Sequential test decompressors with fast variable wide spreading

Usually, test pattern decompressors with dynamic reseeding are reset before starting a new test pattern decoding. The first few scan chain slices are then filled with test vectors that have lower decodability as the number of free variables is limited by the test access mechanism bandwidth. We have found that it is possible to increase the number of free variables in the equations describing the care bits encoding by fast creating and wide spreading as many as possible independent linear combinations of the tester bits and using them for the scan chain loading. We propose a decompressor combining a combinational linear decompressor and an LFSR like automaton that effectively distributes the free variables within the test pattern. The proposed test pattern decompressor outperforms the de-codability of other decompressors with similar hardware overhead and it reduces the test time due to a possible reduction of the decompressor preloading.