Asynchronous circuits as alternative for mitigation of long-duration transient faults in deep-submicron technologies

The use of deeper-submicron technologies in integrated circuits worsens the effects of transient faults. In fact, the transient-fault durations become as important as the clock periods of synchronous circuits. Electronic systems are thus more vulnerable to failure situations. Nevertheless, this paper shows innovatively that such a worse scenario does not happen in asynchronous circuits. This additional novel benefit pushes on the asynchronous design as a better alternative to mitigate transient faults in deep-submicron technology-based circuits.     

Integrating physical level design and high level synthesis for simultaneous multi-cycle transient and multiple transient fault resiliency of application specific datapath processors

Radiation induced faults in digital systems have started gathering major attention in recent years due to increasing reliability concern for future technologies. For future technologies, multiple transient faults (MTF) originating from a single radiation hit are expected to occur more frequently. Further, due to continuous massive scaling in device geometry, a particle with moderate linear energy transfer (LET) values is expected to affect more than one module/device during striking. Additionally, incessant escalation in operating speed with evolution of technology has increased likelihood of multi-cycle transient (MCT) faults in digital circuits. This calls for novel solutions for concurrently tackling multi-cycle transient and multi-transient fault resiliency at a higher design abstraction level such as behavioral level. This paper proposes a novel approach for generating simultaneous multi-cycle transient and multiple transient fault resilient designs during high level synthesis (HLS) of application specific datapath processors using the framework of dual modular redundancy. Results of the proposed approach on benchmarks indicated generation of low cost MCT–MFT resilient designs during HLS within acceptable runtime.     

Defect-oriented testability for asynchronous ICs

For a CMOS manufacturing process, asynchronous ICs are similar to synchronous ICs. The defect density distributions are similar, and hence, so are the fault models and fault-detection methods. So, what makes us think that asynchronous circuits are much harder to test than synchronous circuits? Because the effectiveness of best known test methods for synchronous circuits drops when applied to asynchronous circuits? They may very well be a temporal hurdle. Many test methods have already been reevaluated and successfully adapted from the synchronous to the asynchronous test domain. The paper addresses one of the final hurdles: I_DDQ testing. This type of test method, based on measuring the quiescent power supply current, is very effective for detecting (resistive) bridging faults in CMOS circuits. Detection of bridging faults is crucial, because they model the majority of today's manufacturing defects. I_DDQ fault effects are sensitized in a particular state or set of states and can only be detected if we stop the circuit operation right there. This is a problem for asynchronous circuits, because their operation is self-timed. In the paper, we quantify the impact of self timing on the effectiveness of I_DDQ -based test methods for bridging faults, and propose a Design-for-Test (DfT) approach to develop a low-cost DfT solution. For comparison, we do the same for logic voltage testing and stuck-at faults. The approach is illustrated on circuits from Tangram, the asynchronous design-style employed at Philips Research, but it is applicable to asynchronous circuits in general     

Improving path delay testability of sequential circuits

We analyze the causes of low path delay fault coverage in synchronous sequential circuits and propose a method to improve testability. The three main reasons for low path delay fault coverage are found to be: (A) combinationally false (nonactivatable) paths; (B) sequentially nonactivatable paths; and (C) unobservable fault effects. Accordingly, we classify undetected faults in Groups A, B, and C. Combinationally false paths ran be made testable by modifying the circuit or resynthesizing the combinational logic as discussed by other researchers. A majority of the untestable faults are, however found in Group B, where a signal transition cannot be functionally propagated through a combinational path. A test requires two successive states necessary to create a signal transition and propagate it through the target path embedded in the sequential circuit. We study a partial scan technique in which flip-flops are scanned to break cycles and shun that a substantial increase in the coverage of path delay faults is possible     

Design error diagnosis in digital circuits with stuck-at fault model

In this paper we describe in detail a new method for the single gate-level design error diagnosis in combinational circuits. Distinctive features of the method are hierarchical approach (the localizing procedure starts at the macro level and finishes at the gate level), use of stuck-at fault model (it is mapped into design error domain only in the end), and design error diagnostic procedure that uses only test patterns generated by conventional gate-level stuck-at fault test pattern generators (ATPG). No special diagnostic tests are used because they are much more time consuming. Binary decision diagrams (BDD) are exploited for representing and localizing stuck-at faults on the higher signal path level. On the basis of detected faulty signal paths, suspected stuck-at faults at gate inputs are calculated, and then mapped into suspected design error(s). This method is enhanced compared to our previous work. It is applicable to redundant circuits and allows using incomplete tests for error diagnosis. Experimental data on ISCAS benchmark circuits shows the advantage of the proposed method compared to the known algorithms of design error diagnosis.     

Designing robust threshold gates against soft errors

In applications where issues like power efficiency, high performance, and more noise tolerance are important, asynchronous design methodology can play a significant role. However, as a result of technology shrinkage, combinational asynchronous circuits have become vulnerable in presence of particle strikes. In this paper, we design robust quasi-delay insensitive (QDI) asynchronous circuits against soft errors. Null Convention Logic (NCL) gates used as one of the basic techniques in asynchronous circuits, are redesigned to increase their robustness against Single Event Upset (SEU). We analyze our design for various NCL structures and compare them with another design in Kuang et al. (2007) [4], and show that our proposed approach is more robust against SEU. The effect of some parameters such as power consumption, delay, and the influence of transistor sizing on soft error tolerance are discussed.     

False-Path Removal Using Delay Fault Simulation

Some false paths are caused by redundant stuck-at faults. Removal of those stuck-at faults automatically eliminates such false paths from the circuit. However, there are other false paths that are not associated with any redundant stuck-at fault. All segments of such a false path are shared with other testable paths. We focus on the elimination of this type of false paths. We use a non-enumerative path delay fault simulator based on the path status graph (PSG) data-structure, which duplicates selected gates to separate the detected and undetected path delay faults. The expanded circuit may contain new redundant stuck-at faults, corresponding to those undetected paths that are false. This happens because the expanded circuit has some new interconnects with only false paths passing through them. Such links become the sites for redundant stuck-at faults. Removal of these redundant faults eliminates false paths. The reported results show that the quality of the result may depend on the coverage of testable paths by the vectors that are simulated. When non-enumerative path delay simulation and implication-based redundancy removal techniques are used, the present procedure of false-path elimination can be applied to very large circuits.     

Characterizing,modeling, and analyzing soft error propagation in asynchronous and synchronous digital circuits

Soft errors, due to cosmic radiations, are one of the major challenges for reliable VLSI designs. In this paper, we present a symbolic framework to model soft errors in both synchronous and asynchronous designs. The proposed methodology utilizes Multiway Decision Graphs (MDGs) and glitch-propagation sets (GP sets) to obtain soft error rate (SER) estimation at gate level. This work helps mitigate design for testability (DFT) issues in relation to identifying the controllable and the observable circuit nodes, when the circuit is subject to soft errors. Also, this methodology allows designers to apply radiation tolerance techniques on reduced sets of internal nodes. To demonstrate the effectiveness of our technique, several ISCAS89 sequential and combinational benchmark circuits, and multiple asynchronous handshake circuits have been analyzed. Results indicate that the proposed technique is on average 4.29 times faster than the best contemporary state-of-the-art techniques. The proposed technique is capable to exhaustively identify soft error glitch propagation paths, which are then used to estimate the SER. To the best of our knowledge, this is the first time that a decision diagram based soft error identification approach is proposed for asynchronous circuits.     

Testing of Synchronizers in Asynchronous FIFO

This paper presents a test method for testing two-D-flip-flop synchronizers in an asynchronous first-in-first-out (FIFO) interface. A faulty synchronizer can have different fault behaviors depending on the input application time, the fault location, the fault mechanism, and the applied clock frequency. The proposed test method can apply the input patterns at different time and generate capture clock signals with different frequency regardless of phase-locked loop (PLL) of the design. To implement the proposed test method, channel delay compensator, delayed scan enable signal generator, launch clock generator, and capture clock generator are designed. In addition, a well-designed calibration method is proposed to calibrate all programmable delay elements used in the test circuits. The proposed test method evolves to several test sections to detect all possible faults of the two-D-flip-flop synchronizers in the asynchronous FIFO interface.     

Concurrent Delay Testing in Totally Self-Checking Systems

Prompt detection of even small delay faults, sometimes before causing critical paths to fail, gains importance since stricter test quality requirements for high performance and high density VLSI circuits have to be satisfied in critical applications. This can be achieved by using concurrent delay testing.In this paper a novel idea for concurrent detection of two-rail path delay faults is introduced. It is shown that TSC two-rail code error indicators that monitor pairs of paths with similar propagation delays can be used for concurrent delay testing. Our technique is applied to TSC two-rail code checkers as well as to duplication systems which are the most widely used TSC systems. The design of TSC two-rail code checkers and TSC duplication systems with respect to two-rail path delay faults is achieved for first time in the open literature.     

Classification and Test Generation for Path-Delay Faults Using Single Struck-at Fault Tests

We classify all path-delay faults of a combinational circuit intothree categories: singly-testable (ST), multiply-testable (MT), and singly-testable dependent} (ST-dependent). The classification uses anyunaltered single stuck-at fault test generation tool. Only two runsof this tool on a model network derived from the original network areperformed. As a by-product of this process, we generate single andmultiple input change delay tests for all testable faults. With thesetests, we expect that most defective circuits are identified. All STfaults are guaranteed detection in the case of a single fault, andsome may be guaranteed detection through robust and validatablenon-robust tests even in the case of multiple faults. An ST-dependentfault can affect the circuit speed only if certain ST faults arepresent. Thus, if all ST faults are tested, the ST-dependent faultsneed not be tested. MT faults cannot be guaranteed detection, butaffect the speed only if delay faults simultaneously exist on a setof paths, none of which is ST. Examples and results on several ISCAS89 benchmarks are presented. The method of classification throughtest generation using a model network is complex and can be appliedto circuits of moderate size. For larger circuits, alternativemethods will have to be explored in the future.     

Line coverage of path delay faults

We propose a new coverage metric for delay fault tests. The coverage is measured for each line with a rising and a falling transition, but the test criterion differs from that of the slow-to-rise and slow-to-fall transition faults. A line is tested by a line delay test, which is a robust path delay test for the longest sensitizable path producing a given transition on the target line. Thus, the test criterion resembles path delay test and not the gate or transition delay test. Yet, the maximum number of tests (or faults) is limited to twice the number of lines. In a two-pass test-generation procedure, we first attempt delay tests for a minimal set of longest paths for all lines. Fault simulation is used to determine the coverage metric. For uncovered lines, in the second pass, several paths of decreasing lengths are targeted. We give results for several benchmark circuits     

Universal delay test sets for logic networks

It has been shown earlier that, if we are restricted to unate gate network (UGN) realizations, there exist universal test sets for Boolean functions. Such a test set only depends on the function f, and checks any UGN realization of f for all multiple stuck-at faults and all robustly testable stuck-open faults. In this paper, we prove that these universal test sets are much more powerful than implied by the above results. They also constitute complete delay fault test sets for arbitrary UGN implementations of a given function. This is even true for UGN networks which are not completely testable with respect to the gate or path delay fault model. Our ability to prove the temporal correctness of such circuit realizations comes from the fact that we do not argue the correctness of individual paths, but rather complete path systems     

Enabling testability of fault-tolerant circuits by means ofIDDQ-checkable voters

The reliability of a fault-tolerant circuit may be drastically impaired by the presence of maskable faults that never affect its functionality. Design for testability (DFT) techniques have to be applied to make maskable faults detectable. During the testing phase, traditional DFT schemes inhibit fault masking and/or activate additional observation/control paths through the circuit. Such schemes, however, do not enable on-line testing and cannot be applied to multilevel fault-tolerant circuits, where fault-masking is repeatedly performed inside the circuit. We propose a new approach to the design of testable fault-tolerant CMOS circuits that overcomes both limitations. Our approach is based on the use of I_DDQ-checkable voters (ICVs) that enable a complete test of maskable faults of any multiplicity during normal operations     

Diagnosis Framework for Locating Failed Segments of Path Delay Faults

Diagnosis tools can be used to speed up the process for finding the root causes of functional or performance problems in a VLSI circuit. In this paper, we propose a method to locate possible segments that cause extra delays on circuit paths. We use the delay bounds of the tested paths to build linear constraints. By guiding the solutions of the linear constraints solved by a linear programming solver, we can identify segments with extra delays. Also, with the ranks of segment delays, we can prioritize the search for possible locations of failed segments. Besides, we also propose to reduce the search space by identifying indistinguishable segments. Essentially, we cannot separate segments in the same category no matter which segments have faults. This approach greatly increases the efficiency of the diagnosis process. Three main features of the proposed method are that: 1) it does not assume any delay fault model; 2) it derives diagnosis results directly from test data; and 3) it is able to diagnose failures caused by multiple delay defects. These features make our proposed method more realistic on solving the real problems occurring in the manufacturing process. In the experimental results, for most cases of injecting 5% of the longest path delay, the probabilities are over 90% for locating faulty segments within the list of top-ten suspects, and the average rankings, that is often referred to as first hit rank (FHR), which is defined as the rank of the first hit of the defect in the ranking list, are among the top five suspect locations for single fault injection. In the experimental results of multiple faults injection, the average FHRs are also lower than 5 for all cases of injecting 1% of the longest path delay.      

An advanced diagnostic method for delay faults in combinational faulty circuits

Due to physical defects or process variations, a logic circuit may fail to operate at the desired clock speed. So, verifying the timing behavior of digital circuits is always necessary, and needs to test for delay faults. When a delay fault has been detected, a specific diagnostic method is required to locate the site of the fault in the circuit. So, a reliable method for delay fault diagnosis is proposed in this paper. Firstly, we present the basic diagnostic method for delay faults, which is based on multivalued simulation and critical path tracing. Next, heuristics are given that decrease the number of critical paths and improve diagnosis results. In the second part of this paper, we provide an approximate method to refine the results obtained with the basic diagnostic process. We compute the detection threshold of the potential delay faults, and use statistical studies to classify the faults from the most likely to be the cause of failure to the less likely. Finally, results obtained with ISCAS'85 circuits are presented to show the effectiveness of the method.     

Min-max timing analysis and an application to asynchronous circuits

Modern high-performance asynchronous circuits depend on timing constraints for correct operation, so timing analyzers are essential asynchronous design tools. In this paper we present a 13-valued abstract waveform algebra and a polynomial-time min-max timing simulation algorithm for use in efficient, approximate timing analysis of asynchronous circuits with bounded component delays. Unlike several previous approaches, our algorithm computes separate propagation delay bounds from each circuit input to each internal gate. This is useful for analyzing asynchronous circuits, where the relative transition times of the inputs may not be known a priori, unlike synchronous circuits. We also describe an efficient reconvergent fanout analysis technique that helps in increasing the accuracy of simulation. We have applied our algorithm to build an efficient timing analysis tool for extended burst-mode circuits (a class of timing-dependent asynchronous circuits) implemented in the 30 design style. Our tool analyzes gate-level 30 circuits assuming bounded component delays and determines safe timing constraints for correct operation. Although our results represent conservative approximations to the true timing requirements in the worst case, experiments indicate that our technique is efficient and fairly accurate in practice     

Resynthesis of Combinational Circuits for Path Count Reduction and for Path Delay Fault Testability

Path delay fault model is the most suitable model for detectingdistributed manufacturing defects which can cause delayfaults. However, the number of paths in a modern design can beextremely large and the path delay testability of many practicaldesigns is very low. In this paper we show how to resynthesize acombinational circuit in order to reduce the total number of paths inthe circuit. Our results show that it is possible to obtain circuitswith a significant reduction in the number of paths while notincreasing area and/or delay of the longest sensitizable path in thecircuit.Research on path delay testing shows that in many circuits a largeportion of paths does not have a test that can guarantee detection ofa delay fault. The path delay testability of a circuit would increaseif the number of such paths is reduced. We show that addition of asmall number of test points into the circuit can help reducing thenumber of such paths in the given design.