A Scan Partitioning Algorithm for Reducing Capture Power of Delay-Fault LBIST

Testing for delay faults in heavily gated clock designs has the major test challenges of reduced fault coverage and high test power consumption. In the scan-test method, gated clocks are often simplified and replaced with global test clocks. As such, partial clocking by the gated clocks is not inherited in test operations. Global clocking suffers from delay fault coverage loss because a sensitization state cannot easily be created due to the increased state dependence in functional paths, as compared to partial clocking. The global clocking scheme in the test mode is not adequate for low-power designs either, because the power consumed during a test operation exceeds that used during a normal operation. The power grid may not be sufficient to support the power drawn during testing, perhaps resulting in overkilled devices. It is therefore critical that power consumption be maintained under a safe limit, even during testing. In the proposed method, partial clocking in gated designs is preserved to the maximum possible to create more reachable states, thereby increasing transition fault coverage and reducing test power during launch and capture cycles. A transition fault simulator was developed, and it demonstrated higher transition fault coverage and reduced test power for ISCAS-89 circuits when partial clocking is used.

Abstract—Excessive test power consumption is one of the obstacles which the chip industry currently faces. Peak capture power reduction typically leads to high pattern counts which increase test costs. This paper proposes a new methodology to reduce peak capture power during at-speed scan testing. In this method, a novel X-filling technique Opt-Justification-fill which uses optimization techniques to compute promising X-bits for low-power filling is proposed. This method is tightly integrated into a dynamic compaction flow to create silent test cubes with high compaction ability. By this, X-filling for fault detection and reducing switching activity is balanced. The proposed methodology can be applied during initial compact test set generation as well as a post-ATPG stage for a previously generated test set to reduce switching activity. Experiments show a significant reduction of peak capture power. At the same time, the pattern count increases only moderately which leads to reduced test costs. I.

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

Power management is posing serious challenges for scan-based testing. In this paper, we propose a low power test pattern generation technique which minimizes the peak power consumption associated with the scan and capture operations. Given a set of fully specified test patterns, the proposed technique iteratively replaces the high power consumption patterns with low power ones generated by a PODEM-based low power ATPG. The proposed technique has been validated using ISCAS89 benchmark circuits. Compared to a commercial ATPG using high merge ratio and random-fill options, the proposed technique reduces the peak shift and capture power by 27.3% and 19.6%, respectively, and the average power by 49.9%.

Test compression has become a de facto technique in VLSI testing. Meanwhile, excessive capture power of at-speed testing has also become a serious concern. Therefore, it is important to co-optimize test power and compression ratio in at-speed testing. In this paper, a novel X-filling framework is proposed to reduce capture power of both LoC and LoS at-speed testing, which is applicable for different test compression schemes. The proposed technology has been validated by the experimental results on larger ITC'99 benchmark circuits.

This paper presents a novel approach for selecting optimal pseudo random and deterministic test patterns and minimizing test time for multi-clock domain SoCs based on a hybrid BIST architecture for each core. For test scheduling, a concurrent method considering peak power upper bound is used. A test scheduling graph is presented for modeling concurrent hybrid BIST test scheduling. Furthermore, a heuristic is proposed for selecting cores to be tested concurrently and the order of applying sequence of test patterns to each core. Experimental results show that the proposed heuristics for both selecting groups of cores to be tested concurrently during the SoC test process, and determining the amount of deterministic and pseudo random test patterns for each core, give us an optimized method for multi clock domain SoC testing compared with the existing methods.

The use of deep sub-micron technology raises a number of concerns about reliability in VLSI circuits. Shrinking geometries and reduced power supplies leave the circuits vulnerable to 'soft' and transient errors. The combination of high clock speed and large circuit area result in high power consumption and skew in clock distribution. This paper investigates the use of concurrent error detection (CED) and asynchronous design to overcome these problems. Four pipelined processor designs are compared - two synchronous, two asynchronous with one of each type using CED. Initial results indicate an area overhead of 12% in return for a fault coverage of 98.54% of all unidirectional errors. Additionally, the asynchronous CED processor has an area overhead of only 4% when compared to the synchronous non-CED design.

A strong dl/dt event in a VLSI circuit can induce a temporary voltage drop and consequent malfunctioning of logic as for instance failing speed paths. This event, called power droop, usually manifests itself in at-speed scan test where a surge in switching activity (capture phase) follows a period of quiescent circuit state (shift phase). Power droop is also present during mission mode operation. However, because of the less predictable occurrence of the switching events in mission mode, usually the values of power droop measured during test are different from those measured in mission mode. To overcome the power droop problem, different mitigation techniques have been proposed. The goal of these techniques is to create a uniform current demand throughout the test. This paper proposes a feedback based droop mitigation technique which can adapt to the droop by reading the level of VDD and modifying real time the current flowing on ad-hoc droop mitigators. It is shown that the proposed solution not only can compensate for droop events occurring during test mode but also can be used as a method of mission mode droop mitigation and yield enhancement if higher power consumption is acceptable.

At-speed scan testing, based on ATPG and ATE, is indispensable to guarantee timing-related test quality in the DSM era. However, at-speed scan testing may incur yield loss due to excessive IR-drop caused by high test (shift & capture) switching activity. This paper discusses the mechanism of circuit malfunction due to IR-drop, and summarizes general approaches to reducing switching activity, by which highlights the problem of current solutions, i.e. only reducing switching activity for one capture while the widely used at-speed scan testing based on the launch-off-capture scheme uses two captures. This paper then proposes a novel X-filling method, called double-capture (DC) X-filling, for generating test vectors with low and balanced capture switching activity for two captures. Applicable to dynamic & static compaction in any ATPG system, DC X-filling can reduce IR-drop, and thus yield loss, without any circuit/clock modification, timing/circuit overhead, fault coverage loss, and additional design effort.

To guarantee that an application-specific integrated circuit (ASIC) meets its timing requirement, at-speed scan testing becomes an indispensable procedure for verifying the performance of ASIC. However, at-speed scan test suffers the test-induced yield loss. Because the switching-activity in test mode is much higher than that in normal mode, the switching-induced large current drawn causes severe IR drop and increases gate delay. X-filling is the most commonly used technique to reduce IR-drop effect during at-speed test. However, the effectiveness of X-filling depends on the number and the characteristic of X-bit distribution. In this paper, we propose a physical-location-aware X-identification which redistributes X-bits so that the maximum switching-activity is guaranteed to be reduced after X-filling. We estimate IR-drop using RedHawk tool and the experimental results on ITC'99 show that our method has an average of 9.42% more reduction of maximum IR-drop as compared to a previous work which redistributes X-bits evenly in all test vectors.

At-speed scan testing has become mandatory due to the extreme CMOS technology scaling. The two main at-speed scan testing schemes are namely Launch-Off-Shift (LOS) and Launch-Off-Capture (LOC). As it can be easily implemented, LOC has been widely investigated in the literature in the last few years, especially regarding test power consumption. Conversely, LOS has received much less attention. In this paper, we propose a comparison between the two testing schemes in terms of transition fault coverage and power consumption, in order to quantify the pros and cons of LOS with respect to LOC. This study shows that LOS not only exhibits higher performance in coverage but also does not require as much extra power as predicted, especially in terms of peak power. These facts may represent convincing arguments for its wider use and development.

A parallel test generation algorithm, ProperTEST, for sequential circuits that is portable across a range of MIMD parallel architectures is discussed. It uses prioritized execution to ensure consistent speedups as the number of processors is increased. This consistency is achieved without loss of fault coverage with increase in the number of processors. This also permits the use of parallel processing to improve the fault coverage when the execution time is bounded. Results on ISCAS 89 benchmark programs are provided on a shared memory machine, a message passing machine, and a network of workstations. ProperTEST was run unchanged on these different architectures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

In this paper, we propose a space compression technique for digital circuits with the objective of minimizing the storage for the circuits under test while maintaining the fault coverage information. The technique introduced is called a Modified Dynamic Space Compression method. For a circuit under test, a compaction tree is generated based on its structure. The detectable error probability was calculated by using the Boolean Difference Method. The output data modification was employed to minimize the number of faulty output data patterns which have the same compressed form as the fault-free patterns. The compressed outputs were then fed into a syndrome counter to derive the signature for the circuit. Simulations were performed on known combinational circuits and the results indicate that the loss in fault coverage caused by compression is rather small.

This paper addresses the problem of increasing unknowns in the output response data by exploring reiterative LFSR reseeding based X-masking. This approach takes advantage of the data correlation in the output response data to enable LFSR encoded masks to be reused for multiple scan slices while guaranteeing that all unknowns are masked and all bits required for fault detection are allowed to propagate to the compactor. This paper also investigates a hybrid approach that combines conventional LFSR reseeding based X-masking with fixed-interval reiterative LFSR X-masking. Experimental results for applying this approach to industrial designs indicate that significant amounts of output compression can be achieved without any pattern count inflation or loss in fault coverage.

Test compaction procedures to reduce the test application time for scan designs terminate when they cannot reduce the test application time without reducing the fault coverage. We propose a procedure for placing observation points that allows higher levels of compaction to be achieved without loss of fault coverage. The observation point values are read only at the last time unit of every test, and therefore, they can be scanned-out at the same time as the next-state values.