Constraint extraction for pseudo-functional scan-based delay testing

With the increased densities of integrated circuits, several different types of faults can occur. Faults in digital circuits resulting from random defects can introduce DC (stuck-at) faults as well as AC (delay) faults. Previous work in statistical modeling and analysis for delay fault testing generally assumes that at most a single delay fault can occur along any given path in the circuit under test. In this paper we investigate the statistical effect of multiple delay faults along any path in a circuit under test, and predict the path delay fault probabilities as well as the maximum number of path delay faults for both combinational and sequential benchmark circuits. We begin with the development of a statistical model for path delay faults in VLSI circuits [4], which takes into account multiple delay faults along any signal path.

An efficient delay test generation (DTEST GEN) system for combinational logic circuits is presented. In the DTEST GEN system, delay testing problems are divided into gross delay faults and small delay faults separately so that the tradeoff between the levels of delay testing effort and the confidence levels of proper system operation can be explored. Complete automatic test pattern generation (ATPG) algorithms are proposed for both gross delay faults and small delay faults. A novel timing analysis method for delay test generation which uses a conventional depth-first search technique and a novel functionality analysis technique is introduced. The functionality analysis technique examines, necessary conditions for a given delay fault to be testable and estimates the upper bound of the good circuit propagation delay of the longest sensitizable path passing through the fault site. Several benchmark results are demonstrated for both gross delay fault testing and small delay fault testing. >

The quality of a delay test set hinges not only on test patterns and the distribution of the delay defects but on the variations in process parameters as well. Process variations result in the same delay test set displaying differences from die to die in the detection of particular delay defects at the identical circuit node. The application of an identical test set to all devices independent of process variations consequently results in delivering inefficiencies in test time utilization. This paper proposes a delay test technique that adaptively changes the size of the test set based on the position of the device in the process variation space in order to maximize test quality within a given test time.

Existing delay test generation techniques focus on test generation for combinational blocks, and assume the inputs and outputs of the block to be unconstrained. Test application for delay tests is done by means of enhanced scan, scan shifting or functional justification; all these techniques impose minimal constraints on the inputs and the outputs of the combinational block targeted. This leads to over-testing the components for delay defects. This paper analyzes the gains associated with determining the multi-cycle (sequential) sensitization of delay tests. The advantages of determining multi-cycle sensitization is then illustrated on benchmark designs with and without a delay-specific fault model.

In this paper, we propose a new test generation method for delay faults considering crosstalk-induced delay effects, based on a conventional delay ATPG technique in order to reduce the complexity of previous ATPG algorithm for crosstalk delay faults and to consider multiple aggressor crosstalk faults to maximize the noise of the victim line. Since the proposed ATPG for crosstalk-induced delay faults uses the physical and timing information, the proposed ATPG can reduce the search space of the backward implication of the aggressor's constraints and it is helpful for reducing the time cost of the ATPG than previous works. In addition, since the proposed technique targets on the critical path for the original delay test as the victim lines, it can improve test effectiveness of delay testing. Experimental results demonstrate the effectiveness of the proposed method.

Two-pattern tests for delay faults in standard scan circuits can be of one of two types: skewed-load or broadside. Each type of tests creates different conditions during test application due to the different way in which scan mode and functional mode are interleaved. Therefore, tests that are applicable both as skewed-load tests and as broadside tests are useful for comparing the two types of tests with respect to properties such as defect coverage or overtesting. In this work we investigate the possibility of generating tests that are applicable under both test application schemes. We refer to two-pattern tests that are applicable as both skewed-load and broadside tests as common-mode tests. We show that most benchmark circuits have sufficient numbers of common-mode tests to make them an interesting class of tests. Moreover, we show that the use of multiple scan chains increases the number of common-mode tests.

The author presents a method for generating test sequences to detect delay faults in sequential circuits using the stuck-at-fault sequential test generator STALLION. The method is complete in that it will generate a delay test sequence for a targeted fault given sufficient CPU time, if such a sequence exists. Faults for which no delay test sequence exists are termed sequentially delay redundant. The author describes means of eliminating sequential delay redundancies in logic circuits. He presents a partial-scan methodology for enhancing the testability of difficult-to-test or untestable sequential circuits, wherein a small number of flip-flops are selected and made controllable/observable. The selection process guarantees the elimination of all sequential delay redundancies. It is shown that an intimate relationship exists between state assignment and delay testability of a sequential machine. A state assignment algorithm for the synthesis of sequential machines with maximal delay fault testability is described. Preliminary experimental results using the test generation, partial-scan, and synthesis algorithms are presented. >

We investigate two strategies to guarantee temporal correctness of a combinational circuit. We first propose a new technique to identify and test primitive faults. A primitive fault is a path delay fault that has to be tested to guarantee the performance of the circuit. Primitive faults can consist of single- (SPDF's) or multiple path delay faults (MPDF's). Testing strategies for single primitive faults exist. In this paper, we focus on identifying and testing multiple primitive faults. Identification and testing of these faults is important for at least two reasons: (1) a large percentage of paths in production circuits remain untestable under the SPDF model, and (2) distributed manufacturing defects usually adversely affect more than one path and these defects can be detected only by analyzing multiple affected paths. The SPDF's contained in a multiple primitive fault have to merge at some gate(s). Our methodology can quickly (1) rule out a large number of gates as possible merging gates for primitive faults, and (2) prune the combinations of paths that can never belong to any primitive fault. Our identification procedure also finds a test for the fault. We present a complete algorithm for identifying and testing double path delay faults, Identifying and testing all primitive faults is impractical for large designs. This is because no efficient methods are known for testing primitive faults that include a large number of paths. However, to guarantee that the performance of a digital circuit is not affected by timing defects, it is necessary to test all primitive faults. Our second contribution is a new design for testability method. Our method guarantees that only primitive faults with at most two paths can exist in the circuit in the test mode. The main idea is to efficiently identify a small set of signals for inserting test points to eliminate primitive faults with more than two paths. Our test points only provide controllability. Addition of a single test point can lower the cardinality of several primitive faults. Our approach efficiently re-evaluates primitive delay fault testability of the circuit after insertion of a test point. After a few iterations only primitive faults with at most two paths can exist in the circuit in the test mode. Experimental results on several multilevel combinational benchmark circuits are included to demonstrate the usefulness of our techniques.

Testing delay faults is becoming critical in new deep submicron digital circuits. This paper introduces a new technique for delay and stuck-at fault testing in digital integrated circuits. The proposed technique consists of sensitizing a path in the digital circuit under test and then incorporating it in a ring oscillator to test for delay and stuck-at faults in the path. This procedure should be exercised for all or at least critical paths in the circuit. To establish oscillations, we should make sure that there is an odd number of inverters in the loop. This technique can be used along with scan techniques or be implemented as a built-in self-test technique. Benchmark results confirm the efficiency of the proposed technique. The technique has been implemented in practice for an an 8-bit digital adder on a field programmable device.

Delay faults are an increasingly important test challenge. Modeling bridge faults as delay faults helps delay tests to detect more bridge faults. Traditional bridge fault models are incomplete because these models only model the logic faults or these models are not efficient to use in delay tests for large circuits. In this article, we propose a physically realistic yet economical resistive bridge fault model to model delay faults as well as logic faults. An accurate yet simple delay calculation method is proposed. We also enumerate all possible fault behaviors and present the relationship between input patterns and output behaviors, which is useful in ATPG. Our fault simulation results show the benefit of at-speed tests.

We present an approach to prevent overtesting in scan-based delay test. The test data is transformed with respect to functional constraints while simultaneously keeping as many positions as possible unspecified in order to facilitate test compression. The method is independent of the employed delay fault model, ATPG algorithm and test compression technique, and it is easy to integrate into an existing flow. Experimental results emphasize the severity of overtesting in scan-based delay test. Influence of different functional constraints on the amount of the required test data and the compression efficiency is investigated. To the best of our knowledge, this is the first systematic study on the relationship between overtesting prevention and test compression.

Evaluation of delay fault testability of LUTs for the enhancement of application-dependent testing of FPGAs

A study is presented concerning the efficiency of test pattern sets generated with the transition fault model applied to fine grained delay fault models. The authors have developed the delay fault simulator DELFI-a program which is capable of simulating timing failures of combinational circuits using different delay fault models. For the computer experiments the authors selected transition fault test pattern sets because they are very cost-effective to generate. The simulations of benchmark circuits demonstrate that the transition fault test patterns detect gross delay faults even at nodes with redundant stuck-at faults. Furthermore the results show that the transition fault test patterns are not sufficient for small delay faults in the range of a few gate delays. In order to receive a satisfactory coverage for these delay faults, the transition fault test sets must be extended. >

We propose a functional fault model for delay faults in combinational circuits and describe a functional test generation procedure based on this model. The proposed method is most suitable when a gate-level description of the circuit-under-test, necessary for employing existing gate-level delay fault test generators, is not available or does not accurately describe the circuit. It is also suitable for generating tests in early design stages of a circuit, before a gate-level implementation is selected. In addition, it can potentially be employed to supplement conventional test generators for gate-level circuits to reduce the cost of handling large numbers of paths. A parameter called Δ is used to control the number of funtional faults targeted and thus the number of tests generated. If Δ is unlimited, the functional test set detects every robustly testable path delay fault in any gate-level implementation of the given ciruit. An appropriate subset of tests can be selected once the implentation is known. The test sets generated for various values of Δ are fault simulated on gate-level realizations to demonstrate their effectiveness. The experiments indicate that functional test sets may be able to identify functions whose realizations have low path delay fault coverage.

To guarantee the temporal correctness of a digital circuit a set of multiple path delay faults called primitive faults need to be tested. Primitive faults can contain one or more faulty paths. Existing techniques can identify and test primitive faults containing up to two or three paths. Identifying and testing primitive faults that consist of a larger number of paths is impractical for large designs. We propose a design for testability method that assures the temporal correctness of the circuit without the need to test all primitive faults in the circuit. In the test mode, only primitive faults that contain up to two paths can affect the circuit performance. Our methodology efficiently identifies a small set of potential locations for inserting control points to eliminate primitive faults with more than two paths. Addition of a single control point can lower the cardinality of several primitive faults. Our approach re-evaluates primitive delay fault testability of the circuit after insertion of every control point. After a few iterations only primitive faults with at most two paths can exist in the circuit in the test mode. Experimental results on several circuits are included to demonstrate our method.