This article presents an ultrasmall area on-chip relaxation oscillator with low-temperature sensitivity. In this design, a virtual resistor mainly composed of a complementary to absolute temperature (CTAT) voltage reference circuit is implemented to replace the real resistor for efficient temperature compensation, which counterbalances the inherent proportional to absolute temperature (PTAT) property of the original relaxation circuit of the oscillator. The conventional capacitor is also replaced with a MOS capacitor to complete the ALL-MOS oscillator circuit with two prime advantages, one of which is larger capacitance to area density, and the other is better matching with critical MOSFETs. Implemented in a 0.18-$\mu $m TSMC standard CMOS process, the proposed relaxation oscillator has achieved a temperature coefficient of 28.17 ppm/°C over the temperature range from$- 25~^{\circ }$C to$+ 125~^{\circ }$C at 11.39-MHz oscillation frequency. This circuit consumes$243.1~\mu $W under 1.3-V power supply. Along with the abovementioned excellent performance, the oscillator achieves an ultrasmall core chip area of 0.009 mm2, which is almost one order less than most of the prior arts’ in the same process.

On-chip delay measurement in field is effective for aging detection and prediction of a logic circuit. On the other hand, circuit delay is affected by not only aging but also environment such as temperature and voltage. When comparing a value of measured circuit delay with another value measured in different time to check the progress of aging, we need to correct the measured delay value to allow a fair comparison by eliminating the influence of environmental variations from the delay values. In this paper, we propose a method to convert circuit delay values measured in field into ones measured under the same environment i.e., predetermined temperature and voltage condition. The proposed method, which employs a digital temperature and voltage sensor with ring-oscillators, uses not temperature and voltage values but ROs frequency in the sensor. Furthermore, we also introduce a calibration method for reducing correction errors due to process variation. In evaluation for TEG chips in 65nm CMOS process, the proposed method can improve the correction accuracy for from 83.91ps to 39.78ps. The corrected delay values correspond to 317-day of aging and the prediction accuracy improves 48-day compared with correction using temperature and voltage values.

High power consumption in LSI testing may cause excessive IR-drop. When IR-drop becomes excessive, it causes excessive delay, resulting in test malfunction (over-testing). Excessive IR-drop does not occur in the entire area of a circuit, but in certain areas where a large number of switching activities occur (such areas are called hotspots in this work). In order to avoid test malfunction, it is important to develop a method to reduce or control IR-drop in the hotspots. Locating hotspots is a necessary technique to reduce or control IR-drop effectively and efficiently. In this work, we propose a method to locate hotspots in a logic circuit by switching probability calculation. Experimental results for IWLS2005 OpenCores circuits demonstrate the proposed method can support to locate hotspots.

In this paper, we propose a practical error detection method for combinatorial circuits using three-site implications which satisfies safety integrity level ASIL-B of ISO 26262 standard. The proposed method finds implications which are invariant relationships among internal signals in a logic circuit for any input pattern, and adds checkers to confirm that the implications are satisfied during functional operation. While the checkers playa role of CED (Concurrent Error Detection), higher error detection coverage can be achieved by using three-site implications than only with two-site implications as is the case in previous works. However, as circuit size grows it becomes difficult to find out effective three-site implications in reasonable processing time due to a large number of candidate implications. In the proposed method, along with various speed up techniques utilizing commercial EDA tools, newly developed indexes to narrow down the search space in three-site implication extraction with minimum loss of error detection coverage can achieve reasonable processing time, error detection coverage and area overhead. As preliminary experiments with resynthesized ISCAS85 benchmark circuits, we confirmed that the error detection coverage using all three-site implications under the limit of 50% area overhead is 19.5% higher than only with two-site implications, and the checkers using three-site implications extracted by the proposed indexes can achieve 96% of the error detection coverage derived using all three-site implications. We also applied the proposed method to an industrial CPU core, tinyMicon MatisseCORE™, as complementary error detection of software-based diagnosis. Through this experiment, we confirmed that 90% diagnostic coverage, which is equivalent to ASIL-B requirement in ISO 26262 standard, can be achieved by the checkers added by the proposed method with 15.41 % area overhead. This result implies the proposed method can be applied to actual commercial products.

An aging-tolerant ring oscillator (RO) has been proposed for a digital temperature and voltage sensor. This paper discusses on the effectiveness of aging-tolerance of the ROs through accelerated life test for a test chip with 65nm CMOS technology. The progress of delay degradation of the ROs is examined, and influence of delay degradation on measurement accuracy of the sensor is investigated. Experimental results show that the aging-tolerant ROs can mitigate delay degradation, and that the measurement errors of the sensor can be reduced. Compared with a sensor consisting of an aging-intolerant RO, temperature and voltage errors are reduced 2.5°C and 32mV, respectively.

Periodical delay measurement in field is useful for not only detection of delay-related faults but also prediction of faults due to aging. Logic BIST with variable test clock generation enables on-chip delay measurement in field. This paper addresses a delay measurement scheme based on logic BIST and gives experiment results to observe aging phenomenon of test chips under accelerated life test. The measurement scheme consists of scan-based logic BIST, a variable test clock generator, and digital temperature and voltage sensors. The sensors are used to compensate measured delay values for temperature and voltage variations in field. Evaluation using SPICE simulation shows that the scheme can measure a circuit delay with resolution of 92 ps. The delay measurement scheme is also implemented on fabricated test chips with 180 nm CMOS technology and accelerated test is performed using ATE and burn-in equipment. Experimental results show that a circuit delay increased 552 ps when accelerated the chip for 3000 hours. It is confirmed that the on-chip delay measurement scheme has enough accuracy for detection of aging-induced delay increase.

Path delay measurement in field is useful for not only detection of delay-related faults but also prediction of aging-induced delay faults. In order to utilize the delay measurement results for fault detection and fault prediction, the measured delay must be corrected because the circuit delay is varied in field due to environment such as temperature or voltage variations. This paper proposes a method of BIST-based path delay measurement in which the influence of environmental variations is eliminated. An on-chip sensor measures temperature and voltage during delay measurement. Using information from the temperature and voltage sensor and pre-computed temperature and voltage sensitivities of the circuit delay, the measured delay value is corrected to a delay value that would be obtained under a fixed temperature and voltage. Evaluation for a test chip with 65nm CMOS technology implementing the proposed method shows that errors of measured delays brought by environmental variations could be reduced from 2419 to 211 ps in the range of 30 to 80 °C and 1.05 to 1.35 V. This paper also discusses application and feasibility for degradation detection of the proposed method.

The IP session highlights three innovative test practices in Asia, which include a testing solution for the millimeterwave (76- to 81- GHz) without expensive instruments, an on-chip delay measurement method for in-field test and a power control method of at-speed scan test for IR violation reduction. These would be useful for automotive and IoT application device testing.

An extremely high-resolution, 2-D Vernier field-programmable gate array (FPGA) time-to-digital converter (TDC) with phase wrapping and averaging has been proposed recently to get an extremely fine resolution of 2.5 ps. However, the cell delays in a delay matrix are not fully controlled so that the TDC performance strongly depends on the stochastic distribution of cell delays, and the input range is limited to less than 20 ns. To achieve both high-precision phase division and wide measurement range, a phase-locked loop (PLL)-based delay matrix, which is capable of overclocking and double data rate (DDR), is proposed in this article. All delay cells are under the precise control of PLLs to generate output phases evenly divided within the reference clock period. For a concept proof, the TDC architecture is implemented on an Altera Stratix-IV FPGA chip to achieve 15.6-ps resolution. The differential nonlinearity (DNL), integral nonlinearity (INL), and rms resolution are measured to be merely -0.157 to 0.137 LSB, -0.176 to 0.184 LSB, and 1.0 LSB, which prove the superiority of the proposed structure to its stochastic counterparts. The proposed high-precision phase division technique can be applied to not only the TDC but also the digital-to-time converter (DTC) to enrich its future applications.

While on-chip delay measurement combining logic BIST with a variable test clock is an effective way to secure field reliability of VLSI/FPGAs, validation of the variable test clock generated on the chip is important to guarantee measurement accuracy. This paper addresses a method of on-chip test clock validation using a TDC (Time-to-Digital Converter) for FPGAs. The proposed method has two operation modes, one is a resolution measurement mode and the other is a phase difference measurement mode. The resolution measurement mode is performed first to check the resolution of the TDC circuit. The phase difference measurement mode checks the timing difference between the original clock and the generated test clock. Evaluation experiments using a real FPGA device shows that the resolution of the proposed clock validation method using a TDC is 50.46 ps. For a variable test clock with resolution of 96.15 ps, it was confirmed that INL (Integral Non-Linearity) of the clock is within 10% and it was inconsistent with a result observed by an oscilloscope.

An on-chip digital sensor using three types of ring oscillators (ROs: Ring Oscillators) has been proposed to measure temperature and voltage of a VLSI. Each RO has inherent frequency characteristics with respect to temperature and voltage, which differ from those of the other two ROs. Measurement accuracy of the sensor depends on the combination of the ROs. This paper proposes a RO-selection method for the sensor with high accuracy. The proposed method takes particular note of temperature or voltage sensitivity as well as linearity of the RO characteristics. Evaluation experiments with SPICE simulation in 65 nm CMOS technology show that the temperature and voltage accuracies of the sensor are 2.744 °C and 3.825 mV, respectively, and the selected combination was a nearly optimal from a menu of many different ROs.

Performance degradation caused by high IR-drop in normal functional mode of LSI can be avoided by improving the power supply network in the layout design phase. However, while IR-drop increases much more in test mode than in normal functional mode, excessive IR-drop in test mode is not appropriately considered in the layout design phase. Excessive IR-drop in test mode causes over-testing, which wrongly determines a fault free LSI in normal functional mode to be faulty. In this work, we propose a method for analyzing high IR-drop areas (hotspot distribution), which is necessary to effectively and efficiently reduce excessive IR-drop.

Avoidance of delay-related failures due to aging phenomena is an important issue of current VLSI systems. Delay measurement in field is effective for detection of aging-induced delay increase. This paper proposes a delay measurement method using BIST (Built-In Self-Test) in an FPGA. The proposed method consists of variable test timing generation using an embedded PLL, BIST-based delay measurement, and correction of the measured delay with reflecting temperature variance in field. In on-chip delay measurement of the proposed method, the fastest operating speed is checked by repeating delay test with several test timings. Because circuit delay is influenced by temperature during measurement, the measured delay is then corrected according to the temperature during testing. Based on test log including the corrected delay, delay degradation and aging detection can be grasped. In evaluation experiments of the propose method implemented on an Intel Cyclone IV FPGA device (60nm technology), variable test timing generation realized 96 ps timing step resolution (that is below 1% of the system clock), correction process for measured delay could reduce influence of temperature variation. Furthermore, its feasibility of the proposed method for aging detection is discussed in this paper.

Multi-cycle test with partial observation for scan-based logic BIST is known as one of effective methods to improve fault coverage without increase of test time. In the method, the selection of flip-flops for partial observation is critical to achieve high fault coverage with small area overhead. This paper proposes a selection method under the limitation to a number of flip-flops. The method consists of structural analysis of CUT and logic simulation of test vectors, therefore, it provides an easy implementation and a good scalability. Experimental results on benchmark circuits show that the method obtains higher fault coverage with less area overhead than the original method. Also the relation between the number of selected flip-flops and fault coverage is investigated.

High scan shift power often leads to excessive heat as well as shift timing failures. Partial shift (shifting a subset of scan chains at a time) is a widely adopted approach for avoiding excessive heat by reducing global switching activity, we show for the first time that it may actually cause excessive IR-drop on some clock buffers and worsen shift clock skews, thus increasing the risk of shift timing failures. This paper addresses this problem with an innovative method, namely Clock-Skew-Aware Scan Chain Grouping (CSA-SCG). CSA-SCG properly groups scan chains to be shifted simultaneously so as to reduce the imbalance of switching activity around the clock paths for neighboring scan flip-flops in scan chains. Experiments on large ITC'99 benchmark circuits demonstrate the effectiveness of CSA-SCG for reducing scan shift clock skews to lower the risk of shift timing failures in partial shift.

This paper proposes a test cost reduction method using machine learning techniques. The proposed method tries to predict good dies among the manufactured dies on the way of test process. If a die is predicted as good before completing all of the test process, the die will be allowed to be shipped without going through the remaining test process which contains costly burn-in test and final test. By a SVM-based procedure together with K-fold cross validation, a prediction model to judge certainly good dies is created from known results of the selected test items. In order to evaluate the method in terms of the business effectiveness, we also propose new evaluation measures, "cost reduction rate" and "bad die escape rate", which enable to confirm zero-defect oriented test cost reduction. Experimental results obtained through test data for industrial dies requiring zero-defect show that the proposed method has significant predictability with high test cost reduction capability.

Loading and unloading test patterns during scan testing causes many scan flip-flops to trigger simultaneously. This instantaneous switching activity during shift in turn may cause excessive IR-drop that can disrupt the states of some scan flip-flops and corrupt test stimuli or responses. A common design technique to even out these instantaneous power surges is to design multiple scan chains and shift only a group of the scan chains at a same time. This paper introduces a novel algorithm to optimally group scan chains so as to minimize the probability of test data corruption caused by excessive instantaneous IR-drop on scan flip-flops. The experiments show optimal results on all large ITC'99 benchmark circuits.

The proposed digital sensor measures both temperature and voltage simultaneously in field. The sensor is ring oscillator (RO)-based and its design is fully digital. Its measurement time is shorter than the conventional analog sensors' and it can be placed at any place such as the boundary of a CPU core, GPU core, or Memory. This paper investigates the accuracy of the sensor derived from reduction of temporal and spatial variations. The variations are evaluated by the measurement of a fabricated test chip. When the measurement time is long like the analog sensors, the variations during measurement has a great influence on sensor' accuracy. The comprehensive evaluations show that the total measurement error is smaller than the analog sensors' and it implies the importance of real time and contiguous measurement.

Excessive IR-drop during scan shift can cause localized IR-drop around clock buffers and introduce dynamic clock skew. Excessive clock skew at neighboring scan flip-flops results in hold or setup timing violations corrupting test stimuli or test responses during shifting. We introduce a new method to assess the risk of such test data corruption at each scan cycle and flip-flop. The most likely cases of test data corruption are mitigated in a non-intrusive way by selective test data manipulation and masking of affected responses. Evaluation results show the computational feasibility of our method for large benchmark circuits, and demonstrate that a few targeted pattern changes provide large potential gains in shift safety and test time with negligible cost in fault coverage.

IR-drop induced false capture failures and test clock stretch are severe problems in at-speed scan testing. We propose a new method to efficiently and accurately identify these problems. For the first time, our approach considers the additional dynamic power caused by glitches, the spatial and temporal distribution of all toggles, and their impact on both logic paths and the clock tree without time-consuming electrical simulations.

High power dissipation during scan-based logic BIST is a crucial problem that leads to over-testing. Although controlling test power of a circuit under test (CUT) to an appropriate level is strongly required, it is not easy to control test power in BIST. This paper proposes a novel power controlling method to control the toggle rate of the patterns to an arbitrary level by modifying pseudo random patterns generated by a TPG (Test Pattern Generator) of logic BIST. While many approaches have been proposed to control the toggle rate of the patterns, the proposed approach can provide higher fault coverage. Experimental results show that the proposed approach can control toggle rates to a predetermined target level and modified patterns can achieve high fault coverage without increasing test time.

Measuring temperature and voltage (T&V) in a current VLSI is very important in guaranteeing its reliability, because a large variation of temperature or voltage in field will reduce a delay margin and makes the chip behavior unreliable. This paper proposes a novel method of T&V measurement, which can be used for variety of applications, such as field test, online test, or hot-spot monitoring. The method counts frequencies of more than one ring oscillator (RO), which composes an aging-tolerant monitor. Then, the T&V are derived from the frequencies using a multiple regression analysis. To improve the accuracy of measurement, three techniques of an optimal selection of RO types, their calibration, and hierarchical calculation are newly introduced. In order to make sure the proposed method, circuit simulation in 180-, 90-, and 45-nm CMOS technologies is performed. In the 180-nm CMOS technology, the temperature accuracy is within 0.99 °C, and the voltage accuracy is within 4.17 mV. Furthermore, some experimental results using fabricated test chips with 180-nm CMOS technology confirm its feasibility.

IR-drop induced by launch switching activity (LSA) in capture mode during at-speed scan testing increases delay along not only logic paths (LPs) but also clock paths (Cps). Excessive extra delay along LPs compromises test yields due to false capture failures, while excessive extra delay along CPs compromises test quality due to test clock stretch. This paper is the first to mitigate the impact of LSA on both LPs and CPs with a novel LCPA (Logic/Clock Path-Aware) at-speed scan test generation scheme, featuring (1) a new metric for assessing the risk of false capture failures based on the amount of LSA around both LPs and CPs, (2) a procedure for avoiding false capture failures by reducing LSA around LPs or masking uncertain test responses, and (3) a procedure for reducing test clock stretch by reducing LSA around CPs. Experimental results demonstrate the effectiveness of the LCPA scheme in improving test yields and test quality.

Power-related problems in at-speed scan testing have become more and more serious, since excessive IR-drop caused by excessive power consumption results in overtesting. There are two important factors in low-power testing: one is power estimation, the other is power reduction. Several estimation methods have been proposed based on the analysis of switching activity characteristics. In order to estimate the impact of IR-drop, it is more important to consider the area containing many cells which consume excessive power than to consider the total number of switching activity in a circuit. In this paper, we propose a novel method for identifying areas where excessive IR-drop likely occurs without using test vectors. Visualized experimental results for IWLS 2005 benchmark circuits demonstrate that the proposed method can effectively identify areas containing many cells which consume higher power than others. Such areas identified can be used in low-power test generation so as to achieve effective and efficient results.

An on-chip environment monitor that can estimate a chip temperature and a power supply voltage has been developed to assist accurate circuit delay measurement by field test. The monitor consists of digital circuits and satisfies several features desired for the field test. This paper describes the architecture of the monitor and how to estimate the temperature and voltage in field, and finally shows simulation and TEG results on estimation accuracy.

Fault scrambling technique is considered a promising way to distribute faulty bits into different code words such that the number of faulty cells in each codeword is below the protection capability of the adopted EDAC coding techniques. However, the effectiveness of the scrambling technique depends on the determination of the row/column scrambling control words. Therefore, we propose a heuristic algorithm suitable for built-in implementation for the evaluation of control words based on the fault bitmap and the specifications of the memory. The corresponding built-in scrambling analysis (BISA) circuit is also proposed. The BISA module can be easily integrated into the conventional built-in self-repair (BISR) module. A simulator is developed to evaluate the hardware overhead and repair rate. According to experimental results, the repair rate can be improved significantly with negligible hardware overhead.

Field test is performed in diverse environments, in which temperature varies across a wide range. As temperature affects a circuit delay greatly, accurate temperature monitors are required. They should be placed at various locations on a chip including hot spots. This paper proposes a flexible ring-oscillator-based monitor that accurately measures voltage as well as temperature at the same time. The measurement accuracy was confirmed by circuit simulation for 180 nm, 90 nm and 45 nm technologies. An experiment using test chips with 180 nm technology shows its feasibility.

Ring Oscillators are used for variety of purposes to enhance reliability on LSIs or FPGAs. This paper introduces an aging-tolerant design structure of ring oscillators that are used in FPGAs. The structure is able to reduce NBTI-induced degradation in a ring oscillator's frequency by setting PMOS transistors of look-up tables in an off-state when the oscillator is not working. The evaluation of a variety of ring oscillators using Altera Cyclone IV device (60nm technology) shows that the proposed structure is capable of controlling degradation level as well as reducing more than 37% performance degradation compared to the conventional oscillators.

Ongoing research to shrink feature sizes of LSI circuits leads to an always increasing number of logic gates in a circuit. In general, the complexity of test generation depends on the size of a circuit. Furthermore, modern test generation methods have to consider power reduction in addition to fault detection, since excessive power caused by testing may result in over testing. In this work, we propose a method to reduce the computation time of low-power test generation. The proposed method specifies gates which will cause power issues, consequently reducing the search space for X-filling technique. The reduction of search space for Xfilling also further minimizes the amount of switching activity. Experimental results for circuits of Open Cores provided by IWLS2005 benchmarks show that the proposed method achieves both a reduced computation time and at the same time increased power reduction compared to previous methods.

Electronic systems that consist of various LSIs require very high reliability in field, however, physical degradation phenomena such as NBTI (Negative Bias Temperature Instability) make it difficult to keep reliability in field only by conventional manufacturing-based LSI testing. This paper proposes a stochastic model for NBTI-induced degradation in field and analyzes the effects of parameters that affect LSI failure rate by simulation. Quantitative discussions show a guideline for reliable design and test.

The applicability of at-speed scan-based logic built-in self-test (BIST) is being severely challenged by excessive capture power that may cause erroneous test responses for good chips. Different from conventional low-power BIST, this paper is the first that has explicitly focused on achieving capture power safety with a practical scheme called capture-power-safe BIST (CPS-BIST). The basic idea is to identify all possibly erroneous test responses and use the well-known technique of mask (partial-mask or full-mask) to block them from reaching the MISR. Experiments with large benchmark and industrial circuits show that CPS-BIST can achieve capture power safety with negligible impact on both test quality and area overhead.

Low-power test technology has been investigated deeply to achieve an accurate and efficient testing. Although many sophisticated methods are proposed for scan-test, there are not so many for logic BIST because of its uncontrollable randomness. However, logic BIST currently becomes vital for system debug or field test. This paper proposes a novel low power BIST technology that reduces shift-power by eliminating the specified high-frequency parts of vectors and also reduces capture power. The authors show that the proposed technology not only reduces test power but also keeps test coverage with little loss.

High test power in logic BIST is a serious problem not only for production test, but also for board test, system debug or field test. Many low power BIST approaches that focus on scan-shift power or capture power have been proposed. However, it is known that a half of scan-shift power is compensated by test responses, which is difficult to control in those approaches. This paper proposes a novel approach that directly reduces scan-out power by modifying some flip-flops' values in scan chains at the last capture. Experimental results show that the proposed method reduces scan-out power up to 30% with little loss of test coverage.

This paper proposes a novel technique to measure temperature and voltage on-chip in field test. It consists of three types of NBTI-tolerant ring oscillator and counters constructed with a standard cell library. Temperature and voltage are estimated with high accuracy and in a short time.

Although many electronic safety-related systems require very high reliability, it is becoming harder and harder to achieve it because of delay-related failures, which are caused by decreased noise margin. This paper describes a technology named DART and its implementation. The DART repeatedly measures the maximum delay of a circuit and the amount of degradation in field, in consequence, confirms the marginality of the circuit. The system employing the DART will be informed the significant reduction of delay margin in advance of a failure and be able to repair it at an appropriate time. The DART also equips a technique to improve the test coverage using the rotating test and a technique to consider the test environment such as temperature or voltage using novel ring-oscillator-based monitors. The authors applied the proposed technology to an industrial design and confirmed its effectiveness and availability with reasonable resources.

This paper proposes a novel scheme to manage capture power in a pinpoint manner for achieving guaranteed capture power safety, improved small-delay test capability, and minimal test cost impact in at-speed scan test generation. First, switching activity around each long path sensitized by a test vector is checked to characterize it as hot (with excessively-high switching activity), warm (with normal/functional switching activity), or cold (with excessively-low switching activity). Then, X-restoration/X-filling-based rescue is conducted on the test vector to reduce switching activity around hot paths. If the rescue is insufficient to turn a hot path into a warm path, mask is then conducted on expected test response data to instruct the tester to ignore the potentially-false test response value from the hot path, thus achieving guaranteed capture power safety. Finally, X-restoration/X-filling-based warm-up is conducted on the test vector to increase switching activity around cold paths for improving their small-delay test capability. This novel approach of pinpoint capture power management has significant advantages over the conventionalapproachofglobalcapturepower management, as demonstrated by evaluation results on large ITC'99 benchmark circuits and detailed path delay analysis.

Excessive capture power in at-speed scan testing may cause yield loss due to timing failures. Although reducing the number of clock domains that capture test responses simultaneously is a practical and scalable solution for reducing capture power, no available capture-safety checking metric can assess its effect in an accurate-enough manner, especially when multiple clock domains capture test responses in a short period of time. This paper proposes a novel CLEAR (CLock-Edge-Arrival-Relation-based) capture-safety checking method that, for the first time, takes clock edge arrival times for different clock domains into consideration. The accuracy and usefulness of the proposed method have been clearly demonstrated by simulation-based evaluation with the largest ITC'99 benchmark circuit as well as real-chip-based evaluation with an industrial chip embedded with on-chip delay measurement circuitry.

This paper presents a novel failure prediction technique that is applicable for system-on-chips (SoCs). Highly reliable systems such as automobiles, aircrafts, or medical equipments would not allow any interruptive erroneous responses during system operations, which might result in catastrophes. Therefore, we propose a failure prediction technique that can be applied during an idle time when a system is not working, such as power-on/-off time. To achieve high reliability in the field, the proposed technique should take into consideration various types of aging mechanisms and the testing environment of voltage and temperature which is uncontrollable in the field. Therefore, we propose: 1) an accurate delay measurement technique considering the variation due to voltage and temperature and 2) an adaptive test scheduling that gives more test chances to more probable degrading parts. Experimental results show the required memory space and area cost for implementing the proposed technique.

It has become necessary to reduce power during LSI testing. Particularly, during at-speed testing, excessive power consumed during the Launch-To-Capture (LTC) cycle causes serious issues that may lead to the overkill of defect-free logic ICs. Many successful test generation approaches to reduce IR-drop and/or power supply noise during LTC for the launch-off capture (LOC) scheme have previously been proposed, and several of X-filling techniques have proven especially effective. With X-filling in the launch-off shift (LOS) scheme, however, adjacent-fill (which was originally proposed for shift-in power reduction) is used frequently. In this work, we propose a novel X-filling technique for the LOS scheme, called Adjacent-Probability-based X-Filling (AP-fill), which can reduce more LTC power than adjacent-fill. We incorporate AP-fill into a post-ATPG test modification flow consisting of test relaxation and X-filling in order to avoid the fault coverage loss and the test vector count inflation. Experimental results for larger ITC'99 circuits show that the proposed AP-fill technique can achieve a higher power reduction ratio than 0-fill, 1-fill, and adjacent-fill.

Field test for reliability is usually performed with small amount of memory resource, and it requires a new technique which might be somewhat different from the conventional manufacturing tests. This paper proposes a novel technique that improves fault coverage or reduces the number of test vectors that is needed for achieving the given fault coverage on scan-based BIST structure. We evaluate a multi-cycle test method that observes the values of partial flip-flops on a chip during capture-mode. The experimental result shows that the partial observation achieves fault coverage improvement with small hardware overhead than the full observation.

Excessive capture power in at-speed scan testing may cause timing failures, resulting in test-induced yield loss. This has made capture-safety checking mandatory for test vectors. This paper presents a novel metric, called the TTR (Transition-Time-Relation-based) metric, which takes transition time relations into consideration in capture-safety checking. Capture-safety checking with the TTR metric greatly improves the accuracy of test vector sign-off and low-capture-power test generation.

High power consumption in scan testing can cause undue yield loss which has increasingly become a serious problem for deep-submicron VLSI circuits. Growing evidence attributes this problem to shift timing failures, which are primarily caused by excessive switching activity in the proximities of clock paths that tends to introduce severe clock skew due to IR-drop-induced delay increase. This paper is the first of its kind to address this critical issue with a novel layout-aware scheme based on scan segmentation design, called LCTI-SS (Low-Clock-Tree-Impact Scan Segmentation). An optimal combination of scan segments is identified for simultaneous clocking so that the switching activity in the proximities of clock trees is reduced while maintaining the average power reduction effect on conventional scan segmentation. Experimental results on benchmark and industrial circuits have demonstrated the advantage of the LCTI-SS scheme.

At-speed scan testing may suffer from severe yield loss due to the launch safety problem, where test responses are invalidated by excessive launch switching activity (LSA) caused by test stimulus launching in the at-speed test cycle. However, previous low-power test generation techniques can only reduce LSA to some extent but cannot guarantee launch safety. This paper proposes a novel & practical power-aware test generation flow, featuring guaranteed launch safety. The basic idea is to enhance ATPG with a unique two-phase (rescue & mask) scheme by targeting at the real cause of the launch safety problem, i.e., the excessive LSA in the neighboring areas (namely impact areas) around long paths sensitized by a test vector. The rescue phase is to reduce excessive LSA in impact areas in a focused manner, and the mask phase is to exclude from use in fault detection the uncertain test response at the endpoint of any long sensitized path that still has excessive LSA in its impact area even after the rescue phase is executed. This scheme is the first of its kind for achieving guaranteed launch safety with minimal impact on test quality and test costs, which is the ultimate goal of power-aware at-speed scan test generation.

NBTI, which is one of well-known aging phenomena, brings delay degradation in deep submicron VLSIs. In order to detect NBTI-induced delay faults, we need to estimate delay degradation and apply delay test for the circuit in the field. This paper discusses on estimation of NBTI-Induced delay degradation. We first analyze the effect of the delay degradation, and then give a procedure of path selection in which long paths after the delay degradation are selected for the delay test in the filed. Experimental results show that estimation of delay degradation significantly affects path selection, and accurate estimation is important for the test.

This paper presents a novel failure prediction testing technique that is applicable for system-on-chips (SoCs). Highly reliable systems such as automobiles, aircraft or medical equipments would not allow any interruptive erroneous responses during a system operation, which might result in catastrophe. Therefore, we propose a failure prediction delay testing technique that is applied during the time when the system is not working, such as power-on/-off times. To achieve high reliability in the field, the proposed technique should take into consideration various types of aging mechanisms. Since the testing environment of voltage and temperature is uncontrollable in the field, an accurate delay measurement considering the variation due to voltage and temperature should be developed. Moreover, we propose an adaptive test scheduling that gives more test chances to more possible degrading parts for improving detecting efficiency.

Reducing excessive launch switching activity (LSA) is now mandatory in at-speed scan testing for avoiding test-induced yield loss, and test set modification is preferable for this purpose. However, previous low-LSA test set modification methods may be ineffective since they are not targeted at reducing launch switching activity in the areas around long sensitized paths, which are spatially and temporally critical for test-induced yield loss. This paper proposes a novel CAT (Critical-Area-Targeted) low-LSA test modification scheme, which uses long sensitized paths to guide launch-safety checking, test relaxation, and X-filling. As a result, launch switching activity is reduced in a pinpoint manner, which is more effective for avoiding test-induced yield loss. Experimental results on industrial circuits demonstrate the advantage of the CAT scheme for reducing launch switching activity in at-speed scan testing.

Reducing IR-drop in the test cycle during at-speed scan testing has become mandatory for avoiding test-induced yield loss. An efficient approach for this purpose is post-ATPG test modification based on X-identification and X-filling since it causes no circuit/clock design change and no test vector count inflation. However, applying this approach to test compression has been considered challenging due to the limited availability of X-bits. This paper solves this serious problem by proposing a novel and practical CA (Compression-Aware) test modification scheme for reducing IR-drop in the widely-used broadcast-scan based test compression environment. This unique scheme features (1) CA circuit remodeling for minimizing the effort of applying test modification to broadcast-scan-based test compression, (2) CA X-identification for increasing X-bits for risky test vectors, and (3) CA X-filling for effectively using limited X-bits in reducing IR-drop. As a result, the CA test modification scheme can achieve significant IR-drop reduction even when a test cube only has a small number of X-bits. This advantage is clearly demonstrated by experimental results on three compression configurations created from an industrial circuit.

Power-aware X-filling is a preferable approach to avoiding IR-drop-induced yield loss in at-speed scan testing. However, the quality of previous X-filling methods for reducing launch switching activity may be unsatisfactory, due to low effect (insufficient and global-only reduction) and/or low scalability (long CPU time). This paper addresses this quality problem with a novel, GA (Genetic Algorithm) based X-filling method, called GA-fill. Its goals are (1) to achieve both effectiveness and scalability in a more balanced manner, and (2) to make the reduction effect of launch switching activity more concentrated on critical areas that have higher impact on IR-drop-induced yield loss.Evaluation experiments are being conducted on benchmark and industrial circuits, and initial results have demonstrated the usefulness of GA-fill.

At-speed scan testing is susceptible to yield loss risk due to power supply noise caused by excessive launch switching activity. This paper proposes a novel two-stage scheme, namely CTX (Clock-Gating-Based Test Relaxation and X-Filling), for reducing switching activity when test stimulus is launched. Test relaxation and X-filling are conducted (1) to make as many FFs inactive as possible by disabling corresponding clock-control signals of clock-gating circuitry in Stage-1 (Clock-Disabling), and (2) to make as many remaining active FFs as possible to have equal input and output values in Stage-2 (FF-Silencing). CTX effectively reduces launch switching activity, thus yield loss risk, even with a small number of donpsilat care (X) bits as in test compression, without any impact on test data volume, fault coverage, performance, and circuit design.

Capture-safety, defined as the avoidance of any timing error due to unduly high launch switching activity in capture mode during at-speed scan testing, is critical for avoiding test- induced yield loss. Although point techniques are available for reducing capture IR-drop, there is a lack of complete capture-safe test generation flows. The paper addresses this problem by proposing a novel and practical capture-safe test generation scheme, featuring (1) reliable capture-safety checking and (2) effective capture-safety improvement by combining X-bit identification & X-filling with low launch- switching-activity test generation. This scheme is compatible with existing ATPG flows, and achieves capture-safety with no changes in the circuit-under-test or the clocking scheme.

Test data modification based on test relaxation and X-filling is the preferable approach for reducing excessive IR-drop in at-speed scan testing to avoid test-induced yield loss. However, none of the existing test relaxation methods can control the distribution of identified don’t care bits (X-bits), thus adversely affecting the effectiveness of IR-drop reduction. In this paper, we propose a novel test relaxation method, called Distribution-Controlling X-Identification (DC-XID), which controls the distribution of X-bits identified from a set of fully-specified test vectors for the purpose of effectively reducing IR-drop. Experimental results on large industrial circuits demonstrate the effectiveness and practicality of the proposed method in reducing IR-drop, without any impact on fault coverage, test data volume, or test circuit size.

IR-drop-induced malfunction is mostly caused by timing violations on activated critical paths during the capture cycle of at-speed scan testing. A critical-path-aware X-filling method is proposed for reducing IR-drop, especially on gates that are close to activated critical paths, thus effectively preventing test-induced yield loss.

As a method to evaluate delay test quality of test patterns, SDQM (Statistical Delay Quality Model) has been proposed for transition faults. In order to derive better test quality by SDQM, the following two things are important: for each transition fault, (1) to find out the accurate length of the longest sensitizable paths along which the fault is activated and propagated, and (2) to generate a test pattern that detects the fault through as long paths as possible. In this paper, we propose a method to calculate the length of the potentially sensitizable longest path for detection of a transition fault. In addition, we develop a procedure to extract path information that helps high quality transition ATPG. Experimental results show that the proposed method not only derives more accurate SDQL (Statistical Delay Quality Level) but also enhances the test quality of generated test patterns.

High-quality at-speed scan testing, characterized by high small-delay-defect detecting capability, is indispensable to achieve high delay test quality for DSM circuits. However, such testing is susceptible to yield loss due to excessive power supply noise caused by high launch-induced switching activity. This paper addresses this serious problem with a novel and practical post-ATPG X-filling scheme, featuring (1) a test relaxation method, called path keeping X-identification, that finds don't-care bits from a fully-specified transition delay test set while preserving its delay test quality by keeping the longest paths originally sensitized for fault detection, and (2) an X-filling method, called justification-probability-based fill (JP-fill), that is both effective and scalable for reducing launch-induced switching activity. This scheme can be easily implemented into any ATPG flow to effectively reduce power supply noise, without any impact on delay test quality, test data volume, area overhead, and circuit timing.

We propose a dynamic test compaction procedure to generate high-quality test patterns for path delay faults. While the proposed procedure generates a compact two-pattern test set for paths selected by a path selection criterion, the generated test set would detect not only faults on the selected paths but also faults on many unselected paths. Hence both high test quality by detecting untargeted faults and test cost reduction by reducing test patterns can be achieved. Experimental results show that the proposed procedure could generate a compact test set that detect many untargeted path delay faults certainly, compared with the static test compaction method previously proposed (Kajihara et al., 2005).

X-filling is preferred for low-capture-power scan test generation, since it reduces IR-drop-induced yield loss without the need of any circuit modification. However, the effectiveness of previous X-filling methods suffers from lack of guidance in selecting targets and values for X-filling. This paper addresses this problem with a highly-guided X-filling method based on two novel concepts: (1) X-score for X-filling target selection and (2) probabilistic weighted capture transition count for Y-filling value selection. Experimental results show the superiority of the new X-filling method for capture power reduction.

This paper presents a framework of high-quality test generation for transition faults in full scan circuits. This work assumes a restricted broad-side testing as a test application method for two-pattern tests where control of primary inputs and observation of primary outputs are restricted. Because we use a modified time expansion model of a circuit-under-test during ATPG and fault simulation, conventional ATPG and fault simulation programs can work with minor change. The proposed ATPG method consists of two algorithms, which are activation-first and propagation-first, and for each fault it is decided which algorithm should be applied. Test patterns are generated such that transition faults with small delay can be detected, i.e. a path for fault excitation and propagation becomes as long as possible. In experimental results we evaluate test patterns generated by the proposed method using SDQM that calculates delay test quality, and show the effectiveness of the proposed method

High power dissipation can occur when the response to a test vector is captured by flip-flops in scan testing, resulting in excessive JR drop, which may cause significant capture-induced yield loss in the DSM era. This paper addresses this serious problem with a novel test generation method, featuring a unique algorithm that deterministically generates test cubes not only for fault detection but also for capture power reduction. Compared with previous methods that passively conduct X-filling for unspecified bits in test cubes generated only for fault detection, the new method achieves more capture power reduction with less test set inflation. Experimental results show its effectiveness.

This paper presents a test compression method that effectively derives the capability of a run-length based encoding. The method employs two techniques: scan polarity adjustment and pinpoint test relaxation. Given a test set for a full-scan circuit, scan polarity adjustment selectively flips the values of some scan cells in test patterns. It can be realized by changing connections between two scan cells so that the inverted output of a scan cell, Q, is connected to the next scan cell. Pinpoint test relaxation flips some specified 1s in the test patterns to 0s without any fault coverage loss. Both techniques are applied by referring to a gain-penalty table to determine scan cells or bits to be flipped. Experimental results on ISCAS'89 benchmark circuits show that the proposed method could reduce test data volume by 36%. Switching activities, i.e. test power during scan testing, were also reduced.

In this paper we introduce a quality model that reflects fabrication process quality, design delay margin, and test timing accuracy. The model provides a measure that can predict the level of chip defects that cause delay failure, including marginal delay. We can therefore use the model to make test vectors that are effective in terms of both testing cost and chip quality. The results of experiments using ISCAS89 benchmark data and some large industrial design data reflect various characteristics of our statistical delay quality model.

Recently design for manufacturability (DFM) has been required to achieve higher process yield. Information obtained from silicon by testing and/or fault analysis is sometimes fed back for redesign of VLSI circuits. In this paper we propose a method to maximize defect coverage of a test set initially generated for stuck-at faults in a full scan sequential circuit by using feed back information from fault analysis. If a test set for more complex faults than stuck-at faults is generated, higher defect coverage would be obtained. Such a test set, however, would have a large number of test vectors, and hence the test costs would go up. The proposed method improves defect coverage of the test set by not adding new test vectors but modifying test vectors with the information obtained from fault analysis. Therefore there are no negative impacts on test data volume and test application time. The initial fault coverage for stuck-at faults of the test set is guaranteed with modified test vectors. In this paper we focus on detecting as many as possible non-feedback AND/OR-type bridging faults. Experimental results show that the proposed method significantly decreases the number of non-feedback AND/OR-type bridging faults undetected by a test set generated for stuck-at faults.

In this paper we propose a test compaction method for path delay faults in a logic circuit. The method generates a compact set of two-pattern tests for faults on long paths selected with a criterion. While the proposed method generates each two-pattern test for more than one fault in the target fault list as well as ordinary test compaction methods, secondary target faults are selected from the fault list such that many other faults, which may not be included in the fault list, are detected by the test pattern. Even if faults on long paths in a manufactured circuit are not included in the fault list due to a process variation or noise, the compact test set would detect the longer untargeted faults, i.e., the test set has a noise or variation tolerant nature. Experimental results show that the proposed method can generate a compact test set and it detects longer untargeted path delay faults efficiently.

The quality of delay testing focused on small delay defects is not clear when traditional fault models are used. We therefore evaluated the feasibility of using the statistical delay quality model (SDQM) - which reflects fabrication process quality, design delay quality, test timing accuracy, and test pattern quality - by using a commercial automatic test program generation (ATPG) tool to apply it to a large data set. The SDQM can also provide a measure predicting the defect level of a chip, and by simulating test patterns we show experimentally here that this measure can be calculated within a reasonable CPU time when using a reasonable amount of memory. Because we found when using SDF information to calculate path lengths accurately that the transition test patterns are not good at detecting small delay defects in long paths, a new test algorithm that detects small delay defects should be developed

Scan-based at-speed testing is a key technology to guarantee timing-related test quality in the deep submicron era. However, its applicability is being severely challenged since significant yield loss may occur from circuit malfunction due to excessive IR drop caused by high power dissipation when a test response is captured. This paper addresses this critical problem with a novel low-capture-power X-filling method of assigning 0's and 1's to unspecified (X) bits in a test cube obtained during ATPG. This method reduces the circuit switching activity in capture mode and can be easily incorporated into any test generation flow to achieve capture power reduction without any area, timing, or fault coverage impact. Test vectors generated with this practical method greatly improve the applicability of scan-based at-speed testing by reducing the risk of test yield loss.

Research on low-power scan testing has been focused on the shift mode, with little or no consideration given to the capture mode power. However, high switching activity when capturing a test response can cause excessive IR drop, resulting in significant yield loss. This paper addresses this problem with a novel low-capture-power X-filling method by assigning 0's and 1's to unspecified (X) bits in a test cube to reduce the switching activity in capture mode. This method can be easily incorporated into any test generation flow, where test cubes are obtained during ATPG or by X-bit identification. Experimental results show the effectiveness of this method in reducing capture power dissipation without any impact on area, timing, and fault coverage.

In this paper, we propose techniques for finding Xs in test sequences for sequential circuits. Also we show two applications that utilize the obtained test sequences with Xs: reduction of the power during test and test compaction.

In this paper, we propose a method of test compression for multiple scan designs. Instead of the conventional serial scan chains, the proposed method constructs scan trees in which scan flip-flops are placed and routed in a tree structure. Inputs of the scan trees drive several scan trees of different lengths (height). Since test data volume and test application time are dominated by the scan tree with the maximum height among the constructed scan trees, the proposed method distributes the scan flip-flops to the scan trees so as to minimize the maximum height of the scan trees. In addition, the proposed method modifies the given test vectors to maximize the reduction in test application time. Experimental results for ISCAS-89 benchmark circuits show that the proposed method could reduce, on the average, test data volume by 77% compared with the conventional multiple scan design. The scan tree construction enlarges the number of scan outputs required. However test data volume could be reduced by 66% even if the number of scan outputs is limited.

When test vectors for a full scan logic circuit include unspecified values, conventional 3-valued fault simulation may not compute the exact fault coverage for single stuck-at faults. This paper first addresses the incompleteness of logic/fault simulation based on the conventional 3-valued logic. Then we propose an enhanced method of logic/fault simulation to compute more accurate fault coverage using implicit logic values. The proposed method employs indirect implications. We also propose a new learning criterion to identify indirect implications that are not identified by earlier static learning procedure. Since some indirect implications derived from a fault-free circuit become invalid in the presence of a fault, we use a sufficient condition for an indirect implication to remain valid for the faulty circuit, and give an efficient procedure for more accurate fault simulation. Experimental results demonstrate that the proposed method reduces the number of unknown values at the circuit outputs in logic simulation, and hence it discovers several detected faults that are not declared as detected by the conventional fault simulation.

This work proposes a new per-test fault diagnosis method based on the X-fault model. The X-fault model represents all possible behaviors of a physical defect or defects in a gate and/or on its fanout branches by using different X symbols on the fanout branches. A novel technique is proposed for analyzing the relation between observed and simulated responses to extract diagnostic information and to score the results of diagnosis. Experimental results show the effectiveness of our method.

Given a test set for stuck-at faults of a combinational circuit or a full-scan sequential circuit, some of the primary input values may be changed to the opposite logic values without losing fault coverage. We can regard such input values as don't care (X). In this paper, we propose a method for identifying the X inputs of test vectors in a given test set. While there are many combinations of X inputs in the test set generally, the proposed method finds one including as many X inputs as possible, by using fault simulation and procedures similar to implication and justification of automatic test pattern generation (ATPG) algorithms. Experimental results for ISCAS benchmark circuits show that approximately 69% of the inputs of uncompacted test sets could be X on the average. Even for highly compacted test sets, the method found that approximately 48% of inputs are X.

Since a logic circuit often has too many paths to test delay of all paths in the circuit, it is necessary for path delay testing to limit the number of paths to be tested. Paths to be tested should be ones with large delay that more likely cause a fault. In addition, a test set for the paths are required to detect other models of faults as many as possible. In this paper, we investigate criteria of path selection for path delay testing. We first define typical two criteria to be investigated here, and then experimentally show the feature of paths selected with each criterion, with respect to fault coverage of other delay fault models. From our experiments, we observe that test patterns for the longest paths cannot cover many other faults such as gate delay faults or segment delay faults.

In this paper, we propose a method to estimate fault efficiency for path delay faults based on untestable path analysis. In path delay fault testing, fault coverage of test patterns is usually, very low, because logic circuits often have huge number of paths including many untestable paths. Hence we should compute fault efficiency rather than fault coverage, but it is too difficult to compute exact fault efficiency in a short time, because there is no method to compute total number of untestable paths quickly. The proposed method statistically estimate the number of untestable paths based on untestable path analysis, and compute fault efficiency. Experimental results show that the proposed method can accurately estimate fault efficiency of given test patterns in a reasonable time.

This paper presents a method to reduce test data volume and test application time for a full-scan circuit. The proposed method constructs a scan tree in which scan flip-flops are placed and routed in a tree structure. Although one scan input to the scan tree drives several scan chains with varying length, it is guaranteed that every test vector of a test set can be loaded into the scan tree. Since the height of the scan tree decides test data volume of the test set, the method modifies the test set so as to minimize the height. The procedure of test vector modification consists of don't care identification for the test set and a solution to a vertex coloring problem for an incompatibility graph constructed from the test set including don't cares. Experimental results for ISCAS-89 benchmark circuits show that the proposed method could reduce, on average, test data volume and test application time by 70%.

We propose a method to find don't care (X) values in a test sequence for a sequential circuit. Given a fully specified test sequence generated by a sequential ATPG, the proposed method produces a test sequence containing Xs without losing stuck-at fault coverage of the original test sequence.

We present a pinpoint test set relaxation method for test compression that maximally derives the capability of a run-length encoding technique such as Golomb coding or frequency-directed run-length (FDR) coding. Before encoding a given set of test patterns, we selectively relax some specified bits of the test patterns. By changing a specified bit with value 1 to a don't-care, two consecutive runs of 0s in the test sequence can be concatenated into a longer run of 0, thereby facilitating run-length coding. This procedure retains the fault coverage of the test set. Since the increase in compression depends on the lengths of the two runs that are concatenated with each bit relaxation, a lookup table, referred to as the gain table, is pre-computed and used during the test set relaxation procedure to maximize the likelihood of increasing the amount of test data compression. The gain table is used to pinpoint the bit positions with value 1, which when relaxed to don't-cares, will yield the most compression. In this way, the given test pattern set is appropriately modified as a preprocessing step before test compression. Experimental results for the ISCAS benchmark circuits show that the proposed method can be used to increase the effectiveness of run-length coding methods for test data compression.

We consider issues related to the reduction of scan test data in designs with multiple scan chains. We propose a metric that can be used to evaluate the effectiveness of procedures for reducing the scan data volume. The metric compares the achieved compression to the compression which is intrinsic to the use of multiple scan chains. We also propose a procedure for modifying a given test set so as to achieve reductions in test data volume assuming a combinational decompressor circuit.

This paper describes a method of test data compression for a given test set using statistical encoding. In order to maximize the effectiveness of statistical encoding, the method first converts some specified input values in the test set to unspecified ones without losing fault coverage, and then reassigns appropriate logic values to the unspecified inputs. Experimental results for ISCAS-89 benchmark circuits show that the proposed method can on the average reduce the test data volume to less than 25% of that required for the original test set.

Given a test set for stuck-at faults, a primary input value may be changed to the opposite logic value without losing fault coverage. One can regard such a value as a don't-care (X). The don't care values can be filled appropriately to achieve test compaction, test data compression, or power reduction during testing. However, these uses are better served if the don't cares can be placed in desired/specific bit positions of the test patterns. In this paper, we present a method for maximally fixing Xs on specific bits of given test vectors. Experimental results on ISCAS benchmark circuits show how the proposed method can increase the number of Xs on specific bits compared with an earlier proposed method.

We consider the problem of testing of interconnect open defects in combinational circuits with large fanout nodes. We propose a gate level fault model for interconnect opens. The number of interconnect open faults using the proposed model can be very large, being exponential in the fanout size. We describe methods to effectively consider the very large numbers of open faults. These methods include techniques for implicit consideration of open faults, and the use of information about fanout branches driving each primary output to reduce the list of faults. We present experimental results to demonstrate that fault simulation and test generation for the modeled open faults can be carried out efficiently using these techniques.

This paper presents a test vector modification method for reducing power dissipation during test application for a full-scan circuit. The method first identifies a set of don't care (X) inputs of given test vectors, to which either logic value 0 or 1 can be assigned without losing fault coverage. Then, the method reassigns logic values to the X inputs so as to decrease switching activity of the circuit during scan shifting. Experimental results for benchmark circuits show the proposed method could decrease switching activity of a given test set to 48% of the original test set.

We consider issues related to the reduction of scan test data in designs with multiple scan chains. We propose a metric that can be used to evaluate the effectiveness of procedures for reducing the scan data volume. The metric compares the achieved compression to the compression which is intrinsic to the use of multiple scan chains. We also propose a procedure for modifying a given test set so as to achieve reductions in test data volume assuming a combinational decompressor circuit.

Developed a partially rotational scan (PRS) register used for the n-detection BIST (built-in self-test) that can detect not only delay faults but also unmodeled faults. The developed circuit consists of a shift register with partial rotation. Also presents a procedure for selecting test vectors from ATPG (automatic test pattern generation) ones. This testing method enables at-speed testing and the stuck-at fault coverage of n/spl times/100% by using subset of the ATPG vectors. And it drastically reduces the number of vectors input from an external low-speed tester. Computer simulations of stuck-at fault coverage are conducted on ISCAS'85, ISCAS'89, and ITC'99 circuits for detection times n of 1, 2, 3, 5, 10, and 15. They show that the compaction rates of the ATPG test vectors range from 52.4% (s713) to 0.9% (c499) of origin. This result demonstrates that the PRS register can accomplish low-cost, at-speed testing.

Several methods to reduce the run time and memory requirements of a procedure used to efficiently identify untestable path delay faults are proposed in this work. Based on the correlation between the conditions required for sensitizing subpaths in the fan-out-free regions of a circuit, equivalence relations between the subpaths are defined. Equivalence relations are used to reduce the number of subpaths considered in the identification of untestable paths. Dynamic pruning of the potential search space for identifying pairs of subpaths that cannot be sensitized together is used to achieve additional speedup. Results on benchmark circuits show the effectiveness of the proposed methods.

Given a test set for stuck at faults, some of primary input values may be changed to opposite logic values without losing fault coverage. We can regard such input values as don't care (X). In this paper, we propose a method for identifying X inputs of test vectors in a given test set. While there are many combinations of X inputs in the test set generally, the proposed method finds one including X inputs as many as possible, by using fault simulation and procedures similar to implication and justification of ATPG algorithms. Experimental results for ISCAS benchmark circuits show that approximately 66% of inputs of un-compacted test sets could be X in average. Even for compacted test sets, the method found that approximately 47% of inputs are X. Finally, we discuss how logic values are reassigned to the identified X inputs where several applications exist to make test vectors more desirable.

Logic circuits may have large numbers of untestable paths. Therefore, it is important for path delay fault testing to identify untestable paths prior to test generation. An earlier method, called partial path sensitization, was able to identify large numbers of untestable path delay faults by analyzing pairs of subpaths. We propose to apply this method to the edge graph of the circuit. In the edge graph, an edge corresponds to two consecutive subpaths. Thus, identification of untestable paths is done based on longer subpaths when the edge graph is used than when the original netlist is used. Experimental results presented in this paper show that the proposed method identifies more untestable paths than when the partial path sensitization method is applied to the original netlist.

We present a method of path selection and test generation for path delay faults. The proposed method addresses the fact that logic circuits typically have very large numbers of paths, and a large percentage of these paths are typically untestable. The proposed method selects a set of potentially testable long paths by utilizing non-enumerative identification of untestable paths and removing untestable paths from consideration. Test generation is also applied as part of the proposed method. We demonstrate the effectiveness of the method by presenting results for benchmark circuits.

We consider the problem of validating flip-flop data hold time requirements in sequential circuits. The data hold time violations considered are related to the presence of short paths that allow changes in next-state values to occur fast enough so as to cause latching of erroneous next-states. Three fault models are proposed that are related to the presence of short paths in the circuit. Propagation conditions for robust and non-robust tests for short paths are given. A test generation procedure is described for one of the proposed models, and experimental results are provided for benchmark circuits.

In IDDQ testing, it is important to reduce the number of time-consuming IDDQ measurements. Therefore, it is necessary to select a small number of IDDQ measurement vectors in a test sequence for a sequential circuit while fault coverage is nearly maximum. In this paper, we address the selection problem of measurement vectors by introducing a cost function which is defined by the number of measurement vectors and fault coverage. The proposed method for selecting measurement vectors optimizes the cost function so that high fault coverage is obtained by a small number of measurement vectors.

This paper investigates test sets with high fault coverage for multiple stuck-at faults of large combinational circuits and fully scanned sequential circuits. We show the ability of multiple fault detection of test sets generated for single stuck-at-faults, then give a procedure for generating a compact test set with high multiple fault coverage. Results show that our method can generate a test set with complete fault coverage for many circuits, and the size of the test set is smaller than previously reported ones.

This paper considers a new test generation scheme in which a limitation of the number of tests exists. Since, in this scheme, correct fault coverage cannot be calculated by the representative faults, we present a method for calculating the correct fault coverage by using the weighted fault list. And then we propose a selection-based test generation method which derives a limited number of tests with higher fault coverage. The experimental results for IDDQ testing shows that our test generation method can generate tests with fault coverage close to the maximum fault coverage.

The procedure used in static learning extracts implication relations of the logic circuit. The number of extracted implication relations depends on the order of signal lines processed. In this paper we propose an efficient method to extract implication relations by considering the order of signal lines to be processed. Experimental results show that the proposed order finds more implication relations than others and is effective for redundancy identification.

This paper considers two types of n-bit adders, ripple carry adders and cascaded carry look-ahead adders, with minimum tests for stuck-at-fault models. In the first part, we present two types of full adders consisting of five gates, and show their minimality. We also prove that one of the full adders can be tested by only three test patterns for single stuck-at-faults. We also present two types of 4-bit carry look-ahead adders and their minimum rests. In the second part, we consider the tests for the cascaded adders, an n-bit ripple carry adder and a 4m-bit cascaded carry look-ahead adders. These tests are considerably smaller than previously published ones.

It was recently observed that, in order to improve the defect coverage of a test set, test generation based on fault models such as the single-line stuck-at model may need to be augmented so as to derive test sets that detect each modeled fault more than once. In this work, we report on test pattern generators for combinational circuits that generate test sets to detect each single line stuck-at fault a given number of times. Additionally, we study the effects of test set compaction on the defect coverage of such test sets. For the purpose of experimentation, defect coverage is measured by the coverage of surrogate faults, using a framework proposed earlier. Within this framework, we show that the defect coverage does not have to be sacrificed by test compaction if the test set is computed using appropriate test generation objectives. Moreover, two test sets generated using the same test generation objectives, except that compaction heuristics were used during the generation of one but not the other, typically have similar defect coverages, even if the compacted test set is significantly smaller than the noncompacted one. Test generation procedures and experimental results to support these claims are presented.

This paper presents a design-for-testability technique aimed at test length reduction for scan designed circuits. A new concept, called partially parallel scan chain, is introduced. In the partially parallel scan chain, some flip-flops are arranged in parallel so that the number of scan shift clocks is reduced. Retiming techniques are used to select the flip-flops arranged in parallel. The flip-flops are repositioned only during test vector generation, but not actually. Then the test vectors generated for the retimed circuit are applied to the original circuit. In this paper, the difference in detectability of faults between the retimed circuit and the original circuit is also discussed. Finally experimental results are shown.

We study the effects of test compaction on the defect coverage of test sets for modeled faults. Using a framework proposed earlier, defects are represented by surrogate faults. Within this framework, we show that the defect coverage does not have to be sacrificed by test compaction, if the test set is computed using appropriate test generation objectives. Moreover, two test sets, one compacted and one non-compacted, generated using the same test generation objectives, typically have similar defect coverages, even if the compacted one is significantly smaller than the uncompacted one. Test generation procedures and experimental results to support these claims are presented.

This paper presents a method to compact test sequences for full scan designed circuits by using the reduced scan shift and the retiming. The reduced scan shift, which we previously proposed, can compact test sequences by omitting unnecessary scan shifts. In this work, retiming, which repositions flip-flops, is introduced to enhance the effect of the reduced scan shift. When the number of flip-flops is reduced by the retiming, the test length is also reduced. Furthermore, this paper shows that the test length can be reduced even when the number of flip-flops is not reduced by the retiming. Under applying the reduced scan shift, the change of the control requirement to flip-flops by the retiming causes the reduction of scan shifts. Test vectors for the retimed circuit can be obtained by modifying the test vectors for the original circuit. Then the computing time to newly generate test vectors can be saved. Finally experimental results are given to show the effectiveness of the proposed method.

We propose a procedure to generate compact test sets for bridging faults under I/sub DDQ/ testing. Several techniques are employed to achieve compact test sets. Heuristics developed for stuck-at faults are shown to be effective in this context. The techniques especially designed for bridging faults are based on the observation that the yet-undetected faults can be represented using sets of lines and that a minimum test set size is obtained if the line sets representing yet-undetected faults are halved with every additional test vector. Logic blocks called bit-adders allow the partitioning of the line sets using a test generator for stuck-at faults, without having to determine in advance how the lines in a given set will be divided. Thus partitioning can be performed in a cost effective way for any line set size. Experimental results show that the test sets generated by the proposed procedure are smaller than those obtained by previously proposed procedures.

This paper presents a retiming and redundancy removal method for a sequential circuit with a specified initial state so that the resynthesized circuit has a state corresponding to the initial state and gives same behavior for any input sequences of the original circuit. Experimental results show the proposed method can optimize circuits as well as the method which does not consider the specified initial state.

This paper presents new cost-effective heuristics for the generation of minimal test sets. Both dynamic techniques, which are introduced into the test generation process, and a static technique, which is applied to already generated test sets, are used. The dynamic compaction techniques maximize the number of faults that a new test vector detects out of the yet-undetected faults as well as out of the already-detected ones. Thus, they reduce the number of tests and allow tests generated earlier in the test generation process to be dropped. The static compaction technique replaces N test vectors by M < N test vectors, without loss of fault coverage. During test generation, we also find a lower bound on test set size. Experimental results demonstrate the effectiveness of the proposed techniques.

This paper presents a new testing method for sequential circuits, called reduced scan shift, which generates short test sequences. In this method, only part of flip-flops close to the scan input line are controlled and another part of flip-flops close to the scan output line are observed by scan shift operations as small as possible. For the purpose of reducing scan shift operations, the following points are considered: (1) how to decide target faults which each test vector should detects, (2) how to arrange flip-flops in the scan chain, (3) how to decide the order of test vectors. Experimental results for ISCAS'89 benchmark circuits are given to show the effectiveness of this method.

This paper presents a method of test compaction for stuck-at faults in combinational circuits, that complements previously proposed methods and allows further reduction in test set size in a cost-effective way. A given test set is compacted by generating additional test vectors. Each test vector added allows the removal of two or more test vectors from the existing test set, thus reducing its size. Experimental results for benchmark circuits demonstrate the effectiveness of the method.>

Article Cost-effective generation of minimal test sets for stuck-at faults in combinational logic circuits Share on Authors: Seiji Kajihara View Profile , Irith Pomeranz View Profile , Kozo Kinoshita View Profile , Sudhakar M. Reddy View Profile Authors Info & Claims DAC '93: Proceedings of the 30th international Design Automation ConferenceJuly 1993 Pages 102–106https://doi.org/10.1145/157485.164617Online:01 July 1993Publication History 45citation336DownloadsMetricsTotal Citations45Total Downloads336Last 12 Months1Last 6 weeks0 Get Citation AlertsNew Citation Alert added!This alert has been successfully added and will be sent to:You will be notified whenever a record that you have chosen has been cited.To manage your alert preferences, click on the button below.Manage my AlertsNew Citation Alert!Please log in to your account Save to BinderSave to BinderCreate a New BinderNameCancelCreateExport CitationPublisher SiteGet Access

This paper presents a method for generating test sets to detect multiple stuck-at faults in combinational circuits. The proposed method is based on parallel vector pair analysis which finds faulty status of each line. The parallel analysis can bring higher fault coverage and smaller test sets than the serial vector pair analysis. In our method, redundant lines are removed firstly. An then a compacted test set for single stuck-at faults is generated in order to construct vector pairs used in the parallel analysis. Experimental results show that test sets to detect all multiple stuck-at faults were generated for 25 benchmark circuits and the parallel analysis was very effective for test generation.