On Common-Mode Skewed-Load and Broadside Tests

This paper studies the effect of the type of scan-based delay fault tests used for a circuit on the ability to diagnose delay defects by studying its effect on diagnosis of transition faults. The authors consider enhanced scan tests, skewed-load tests, broadside tests, functional broadside tests, and a combination of skewed-load and broadside tests. The results indicate that while functional broadside tests should be used for fault detection to avoid overtesting, the test set should be extended for fault diagnosis by adding other types of tests. Adding a small number of skewed-load tests is especially useful for diagnosis if enhanced scan is not available

The use of both broadside (launch-on-capture) and skewed-load (launch-on-shift) tests for delay faults results in increased delay fault coverage and better test compaction than the use of a single test type. Two-cycle broadside and skewed-load tests differ in the sequence of length two applied to the scan-enable input between the scan-in and scan-out operations of a test. Considering a circuit with a single clock domain, the question that this article attempts to answer is whether it is possible to generate a complete test set for transition faults where all the tests use the same scan-enable sequence of length three or more. The use of a single scan-enable sequence simplifies the test application process. Experimental results demonstrate that there is a significant number of benchmark circuits for which a test set with a single scan-enable sequence achieves the same transition fault coverage as a test set that consists of both broadside and skewed-load tests. For other benchmark circuits, a small loss in transition fault coverage compared with the use of both test types allows a single scan-enable sequence to be used.

Skewed-load and broadside tests complement each other and allow higher delay fault coverage to be achieved for a standard-scan circuit that supports both types of tests. The difference between the two types of tests is mainly in the test application process. The input test data required for both of them are similar. This similarity is used in this work to compute compact input test data that can be used as a basis for forming both types of tests. It results in improved fault coverage compared to the use of broadside (or skewed-load) tests alone, and in reduced test data volume compared to the case where broadside and skewed-load tests are stored separately. Experimental results are presented using a procedure that accepts a test set of any type, and computes input test data suitable for the application of both types of tests. The procedure modifies the test data so as to compact it as well as increase the fault coverage. The procedure is applied to a broadside test set and to mixed test sets that consist of both types of tests.

Tester limitations may impose certain constraints on the primary input vectors applicable as part of a two-pattern test for delay faults. Under these constraints, the primary input vectors may be held constant, or the second primary input vector of a test may be obtained by a single shift of a scan chain relative to the first. The goal of this brief is to study the differences in achievable transition fault coverage between various primary input constraints that are similar to the commonly used ones of holding or shifting primary input vectors. This brief also studies the possibility of combining the constraints in order to increase the transition fault coverage. The combination requires a fixed and circuit-independent hardware structure similar to the case where shifting of primary input vectors is used. This study is done using test sets that consist of both broadside and skewed-load tests in order to maximize the transition fault coverage.

The use of multiple scan chains was shown to improve the coverage of transition faults achieved by skewed-load tests. For broadside tests, the number of scan chains does not affect the transition fault coverage. We describe an enhanced broadside configuration under which increasing the number of scan chains helps increase the fault coverage. In the enhanced configuration, the first flip-flop of a scan chain operates in skewed-load mode while the other flip-flops operate in broadside mode. This provides flexibility in determining the value of the first flip-flop of every scan chain under the second pattern of a broadside test, thus increasing the transition fault coverage. We also describe a procedure that makes small modifications to a given scan chain configuration in order to improve the transition fault coverage.

SAT-based generation of compressed skewed-load tests for transition delay faults

Test generation procedures target a variety of fault models in order to produce test sets that are effective for defect detection. This paper considers the likely scenario where two-cycle skewed-load tests are generated to detect single transition faults, and the test set is complemented with tests for single stuck-at faults that are not detected by the transition fault test set. For this scenario, this paper makes several unique observations that can be utilized to produce a single compact test set that consists only of two-cycle skewed-load tests for both fault models. The first observation is that a single-cycle test for a stuck-at fault can be transformed into a skewed-load test that is guaranteed to detect the stuck-at fault without performing logic or fault simulation. The second observation is that skewed-load tests, which are transformed from single-cycle tests for stuck-at faults, sometimes detect more transition and stuck-at faults than tests that were generated for transition faults. The third observation is that a static test compaction procedure, which is based on the modification and removal of tests, is effective in this context because it allows tests for stuck-at faults to detect more transition faults and vice versa. This paper describes a test compaction procedure based on these observations and presents experimental results for benchmark circuits to demonstrate the effectiveness of the procedure.

A test for a delay fault can be considered as covering a transition on one or more lines. A bias in the transition coverage of a delay test set implies that more rising or more falling transitions are covered by the test set. Such a bias is not captured by fault coverage metrics that consider both types of transitions together. We study the bias in the transition coverage of test sets for path delay faults. The results demonstrate that the bias is circuit-dependent. It also depends on the type of two-pattern tests used. In general, broadside tests show more bias than skewed-load tests, while enhanced-scan tests show little bias. We also consider the use of partial-enhanced-scan for reducing the bias exhibited by broadside tests.

Most previous DFT-based techniques for low-capture-power broadside testing can only reduce test power in one of the two capture cycles, launch cycle and capture cycle. Even if some methods can reduce both of them, they may make some testable faults in standard broadside testing untestable. In this paper, a new test application scheme called partial launch-on-capture (PLOC) is proposed to solve the two problems. It allows only a part of scan flip-flops to be active in the launch cycle and capture cycle. In order to guarantee that all testable faults in the standard broadside testing can be detected in the new test scheme, extra efforts are required to check the overlapping part. In addition, calculation of the overlapping part is different from previous techniques for the stuck-at fault testing because broadside testing requires two consecutive capture cycles. Therefore, a new scan flip-flop partition algorithm is proposed to minimize the overlapping part. Sufficient experimental results are presented to demonstrate the efficiency of the proposed method.

Approximations of functional broadside tests maintain close-to-functional operation conditions in order to avoid overtesting of delay faults. Approximations were used earlier for increasing the fault coverage of functional broadside tests. This paper describes the first procedure that uses an approximation, referred to as piecewise-functional broadside tests, to achieve test compaction. Piecewise-functional broadside tests are characterized by a parameter p. A lower value of p indicates a closer proximity to functional operation conditions. The test compaction procedure combines a pair of tests, t i and t j , to form a test, t i,j , that can replace t i and t j without reducing the fault coverage, and thus reduce the number of tests. With t i and t j having parameter values p i and p j , respectively, the combined test t i,j has parameter value p i,j ≤ p i + p j . The procedure increases the bound p on p i,j gradually as it compacts the test set. Experimental results for benchmark circuits show that significant levels of test compaction can be achieved with p = 2. In addition, the procedure converges for a small value of p.

Recent research results, have shown that the traditional structural testing for, delay and crosstalk faults may result in over-testing due to the non-trivial number of such faults that are untestable in the functional mode while testable in the test mode. This paper presents a pseudo-functional test methodology that attempts to minimize the over-testing problem of the scan-based circuits for the delay faults. The first pattern of a two-pattern test is still delivered by scan in the test mode but the pattern is generated in such a way that it does not violate the functional constraints extracted from the functional logic. In this paper, we use a SAT solver to extract a set of functional constraints which consists of illegal states and internal signal correlation. Along with the functional justification (also called broad-side) test application scheme, the functional constraints are imposed to a commercial delay-fault ATPG tool to generate pseudo-functional delay tests. The experimental results indicate that the percentage of untestable delay faults is non-trivial for many circuits which support the hypothesis of the over-testing problem in delay testing. The results also indicate the effectiveness of the proposed constraint extraction method.

We present a test generation procedure for transition faults that minimizes the detection of functionally redundant transition faults in scan circuits. The procedure uses broadside testing. We also propose rules for identifying dominance relations between functionally redundant transition faults and functionally detectable transition faults. Dominance relations can provide two types of lower bounds. (1) A lower bound on the number of functionally detectable transition faults that cannot be detected without detecting any functionally redundant transition faults. (2) A lower bound on the number of functionally redundant faults that have to be detected if all the functionally detectable faults are detected. In our experiments with ISCAS-89 and ITC-99 benchmark circuits we achieve both of the lower bounds for almost all the circuits considered.

Testing for delay and CMOS stuck-open faults requires two pattern tests and test sets are usually large. Built-in self-test (BIST) schemes are attractive for such comprehensive testing. The BIST test pattern generators (TPGs) for such testing should be designed to ensure high pattern-pair coverage. In this paper, necessary and sufficient conditions to ensure complete/maximal pattern-pair coverage for linear feedback shift register (LFSR) and cellular automata (CA) have been derived. The theory developed here identifies all LFSR/CA TPGs which maximize pattern-pair coverage under any given TPG size constraints. It is shown that LFSRs with primitive feedback polynomials with large number of terms are better for two-pattern testing. Also, CA are shown to be better TPGs than LFSRs for two-pattern testing. Results derived in this paper provide practical algorithms for the design of optimal TPGs for two-pattern testing. Experiments on some benchmark circuits indicate the TPGs designed using the procedures outlined in this paper provide much higher delay fault coverage than other TPGs. >

In enhanced-scan circuits, a two-pattern test < t i , t j > for a transition fault can be obtained by using a test t j that detects a stuck-at fault, and preceding it by a test t i that activates another stuck-at fault. Thus, test generation for transition faults can be done by combining pairs of stuck-at tests. This provides an alternative to deterministic test generation, as well as reduces the test storage requirements for transition fault tests. We study the possibility of generating scan-based tests for transition faults in standard-scan circuits in a similar way, by combining pairs of stuck-at tests. Since it is not always possible to obtain a standard-scan test that is equivalent to a two-pattern test < t i , t j > based on stuck-at tests t i and t j , it is not always possible to guarantee that the combination of t i and t j will detect a transition fault. To compensate for this, it is necessary to try combinations of different stuck-at test pairs, resulting in an increased simulation effort to compute effective standard-scan tests. Our focus in this work is on reducing this simulation effort by reducing the number of stuck-at test pairs that need to be considered.

Many X-Filling strategies are proposed to reduce test power during scan based testing. Because their main motivation is to reduce the switching activities of test patterns in the test process, some of them are prone to reduce the test ability of test patterns, which may lead to low defect coverage. In this paper, we propose a segment based X-filling(SBF) technique to reduce test power using multiple scan chains, with minimal impact on defect coverage. Different from the previous filling methods, our X-filling technique is segment based and defect coverage aware. The method can be easily incorporated into traditional ATPG flow to keep capture power below a certain limit and keep the defect coverage at a high level.