circuit can be uniquely attributed to a single block. This happens when every pair of faults belonging to different blocks is distinguishable. If faults of different blocks remain undistinguished, fault isolation is not possible when responses equal to the responses produced by these faults are produced by the circuit-under-test. It may appear that tests for several non-isolated blocks will not be able to isolate faults. In this work, we study this issue and demonstrate that perfect or close-to-perfect fault isolation is possible with tests that propagate fault effects through several blocks