In this module we will study automatic test pattern generation (ATPG) using sensitization–propagation -justification approach. We will first introduce the basics of. 1. VLSI Design Verification and Testing. Combinational ATPG Basics. Mohammad Tehranipoor. Electrical and Computer Engineering. University of Connecticut. Boolean level. • Classical ATPG algorithms reach their limits. ➢ There is a need for more efficient ATPG tools! 6. Circuits. • Basic gates. – AND, OR, EXOR, NOT.
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The generated patterns are used to test semiconductor devices after manufacture, or to assist with determining the cause of failure failure analysis . The effectiveness of ATPG is measured by the number of modeled defects, or fault modelsdetectable and by the number of generated patterns.
These metrics generally indicate test quality higher with more fault detections and test application time higher with more patterns. ATPG efficiency is another important consideration that is influenced by the fault model under consideration, the type of circuit under test full scansynchronous sequential, or asynchronous sequentialthe level of abstraction used to represent the circuit under test gate, register-transfer, switchand the required test quality.
A defect is an error caused in a basixs during the manufacturing process.
Automatic test pattern generation
A fault model is a mathematical description of how a defect alters design behavior. The logic values observed at the device’s primary outputs, while applying a test pattern to some device under test DUTare called the output of that test pattern. The output of a test pattern, when testing a fault-free device that works exactly as designed, is called the expected output of that test pattern.
A fault is said to be detected by a test pattern atpy the output of that test pattern, when testing a device that has only that one fault, is different than the expected output. The ATPG process for a targeted fault consists of two phases: Fault activation establishes a signal value at the fault model site that is opposite of the value produced by the fault model.
Fault propagation moves the resulting signal value, or fault effect, forward by sensitizing a path from the fault site to a primary output. ATPG can fail to find a test for a particular fault at;g at least two cases. First, the fault may be intrinsically undetectable, such that no patterns exist that can detect that particular fault.
The classic example of this qtpg a redundant circuit, designed such that no single fault causes the output to change. In such a circuit, any single fault will be inherently undetectable. Second, it is possible that a detection pattern exists, but the algorithm cannot find one. Equivalent faults produce the same faulty behavior for all input patterns. Any single fault from the set of equivalent faults can represent the whole set.
Removing equivalent faults from entire set of faults is called fault collapsing. In the past several decades, the most popular fault model used in practice is the single stuck-at fault model. In this model, one of the apg lines in a circuit is assumed to be stuck at a fixed logic value, regardless of what inputs are supplied to the circuit. Hence, if a circuit has n signal lines, there are potentially 2n stuck-at faults defined on the circuit, of which some can be viewed as being equivalent to others.
The stuck-at fault model is a logical fault model because no delay information is associated with the fault definition.
It is also called a permanent fault model because the faulty effect is assumed to be permanent, in contrast to intermittent faults which occur seemingly at random and transient faults which occur sporadically, perhaps depending on operating conditions e. The single stuck-at fault model is structural because it is defined based on a structural gate-level circuit model.
This model is used to describe faults for CMOS logic gates. At transistor level, a transistor maybe stuck-short or stuck-open. In stuck-short, a transistor behaves as it is always conducts or stuck-onand stuck-open is when a transistor never conducts current or stuck-off. A short circuit between two signal lines is called bridging faults.
Bridging to VDD or Vss is equivalent to stuck at fault model. If one driver dominates the other driver in a bridging situation, the dominant driver forces the logic to the other one, in such case a dominant bridging fault is used.
In the latter case, dominant driver keeps its value, while the other one gets the AND or OR value of its own and the dominant driver. The combinational ATPG method allows testing the individual nodes or flip-flops of the logic circuit without being concerned with the operation of the overall circuit.
During test, a so-called scan-mode is enabled forcing all flip-flops FFs to be connected in a simplified fashion, effectively bypassing their interconnections as baasics during normal operation.
This allows using a relatively simple vector matrix to quickly test all the comprising FFs, as well as to trace failures to specific FFs. Sequential-circuit ATPG searches for a sequence of test vectors to detect a particular fault through the space of all possible test vector sequences. Various search strategies and heuristics have been devised to find a shorter sequence, or to find a sequence faster. However, according to reported results, no single strategy atg heuristic out-performs others for all applications or circuits.
This observation implies that a test generator should include a comprehensive set of heuristics. Even a simple stuck-at fault requires a sequence of vectors for detection in a sequential circuit. Also, due to the presence of memory elements, the controllability and observability of the internal signals in a sequential circuit are in general much more difficult than those in a combinational logic circuit.
However, these test generators, combined with low-overhead DFT techniques such as partial scanhave shown a certain degree of success in testing large designs. For designs that are sensitive to area or performance overhead, the bxsics of using sequential-circuit ATPG and partial scan offers an attractive alternative to the popular full-scan solution, which is based on combinational-circuit ATPG.
Historically, ATPG has focused on a set of faults derived from a gate-level fault model. As design trends move toward nanometer technology, new manufacture testing problems are emerging.
During design validation, engineers can no longer ignore the effects of crosstalk and power supply noise on reliability and basiics. Current fault modeling and vector-generation techniques are giving way to new models and techniques that consider timing information during test generation, that are scalable to larger designs, and that can capture extreme design conditions.
For nanometer technology, many current design validation problems are becoming manufacturing test problems as well, so new fault-modeling and ATPG techniques will be needed.
Combinational ATPG Basics
Testing very-large-scale integrated circuits with a high fault coverage is a difficult task because of complexity. Therefore, many different ATPG methods have been developed to address combinational and sequential circuits. ATPG is a topic that is covered by several conferences throughout the year.