A Literature Review and Over View of Built in Self Testing in VLSI

Transcription

1 Volume-5, Issue-4, August-2015 International Journal of Engineering and Management Research Page Number: A Literature Review and Over View of Built in Self Testing in VLSI Jalpa Joshi 1, Prof. Jaikaran Singh 2 1 Electronics & Communication. VLSI, SSSIST, Sehore, INDIA 2 Electronics and Communication, SSSIST, Sehore, INDIA ABSTRACT Now a day due to the technology advancement the number of component on a single chip is increased and that s why complexity of the circuit is also increased day by day. Due to this the problem of fault is also increased. So detection and elimination of the faults in very large scale integrated system (VLSI) is the major factor. For the analysis of fault we have required Testing. Testing is process in which we have testing the circuit improve the fault tolerance in circuits. In the last decade there are different have been introduced for testing here in the paper we focused one of the most famous technique that is the built in self testing (BIST). Built in self testing is technique that is used different circuit testing. There are many improvement in the field of BIST. So in this we have to the working phenomena of BIST, and its different technique also with the application of BIST. The main focus is done on BIST, online and offline method of detection and elimination of faults. Keywords BIST, VLSI, UART, Testing, Circuit fault. I. INTRODUCTION It is necessary to ensure the fault free about the circuit in all type of circuits system like small scale integration system (SSI) large scale integration system (LSI) and also in very large scale integration system (VLSI) before finalizing the circuit. For this fault detection and elimination is done many number of times in circuits. Detection of fault and the type of fault present in a circuit is known as fault diagnosis. Some processes are done during manufacturing and some are during use of the system. It is also necessary to care about the time taking in detection. In this paper many works done related to testing and elimination of faults are reviewed. We are mainly focusing on BIST method of detection of faults which is most easy and suitable method. By using the BIST no extra hardware is required for testing the circuit that is main advantage of BIST. For this only an extra BIST chip is introduced in the circuit, itself. It also helps in saving the time. But BIST increases the internal complexity of the circuit that is the main problem with this. Fault tolerant circuits are receiving attention in many major application stores. In the case of complicated fault tolerant systems, fault injection has been recognized to be significantly attractive and valuable methodology wherever faults are inserted into a system and supervising the system to look at its function in response to a fault. This paper evaluates digital circuits employing a methodology where fault injection may be used among VHDL description. Also discuss on the design of a UART chip with embedded BIST architecture using simple LFSR with the help of VHDL language. The paper describes the problems of (VLSI) testing followed by the behavior of UART that includes both transmitter and receiver section using VHISC Hardware Description Language (VHDL). Similarly, in high-speed circuits that process digital audio and video signals, the inputs to most of those modules change relatively slowly. In fact, designers of low-power circuits take advantage of this consistent behavior when they determine a circuit s thermal and electrical limits, and system packaging requirements. In contrast, there is no definite correlation between the successive test patterns generated by an automatic testpattern generator for external testing or the patterns produced by a linear feedback shift register (LFSR) for built-in self-test (BIST). This lack of correlation can result in significantly greater switching activity in the circuit during test than during normal operation. Because power dissipation in CMOS circuits is proportional to switching activity, test s excessive switching activity can cause catastrophic 390 Copyright Vandana Publications. All Rights Reserved.

2 problems, as detailed later. Although academic research on low-power design remains nearly independent of that for test, industrial practice requires ad hoc solutions for considering power consumption during test application.[7] Practiced solutions include over sizing power supply, package, and cooling to withstand the increased current during testing (test engineers insert breaks into the test process to avoid hot spots);testing with reduced operating frequency; and system-under-test partitioning and appropriate test planning. The initial solution increases both hardware costs and test time. Although the second proposal uses less hardware, the reduced frequency increases test time and might lead to a loss of defect coverage because the reduced frequency can mask dynamic faults. Moreover, this solution reduces power consumption but lengthens test time, so it does not reduce the total energy consumed during test. The third solution of test partitioning and test planning detects dynamic faults, but increases hardware costs and test time. To provide an adequate response to these industrial needs, various researchers have proposed solutions for power problems encountered during test. [1-3] II. TYPE OF FAULT IN IC When we talk about the fault in integrated circuit. There are mainly two types of faults. The first one is the Temporary fault and one is the Permanent fault. Temporary faults are again of two types- (i) Transient fault and (ii) intermittent fault. Transient fault are appears for short duration of time which disappear itself after some duration. Intermittent faults are detected by online method while the permanent faults are detected by offline method. In another way of classification of faults there are several types of faults, for example bridging fault, stuck-at fault, stuck-open and stuck-short faults, delay fault. A. Bridging Fault: In bridging faults short circuit occurs between groups of signals. This may cause an OR bridge or an AND bridge which are ones dominant or zeros dominant respectively. These faults can be found with tests used for finding stuck-at-faults. B. Stuck-at-fault: This fault occurs when a signal line within a circuit is permanently set to either logic high 1 or a logic low 0. This fault model does not have a specific cause; rather, it is an abstract fault model with numerous causes. C. Stuck-Open and Stuck-Short Faults: This fault is defined as a single transistor that has either been stuck open, no current will ever pass through, or stuck short, lacking the ability to stop current. Like bridge faults, the stuck-open faults can be detected by running a sequence of stuck-at fault detection vectors. D. Delay Fault: This fault is active when the combinational logic delay exceeds the specified clock period. The variation in the manufacturing process can cause certain portions of a circuit to be slower than other parts of the circuits causing internal signals to arrive at different times and cause functional failure. The fault is usually modeled as either a gate delay fault model where a single gate is assumed responsible for producing the slow response or a path delay fault model where certain interconnects and paths are responsible for slow propagation of a signal. E. Motivation for testing: Testing has become an important issue in the production process of each electronic system, board or VLSI chip. Design for test is increasingly being preferred over tricky ad-hoc design solutions, and all major electronic companies spend a considerable proportion of production cost and engineering resources for testing. The motivation for this becomes obvious from a more global point of view: Although testing incurs a lot of efforts it is finally an important means to reduce overall cost significantly. This is illustrated by the following examples: While the actual material cost is only a negligible proportion of the product value, the cost of repair increases by a factor of 10 with each production stage [1]. It is much cheaper to reject several dies than to locate and exchange a defective chip in a complete system. As a consequence no customer is willing to bear the risk of using defective components and therefore (a) only accepts suppliers that guarantee low defect rate and (b) often performs an incoming test for supplied parts. Low defect rate of the product can be guaranteed by extensive outgoing product tests only. VLSI chips have reached an enormous complexity, and still their density doubles every 2 years [2]. This makes it impossible to rule out faults during design and production, even with the best design tools and fabrication processes available. However, short time-tomarket is critical to profitability. If testing facilitates rapid diagnosis and thus provides a means to avoid fatal production delays resulting from excessive debug time or shipping defective products, it is worth the additional cost. 391 Copyright Vandana Publications. All Rights Reserved. III. BUILT IN SELF-TEST Till now we have been trying into VLSI testing, solely from the context wherever the circuit has to be place to a test mode for validating that it is free of faults. Following that, the circuits tested OK are shipped to the clients with the idea that they would not fail within their expected life time; it can be known as off-line testing. In other words, in off-line testing, a circuit is tested once and for all, with the hope that once the circuit is verified to be fault free it would not fail throughout its expected lifetime. However, this assumption does not hold for modern day ICs, based on deep sub-micron technology, as a result of they will develop failures even during operation within

3 expected life time. To cater to this problem sometimes redundant circuitry are kept on-chip which replace the faulty parts. To enable replacement of faulty circuitry, the ICs are tested before each time they startup. If a fault is found, a part of the circuit (having the fault) is replaced with a corresponding redundant circuit part (by readjusting connections). Testing a circuit every time before they startup, is called Built-In-Self-Test (BIST) A. General Bist Architecture: BIST is a design-for-testability technique that places the testing functions physically with the circuit under test (CUT), as illustrated in Figure 1. The basic BIST architecture requires the addition of three hardware blocks to a digital circuit: a Circuit under Test (CUT), TPG (test pattern generator), a response analyzer, and a test controller. B. Circuit under Test (CUT): It is the portion of the circuit tested in BIST mode. It can be sequential, combinational or a memory. It is delimited by their Primary Input (PI) and Primary Output (PO). C. Test Pattern Generator (TPG): It generates patterns for the CUT. It is a dedicated circuit or a microprocessor. The patterns may be generated in pseudorandom or deterministically. D. Test Response Analysis (TRA): It analyses the value sequence on PO and compares it with the expected output. E. BIST Controller Unit (BCU): It controls the test execution; it manages the TPG, TRA and reconfigures the CUT and the multiplexer. IV. TESTING PARAMETER Test power is a possible major engineering problem in the future of development. As both the designs and the deep-submicron geometry become prevalent, larger designs, tighter timing constraints, higher operating frequencies, and lower applied voltages all affect the power consumption systems of silicon devices. [4] A. Energy The total switching activity generated during test application, energy affects the battery lifetime during power up or periodic self-test of battery-operated devices. B. Average Power Average power is the total distribution of power over a time period. The ratio of energy to test time gives the average power. Elevated average power increases the thermal load that must be vented away from the device under test to prevent structural damage (hot spots) to the silicon, bonding wires, or package. C. Instantaneous Power Instantaneous power is the value of power consumed at any given instant. Usually, it is defined as the power consumed right after the application of a synchronizing clock signal. Elevated instantaneous power might overload the power distribution systems of the silicon or package, causing brown-out. D. Peak Power The highest power value at any given instant, peak power determines the component s thermal and electrical limits and system packaging requirements. If peak power exceeds a certain limit, designers can no longer guarantee that the entire circuit will function correctly. In fact, the time window for defining peak power is related to the chip s thermal capacity, and forcing this window to one clock period is sometimes just a simplifying assumption. For example, consider a circuit that has a peak power consumption during only one cycle but consumes power within the chip s thermal capacity for all other cycles. In this case, the circuit is not damaged, because the energy consumed which corresponds to the peak power consumption times one cycle will not be enough to elevate the temperature over the chip s thermal capacity limit (unless the peak power consumption is far higher than normal). V. TYPE OF TESTING A. Bit-Swapping Algorithm for Low-Power BIST A modified linear feedback shift register (LFSR) is presented that reduces the number of transitions at the inputs of the circuit-under test by 25% using a bitswapping technique. They also show that the proposed design can be combined with other techniques to achieve a very substantial power reduction of up to 63%. An important application of Linear Feedback Shift Registers (LFSRs) is in the generation of test vectors for digital circuits. This is because, with little overhead in the hardware area, a normal register can be configured to work as a test generator and, with an appropriate choice of the tap sequence (XOR locations), the LFSR can generate all possible output test vectors. Furthermore, the pseudorandom behavior of the LFSR reduces the correlation between successive test vectors, which means that it can achieve a high fault coverage in a relatively short run of test vectors. However, this lack of correlation substantially increases the Weighted Switching Activity (WSA) within the Circuit- Under-Test (CUT). This often causes the power consumed during test mode operation to be much higher than during normal mode operation, which can lead to problems with battery lifetime and system reliability. B. Adaptive Shift Power Control technique To reduce the scan shift power consumption in logic BIST by using highly correlated test stimulus bits among adjacent scan cells, all existing methods only manipulate test stimulus sequences generated by LFSR in various ways and the test responses are ignored completely. Although it has been observed that the Hamming distance between a test stimulus and its captured test response is typically small, the test stimulus of a test 392 Copyright Vandana Publications. All Rights Reserved.

4 pattern is loaded into the scan chains at the same time as the test response of the previous test pattern is unloaded from the scan chains. Therefore, they are irrelevant. As a result, the scenarios listed below may happen when shifting the test stimulus and the test response simultaneously: The number of transitions contributed by the test response is much larger than that contributed by the test stimulus. [27] C. Increasing Encoding Efficiency of LFSR Reseeding- Based Test Compression Usually, the deterministic test set to be encoded by LFSR reseeding tends to have a biased probability for the logic value 1 or 0 at each primary input. The biased inputs are fixed to the logic value 1 or 0 with some combinational logic, so that the amount of data to be encoded by the LFSR can considerably be reduced. The combinational logic for bit fixing has to set some primary input to the logic value 0 (or 1), if the corresponding probability of the logic value 0 (or 1) is one. Otherwise, the test pattern from the pseudorandom test pattern generator, such as an LFSR, is directly applied to the CUT. Fig 5 shows example of applying the bit fixing scheme. In contrast to the original bit-fixing technique in, the bitfixing scheme in this application fixes bits for the complete test set so that the bit-fixing sequence generator is controlled only by a bit counter. The bit positions 0 and 3are biased to the logic values 0 and 1, respectively, so they can be fixed. [28] VI. CONCLUSION This paper reviews the various faults in digital and analog circuits and also the elimination of those. But we have mainly focused on BIST techniques because this method is useful for detecting faults and also it takes less time as well as required less hardware. Using memory BIST has various advantages such as no external test equipment, reduced development efforts, at-speed tests. However, there are many challenges associated with it such as silicon area overhead, extra pins and routing. In addition, the testability of the test hardware itself is another difficult task. Also analysis the qualitative survey on low power testing techniques and its methodology was carried out. While analyzing, all dimensions of power during chip testing was considered as parameters. Further this paper gives a survey not only on algorithmic side but also on hardware approaches. REFERENCES [1] W. Y. Zorian, A Distributed BIST Control Scheme for Complex VLSI Devices, Proc. 11th IEEE VLSI Test Symp.(VTS 93), IEEE CS Press, Los Alamitos,Calif., 1993, pp.4-9. [2] M.L. Bushnell and V.D. Agrawal, Essentials of Electronic Testing, Kluwer Academic, Boston, 2000, p. 14. [3] S. Wang and S.K. Gupta, DS-LFSR: A New BISTTPG for Low Heat Dissipation, Proc. Int l Test Conf. (ITC 97), IEEE Press, Piscataway, N.J., 1997, pp [4] B. Pouya and A. 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