Improving test compression with scan feedforward techniques

Scan feedforward techniques are proposed for improving test compression for both sequential linear decompressors based on solving linear equations and for broadcast scan based decompressors in which the decompressor constraints are included in the automatic test pattern generation (ATPG) backtrace. The conventional approach is to first pre-load a sequential linear decompressor with free variables from the tester for some number of clock cycles before beginning to shift the scan chain. A scan feedforward scheme is proposed here which can shorten or eliminate the need for pre-loading the decompressor. More importantly, even if the pre-load phase is not changed, the proposed scheme significantly expands the free-variable dependence of each scan cell, which increases the probability of encoding a test cube. This permits more faults to be targeted by each test cube generated during ATPG (i.e., improved dynamic compaction) thereby leading to fewer test cubes for the same fault coverage. For broadcast scan based schemes, a methodology is presented for using both feedforward and feedback in the scan chains to increase encoding flexibility. The proposed scheme is effective even when narrow TAMs (even 1-bit TAMs) are feeding the decompressor. There is no need to pre-load pipeline flip-flops as is conventionally required in existing solutions thereby reducing test time and improving compression. Experimental results are presented demonstrating the effectiveness of the proposed schemes.