Test Sequence Length Requirements for Scan-based Testing An Efficient Test Set Embedding Scheme with Reduced Test-Data Storage and Test Sequence Length Requirements for Scan-based Testing D. Kaseridis1, E. Kalligeros1, X. Kavousianos2 & D. Nikolos1 2Dept. of Computer Science University of Ioannina, 45110, Ioannina, Greece 1Dept. of Computer Engineering & Informatics University of Patras, 26500, Patras, Greece e-mails: kaserid@ceid.upatras.gr, kalliger@ceid.upatras.gr, kabousia@cs.uoi.gr, nikolosd@cti.gr I. Test set Embedding Core-oriented way of designing contemporary SoCs leads to larger and denser circuits that require greater test data volumes and longer test-application times The introduction of new, embedded testing solutions that overcome these problems is of great importance. Test set embedding techniques that combine both reduced hardware and test-data storage requirements with short test sequences are desirable II. Seed Selection Algorithm We consider the classical LFSR-based reseeding scheme: LFSR, Bit and Vector Counter The algorithm receives as input the size L of the window (number of test-vectors) that each seed expands to and a set of test cubes T For determining a new seed the seed-algorithm makes uses of the well-known concept of solving systems of linear equations (i.e. assuming Gauss-Jordan elimination) The algorithm examines all possible linear systems and chooses one to solve Table1. Seed-selection algorithm's criteria Criterion Description 1st Select the solvable systems that corresponds to the test cubes containing the maximum number of defined bits 2nd If there are more than one solvable systems selected by the 1st criterion, choose the solvable system that its solution leads to the replacement of the fewest variables ai in the L-vector window 3rd If there are more than one solvable systems selected by the 2nd criterion, select the solvable system that is nearest to the first vector of the window Since at each step of algorithm, linear systems corresponding to more than one test cubes will be solvable at more than one positions of the window, a set of heuristics is used (Table 1) for selecting the system that will be actually solved III. Test-sequence reduction scheme Seed-selection algorithm assumes a window of L successive test vectors for each selected seed. (Fig. 1) If the last vector of a window is not a useful one then all vectors from the last useful one to the last vector of each window are redundant Rearrangement technique Main idea: Order the seeds according to the number of useful segments If these volumes for every two successive windows differs at most by one  Only a single extra bit per seed is needed for indicating this relation. Extra bit=0  Same number of useful segment Extra bit=1  One segment difference Example of rearrangement technique Fig. 1. A window of L stages Proposed window segmentation approach Each window is segmented into a number (m) of equal-sized groups of test vectors (Fig. 2) The useful vectors of the window are included in the first k segments and thus the remaining m-k segments contain redundant test vectors and can be dropped during test generation Load Counter: Down counter that maintains the necessary number of segments for each window Bit Counter: controls the scan-in operation of each vector's bits Segment-Vectors Counter: controls the generation of the test vectors of a single segment Segment Counter: counts for the required number of segments for each window and is initialized for each seed with the value of Load Counter Fig. 2. The proposed window segmentation technique Fig. 3. The proposed test-sequence reduction scheme IV. Comparisons The proposed approach compares favorably against the most recent and efficient test set embedding technique in the literature (Reconfigurable Interconnection Networks–TCAD’04) The comparison shows that the proposed scheme requires substantially smaller test sequences (Fig. 4) and hardware overhead (Fig. 5) for both 32 () and 64 () scan chains Fig. 4. Test sequence length reductions Fig. 5. Hardware overhead reductions