Enhancing encoding capacity of combinational test stimulus decompressors

While scan-based compression is widely utilized in order to alleviate the test time and data volume problems, the overall compression level is dictated not only by the chain to channel ratio but also the ratio of encodable patterns. Aggressively increasing the number of scan chains in an effort to raise the compression levels may reduce the ratio of encodable patterns, degrading the overall compression level. In this paper, we present various methods to improve the ratio of encodable patterns. These methods are based on manipulating the care bit distribution of an unencodable pattern, thereby rendering it compliant with the correlation induced by the decompressor, and thus converting it into an encodable pattern. The proposed transformations, which are simple yet effective, target improvements over fanout and XOR decompressors, while they can be utilized to enhance other types of decompressors, such as multiplexer-based ones; simple nature of these transformations help preserve the simplicity benefits of combinational decompressors. Care bit manipulation is effected in the form of selective chain delay, selective slice rotate/invert, or both. By developing computationally efficient algorithms and cost-effective hardware blocks for these manipulation methods, we show that the encodability, and thus the compression levels, of stimulus decompressors can be significantly improved through the practical and design flow compatible solution that we propose.