State Skip LFSRs: Bridging the Gap between Test Data Compression and Test Set Embedding for IP Cores

Abstract

We present a new type of linear feedback shift registers, state skip LFSRs. state skip LFSRs are normal LFSRs with the addition of a small linear circuit, the State Skip circuit, which can be used, instead of the characteristic-polynomial feedback structure, for advancing the state of the LFSR. In such a case, the LFSR performs successive jumps of constant length in its state sequence, since the State Skip circuit omits a predetermined number of states by calculating directly the state after them. By using State Skip LFSRs we get the well- known high compression efficiency of test set embedding with substantially reduced test sequences, since the useless parts of the test sequences are dramatically shortened by traversing them in state skip mode. The length of the shortened test sequences approaches that of test data compression methods. A systematic method for minimizing the test sequences of re- seeding-based test set embedding methods, and a low overhead decompression architecture are also presented.