X-filter: filtering unknowns from compacted test responses

Using off-the-shelf error correcting codes for compacting test response (Patel, 2003) is attractive because it provides multiple error detection and diagnosis support in the presence of multiple unknowns. However, this technique is not easily incorporated in a conventional scan testing environment because handling unknowns requires special ATE support or test response postprocessing to determine if the test passed. In this paper we solve this problem using a novel technique, X-filter, which removes the effect of unknowns on compacted test response. X-filter achieves this by using only O(mx) additional hardware with mx inputs. (x=maximum number of unknowns that can be tolerated, m=number of bits in the compacted response). Unlike compaction methods that require creation of special codes (Mitra, 2004), (Wohl, 2003), X-filter is more flexible in terms of number of errors and unknowns handled, and it does not increase the number of output pins over those used by the error correcting code itself