On don't cares in test compression

Both test compression tools and ATPGs directly producing compressed test greatly benefit from don't care values present in the test. Actually, presence of these don't cares is essential for success of the compression. Contemporary ATPGs produce tests having more than 97% of don't cares for large industrial circuits, thus high compression ratios can be expected. However, these don't cares are placed in the test in an “uninformed” way. There are many possibilities of constructing a complete test for a circuit, while the ATPG chooses just one particular, without respect to the subsequent compression process. Therefore, the don't cares cannot be fully exploited. In this paper we show how severe this issue is. A novel ATPG algorithm directly producing compressed test patterns for the RESPIN decompression architecture is presented. Test don't cares are placed in an informed way, so that they are maximally exploited by compression. We compare the results with several ways of uninformed don't care generation to show the benefits of the proposed method. Results for the ISCAS and ITC'99 benchmark circuits are shown and compared to state-of-the-art test compression techniques.