Testing tri-state and pass transistor circuit structures



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Texas A&M University


Tri-state structures are used to implement multiplexers and buses because these structures are faster than AND/OR logic structures. But testing of tri-state structures has some issues associated with it. A stuck open control line of a tri-state gate will cause some lines in the circuit to float and take unknown values. A stuck-on control line can cause fighting when the two drivers connected to the same node drive different values. This thesis develops new gate level fault models and dynamic test patterns that take care of these problems. The models can be used with traditional stuck-at and transition fault automatic test pattern generation (ATPG) to ensure high fault coverage. This research focuses on producing good test coverage with reduced effort for tristate and pass transistor structures. We do circuit level modeling to help develop and validate gate level models, which can be used in production ATPG. We study the two primary effects of interest, capacitive coupling and leakage, and analyze the tri-state structures using these two effects. Coupling and leakage can cause a Z or X state to be seen as 0 or 1 in some cases. We develop parameterized models of behavior of common structures using these effects and some parameters such as number of fan-ins. We also develop gate level models of tri-state circuits that would replace the tri-state library cells in the ATPG engine. This work develops a methodology to make tri-state and pass transistor circuit structures more usable in the industry.