June 14, 2015
June 14, 2015
June 17, 2015
Computers in Education
26.1252.1 - 26.1252.19
Presenting Test Benches and Device Characteristics of Programmable Logic In An Introductory Logic Circuits CourseIn the fall semester of 2011, we introduced the complex programmable logic device (CPLD) inour introductory logic circuits course. We specifically chose to use a CPLD as it includes the useof modern CAD tools and logic devices, as well as hands-on activities and allows for theconsideration of device characteristics. This paper considers test benches as well as devicecharacteristics which are each important to students' learning experience in our use of CPLDs. Inour prior research we identified test benches as a critical aspect in the use of CAD tools. Thispaper first outlines our effort to better introduce students to test benches.In teaching with traditional TTL logic, the presentation of device characteristics, such as signalVoltage levels, device loading, and propagation delay was immediately available. With the movefrom such a hands-on, to a hands-off approach using a development board, the presentation ofdevice characteristics appears to have been missing from the curriculum. In using our CPLDmodule we are discovering new ways to reintroduce long standing basic concepts in the contextof CMOS devices, back into our introductory logic circuits course.Students in such an introductory course must be aware that they are dealing with real circuits andthat logic signals are represented with physically measurable quantities. The logic circuits labmust be tangible, demonstrating the connection between digital and analog concepts, such asVoltage and current. Our students take their first electric circuits course the same semester asintroductory logic circuits which means that these ideas are new to our students. As such we limitour discussion to presenting the device terminal characteristics of logic circuits.The device characteristics of CMOS gates differ from traditional TTL devices in severalimportant ways. Students discover that for CMOS the transition region between logic high andlogic low is extremely narrow which causes a gate to be sensitive to noise present in a slowlychanging input and makes it difficult to use a potentiometer and Voltmeter to direct measure theinput thresholds associated with logic-high and logic-low. This paper presents a feedback testcircuit that students use to investigate the device Voltage characteristic and use a Schmitt triggerto investigate how such capability reduces sensitivity to noise. Students also learn aboutpropagation delay and static loading. After each topic we outline a practical hands-on exercise.Our students use an oscilloscope to measure propagation delay and learn how to select a loadresistor for a device such as an LEDIn performing our research we assessed our students' learning experience with our newpresentation of test benches and assessed if device characteristics should be specifically includedamong the required course outcomes for our logic circuits course. We include results andanalysis from a student focus group, an anonymous exit survey, and include our ownobservations.
Hill, K. M., & Yu, Y. (2015, June), Presenting Test Benches and Device Characteristics of Programmable Logic in an Introductory Logic Circuits Course Paper presented at 2015 ASEE Annual Conference & Exposition, Seattle, Washington. 10.18260/p.24589
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