The run length based coding schemes have been very effective for the test data compression in case of current generation SoCs with a large number of IP cores. The first part of paper presents a survey of the run length based codes. The data compression of any partially specified test data depends upon how the unspecified bits are filled with 1s and 0s. In the second part of the paper, the five different approaches for “don't care” bit filling based on nature of runs are proposed to predict the maximum compression based on entropy. Here the various run length based schemes are compared with maximum data compression limit based on entropy bounds. The actual compressions claimed by the authors are also compared. For various ISCAS circuits, it has been shown that when the X filling is done considering runs of zeros followed by one as well as runs of ones followed by zero (i.e., Extended FDR), it provides the maximum data compression. In third part, it has been shown that the average test power and peak power is minimum when the don't care bits are filled to make the long runs of 0s as well as 1s. 1. Introduction As a result of the emergence of new fabrication technologies and design complexities, standard stuck-at scan tests are no longer sufficient. The number of tests, corresponding to data volume and test time, increases with each new fabrication process technology just to maintain test quality requirements. Conventional external testing involves storing all test vectors and test response on ATE. But these testers have limited speed, memory, and I/O channels. Testing cannot proceed any faster than the amount of time required to transfer the test data: ？Test time ≥ (amount of test data on tester)/(number of tester channels × tester clock rate) . As a result, some companies are looking for compression well beyond 100X tester cycle reduction [2–4]. The paper is organized as follows. Section 2 describes the test data compression techniques and the qualities of a good technique. Section 3 presents existing run-length-based codes. Section 4 introduces the different methods of do not care bit filling for run-length-based code. Section 5 introduces entropy. Sections 6 and 7 present the experimental results of test data compression and test power with different methods of X filling. Section 8 compares the actual data compression for various methods claimed in literature with maximum possible compression predicted on the basis of entropy. Section 9 analyzes the nature of test data on the basis of various experimental results. Finally conclusions and future work
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