week5_COD.pdf

5.1 SEQUENTIAL CIRCUITS Jean-Pierre Deschamps University Rovira i Virgili, Tarragona, Spain  5.1 Motivation Most digital system specifications cannot be implemented by combinational circuits because the value of...

5.1 SEQUENTIAL CIRCUITS Jean-Pierre Deschamps University Rovira i Virgili, Tarragona, Spain  5.1 Motivation Most digital system specifications cannot be implemented by combinational circuits because the value of an output signal could be a function of  (not only) the value of the input signals at the same time,  (but also) the value of the input signals at preceding times. Example: vehicle access control system. Input signals:  entrance request,  gate lower and upper positions,  vehicle present within the gate area, Output signals:  motor control (on/off, up/down). 2  5.1 Motivation The values of the signals that control the motor depend on the sequence of events:  entrance request,  open the gate,  wait for upper gate position,  wait for clear gate area,  wait for lower gate position. A new entrance request is not attended until this sequence of events is completed. Conclusion: Some type of memory is necessary. 3  5.1 1. Sequential systems Digital circuits with memory:  Output values at time t depend on input values at times t, t-1, t-2, etc.  Able to generate sequences.  Able to detect sequences. Example of sequence detection: implement a circuit that detects the following sequence: 1557. if x(t-3) = 1 and x(t-2) = 5 and x(t-1) = 5 and x = 7 then y