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It is similar to the stuck-at model in that there are two faults for every node location in the design, classified as slow-to-rise and slow-to-fall faults. The stuck-at model is classified as a static model because it is a slow speed test and is not dependent on gate timing (rise and fall times and propagation delay).Ī second common type of fault model is called the “transition” or “at-speed” fault model, and is a dynamic fault model, i.e., it detects problems with timing. The stuck-at model can also detect other defect types like bridges between two nets or nodes. For example, if a NAND gate in the design had an input pin shorted to ground (logic value 0) by a defect, the stuck-at-0 test for that node would catch it. The most basic and common is the “stuck-at” fault model, which checks each node location in the design for either stuck-at-1 or stuck-at-0 logic behavior. There are a number of different fault models that are commonly used. The ATPG tool then uses the fault models to determine the patterns required to detect those faults at all points in the circuit (or almost all-coverage of 95% or more is typical).
#Acad 2016 debug software#
To enable automatic test pattern generation (ATPG) software to create the test patterns, fault models are defined that predict the expected behaviors (response) from the IC when defects are present. Many designs do not connect up every register into a scan chain. Any mismatches are likely defects and are logged for further evaluation. The ATE then compares the captured test response with the expected response data stored in its memory. The design is again put in test mode and the captured test response is shifted out, while the next test pattern is simultaneously shifted in to the scan cells. After the test pattern is loaded, the design is placed back into functional mode and the test response is captured in one or more clock cycles. The scan chains are used by external automatic test equipment (ATE) to deliver test pattern data from its memory into the device.
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The scan cells are linked together into “scan chains” that operate like big shift registers when the circuit is put into test mode. Since scan test modifies flip flops that are already in the design to enable them to also act as scan cells, the impact of the test circuitry is relatively small, typically adding about only 1-5% to the total gate count. The test software doesn’t need to understand the function of the logic-it just tries to exercise the logic segments observed by a scan cell. Segmenting the logic in this manner is what makes it feasible to automatically generate test patterns that can exercise the logic between the flops. For a design with a million flops, introducing scan cells is like adding a million control and observation points. The modified flip-flops, or scan cells, allow the overall design to be viewed as many small segments of combinational logic that can be more easily tested. The design’s flip-flops are modified to allow them to function as stimulus and observation points, or “scan cells” during test, while performing their intended functional role during normal operation.
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The approach that ended up dominating IC test is called structural, or “scan,” test because it involves scanning test patterns into internal circuits within the device under test (DUT). So the industry moved to a design for test (DFT) approach where the design was modified to make it easier to test. The technique is referred to as functional test. As logic devices become more complex, it took increasing amounts of time and effort to manually create and validate tests, it was too hard to determine test coverage, and the tests took too long to run.