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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A defect is an error caused in a device during the manufacturing process. Fault propagation moves the resulting signal value, or fault effect, forward by sensitizing a path from the fault site to a primary output. During design validation, engineers can no longer ignore the bascis of crosstalk and power supply noise on reliability and performance. 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.
Combinational ATPG Basics
Current fault modeling and vector-generation techniques are giving way to new models and techniques that consider timing information during test generation, that are basocs to larger designs, and that can capture extreme design conditions.
Even a simple stuck-at fault requires a sequence of vectors for detection in a sequential circuit.
Removing equivalent faults from entire set of faults is called fault collapsing. At transistor level, a transistor maybe stuck-short or stuck-open. 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. 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. The ATPG process for a targeted fault consists of two phases: 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. Views Read Edit View history. Historically, ATPG has focused on a set of faults derived from a gate-level fault model.
In this model, one of the signal 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. However, these test generators, combined with low-overhead DFT techniques such as partial scanhave shown a certain degree of success in testing large designs.
First, the fault may be intrinsically undetectable, such that no patterns exist that can detect that particular fault. From Wikipedia, the free encyclopedia. Testing very-large-scale integrated circuits with a high fault coverage is a difficult task because of complexity.
Automatic test pattern generation
Various search strategies and heuristics have been devised to find a shorter sequence, or to find a sequence faster. 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. In such a circuit, any single fault will be inherently undetectable.
Sequential-circuit ATPG searches for a sequence of test vectors to detect a particular fault through the space of all possible test vector sequences. This model is used to describe faults for CMOS logic gates. The combinational ATPG method allows testing the individual nodes or flip-flops of basicw logic circuit without being concerned with the operation of the overall circuit.
This observation implies that a test generator should include a comprehensive set ztpg heuristics. The generated patterns are used to test semiconductor devices after manufacture, or basicz assist with determining the cause of failure failure analysis .
Combinational ATPG Basics
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. 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.
The baaics fault model is a logical fault model because no delay information is associated with the fault bsics. Any single fault from the set of equivalent faults can represent the whole set.
Retrieved from ” https: In the past several decades, the most popular fault model used in practice is the single stuck-at fault model. Equivalent faults produce the same faulty behavior for all input 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.
These metrics generally indicate test quality higher with more fault detections and test application time higher with more patterns. ATPG is a topic that is covered by several conferences throughout the year.
The single stuck-at fault basic is structural because it is defined based on a structural gate-level circuit model. Bridging to VDD or Vss is equivalent to stuck at fault model.
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. 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.
Second, it is possible that a detection pattern exists, but the algorithm cannot find one. 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.
Therefore, many different ATPG methods have been developed to address combinational and sequential circuits. This page was last edited on 23 Novemberat However, according to reported results, no single strategy or heuristic out-performs others for all basicw or circuits. The classic example of this is a redundant circuit, designed such that no single fault causes the output to change.