X-canceling MISR — An X-tolerant methodology for compacting output responses with unknowns using a MISR

A new X-tolerant multiple-input signature register (MISR) compaction methodology is proposed which can compact output streams containing unknown (X) values. Unlike conventional X-masking approaches, it does not require any masking logic at the input of the MISR. Instead it uses symbolic simulation to express each bit of the MISR signature as a linear equation in terms of the X´s. Linearly dependent combinations of the signature bits are identified with Gaussian elimination and XORed together using a programmable XOR to cancel out all X values thereby yielding deterministic values that are invariant of what the final values of the X´s end up being during the test. These X-canceled values can be compacted in a separate MISR to generate a final X-free signature. Each intermediate signature for an m-bit MISR can tolerate k X´s present anywhere in the output stream with error detection capability equivalent to using an m-k bit MISR with no unknowns. The tester storage requirement is a small constant times the total number of unknowns in the test set and thus does not depend on the scan architecture, the number of test vectors, or the distribution of X´s which is a key advantage compared with other X-tolerant compaction schemes.