Adjustable width linear combinational scan vector decompression

A new scheme for combinational linear expansion is proposed for decompression of scan vectors. It has the capability to adjust the width of the linear expansion each clock cycle. This eliminates the requirement that every scan bit-slice be in the output space of the linear decompressor. Depending on how specified the current bit-slice is, the decompressor may load all scan chains or may load only a subset of the scan chains. This provides the nice feature that any scan vector can be generated using the proposed scheme regardless of the number or distribution of the specified bits. Thus, the proposed scheme allows the use of any ATPG procedure without any constraints. Moreover, it allows greater compression to be achieved than fixed width expansion techniques since the ratio of the number of scan chains to the number of tester channels can be scaled much larger. A procedure for designing and optimizing the adjustable width decompression hardware and obtaining the compressed data is described. Experimental data indicates that the proposed scheme is simple yet very effective.