X-canceling MISR — An X-tolerant methodology for compacting output responses with unknowns using a MISR
Nur A. Touba · 2007
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.