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 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. 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. 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.
Also, due to the presence of memory elements, the controllability and observability atpf the internal signals in a sequential circuit are in general much more difficult than those in a combinational logic circuit. ATPG can fail to find a test for a particular fault in at least two cases.
Combinational ATPG Basics
From Wikipedia, the free encyclopedia. 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 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.
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. Removing equivalent faults from entire set of faults is called fault collapsing. Testing very-large-scale integrated circuits with a high fault coverage is a difficult task because of complexity.
Historically, ATPG has focused on a set of faults derived from a gate-level fault model. Views Read Edit View history.
Bridging to VDD or Vss is equivalent to stuck at fault model.
Automatic test pattern generation – Wikipedia
This observation implies that a test generator should include a comprehensive set of heuristics. Second, it is possible that a detection pattern exists, but the algorithm cannot find one. The classic example of this is a redundant circuit, designed such that no single fault causes the output to change. 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 sequential bzsics, the level of abstraction used to represent the circuit under test gate, register-transfer, switchand the required test quality.
The output of a test pattern, when testing a fault-free bssics that baasics exactly as designed, is called the expected output of that test pattern. During design validation, engineers can no longer ignore the effects of crosstalk and power supply noise on reliability and performance.
This page was last edited on 23 Novemberat 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.
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 combinational ATPG method allows testing the individual nodes or flip-flops of the logic circuit without being concerned with the operation of the overall qtpg.
This model is used to describe faults for CMOS logic gates. In the past several decades, the most popular stpg model used in practice is the single stuck-at fault model. 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.
Automatic test pattern generation
However, these test generators, combined with low-overhead DFT techniques such as partial scanhave shown a certain degree of success atpb testing large designs.
This allows using a relatively simple vector matrix to quickly test all the comprising FFs, as well as to trace failures to specific FFs. A defect is an error caused in a device during the manufacturing process. Various search strategies and heuristics have been devised to find a shorter sequence, or to find a sequence faster.
In this model, one of the signal lines in a circuit is assumed to be stuck at a bascs logic value, regardless of what inputs are supplied to the circuit.
Combinational ATPG Basics
The stuck-at fault model is a logical fault model because no delay information is associated with the fault definition. The single stuck-at fault model is structural because it is defined based on a structural gate-level circuit model.
A fault model is a mathematical description of how a defect alters design behavior. However, according to reported results, no single strategy or heuristic out-performs others for all applications or circuits.
The effectiveness of ATPG is measured by the number of modeled defects, or fault modelsdetectable and by the number of generated patterns. 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 generated patterns are used to test semiconductor devices after manufacture, or to assist with determining the cause of failure failure analysis . First, the fault may be intrinsically undetectable, such that no patterns exist that can detect that particular fault.
Fault activation establishes a signal value at the fault model site that is opposite of the value produced by the fault model. Sequential-circuit ATPG searches for a sequence of test vectors to detect a particular fault through the space of all possible test vector sequences. Even a simple stuck-at fault requires a sequence of vectors for detection in a sequential circuit. These metrics generally indicate test quality higher with more fault detections and test application time higher with more patterns.
Equivalent faults produce the same faulty behavior for all input patterns. Fault propagation moves the resulting signal value, or fault effect, forward by sensitizing a path from the fault site to a primary output.
Any single fault from the set of equivalent faults can represent the whole set. A short circuit between two signal lines is called bridging faults.