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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For designs that are sensitive to area or performance overhead, the solution 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.
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. Views Read Edit View history.
A short circuit between two signal lines is called bridging faults. The classic example of this is a redundant circuit, designed such that no single fault causes the output to change.
Historically, ATPG has focused on a set of faults derived from a gate-level fault model. During design validation, engineers can no longer ignore the effects of crosstalk and power supply noise on reliability and performance. 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. The ATPG process for a targeted fault consists of two phases: At transistor level, a transistor maybe stuck-short or stuck-open.
Testing very-large-scale integrated circuits with a high fault coverage is a difficult task because of complexity. 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. As design trends move toward nanometer technology, new manufacture testing problems are emerging. Sequential-circuit ATPG searches for a sequence of test vectors to detect a particular fault through the space of all possible test vector sequences.
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. Removing equivalent faults from entire set of faults is called fault collapsing. 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. ATPG is a topic that is covered by several conferences throughout the year.
Automatic test pattern generation
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. However, these test generators, combined with low-overhead DFT techniques such as partial scanhave shown a certain degree of success in testing large designs. These metrics generally indicate test quality higher with more fault detections and test application time higher with more patterns.
Combinational ATPG Basics
A defect is an error caused in a device during the manufacturing process. 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. Bridging to VDD or Vss is equivalent to stuck at fault model. In this model, one of the signal lines in a circuit is assumed to tapg stuck at a fixed logic value, regardless of what inputs are supplied to the circuit. Fault propagation moves the resulting signal value, or fault effect, forward by sensitizing a path from the fault site to a primary output.
The output of a atph pattern, when testing a fault-free device that works exactly as designed, is called the expected output of that test pattern. Various search strategies and heuristics have been devised to find a shorter sequence, or to find a sequence faster.
First, the fault may be intrinsically undetectable, such that no patterns exist that can detect that particular fault. This observation implies that a test generator should include a comprehensive set of heuristics.
This model is used to describe faults for CMOS logic gates.
Second, it is possible that a detection pattern exists, but the algorithm cannot find one. A fault model is a mathematical description of how a defect alters design behavior.
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.
In the past several decades, the most popular fault model used in practice is the single stuck-at fault model. From Wikipedia, the free encyclopedia. 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.
Fault activation establishes a signal value at the fault model site that is opposite atp the value produced by the fault model. Even a simple stuck-at fault requires a sequence of vectors for detection in a sequential circuit.
The single stuck-at fault model is structural because bssics is defined based on a structural gate-level circuit model. 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 intended during normal operation.
However, according to reported results, no single strategy or heuristic out-performs others for all applications or circuits. This page was last edited basic 23 Novemberat The effectiveness of ATPG is measured by the number of modeled defects, or fault modelsdetectable and by the number of generated patterns. ATPG can fail to find a test for a particular fault in at least two cases.
Retrieved from ” https: This allows using a relatively simple vector matrix to quickly test all the comprising FFs, as well as to trace failures to specific FFs. The basicx patterns are used to test semiconductor devices after manufacture, or to assist with determining the cause of failure failure analysis . In such a circuit, any single fault will be inherently undetectable.
A fault is said to be detected by a test pattern if the output of that test pattern, when testing a device that has only that one fault, is different than the expected output. 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.