ECEN 248: INTRODUCTION TO DIGITAL SYSTEMS DESIGNSlides courtesy Dr. A. DEMOSTHENOUS UNIVERSITY COLLEGE OF LONDONAsynchronous sequential circuits:DefinitionsExampleTransition TableState TableSlide 8Flow Tables of Previous ExamplesRaceCritical raceCycles to avoid RacesExample CircuitSR LatchConstraints on InputsECEN 248: INTRODUCTION TO DIGITAL SYSTEMS DESIGNDr. ShiDept. of Electrical and Computer EngineeringSlides courtesyDr. A. DEMOSTHENOUSUNIVERSITY COLLEGE OF LONDONAsynchronous sequential circuits:Do not use clock pulses. The change of internal state occurs when there is a change in the input variable. Their memory elements are either unclocked flip-flops or time-delay elements. They often resemble combinational circuits with feedback. Their synthesis is much more difficult than the synthesis of clocked synchronous sequential circuits. They are used when speed of operation is important.DefinitionsThe present state variables (y1 to yk) are called secondary variables. The next state variables (Y1 to Yk) are called excitation variables.Assumption: The input signals change one at a time and only when the circuit is in a stable condition.ExampleThe analysis of the circuit starts by considering the excitation variables (Y1and Y2) as outputs and the secondary variables (y1 and y2) as inputs.Transition TableState TableExampleFlow Tables of Previous ExamplesRaceA race condition exists in an asynchronous circuit when two or more binary state variables change value in response to a change in an input variable.Non CriticalCritical raceCritical RaceCycles to avoid RacesUnique pathExample CircuitSR LatchConstraints on
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