lab9 - State University of New York at Stony Brook...

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State University of New York at Stony Brook Department of Electrical and Computer Engineering ESE 218 Digital Systems Design Spring 2009 Lab 9 . Counters 1. Objectives Design a synchronous modulo-3 counter which provides the frequency of pulses at output Z to be 1/3 of the clock frequency. The counter should be operational in the count and wait modes. Input X controls the operation mode so that X = 1 allows counting while with X = 0 the current state values do not change with clock pulses. You have to design two alternative versions of the circuit that utilize D- flip-flops (74LS74) in version 1 and JK- flip-flops (74LS112A) in version 2 respectively. Based on the flip-flop and logic gate AC specifications, calculate the maximum operation frequencies for both versions and comment on the difference. An example of the circuit timing diagram for X =1 in the case of positive–edge triggered flip-flops is shown in Figure 1. In the case of negative-edge triggered flip-flops, the timing diagram for Z will be shifted to the right for ½ of the clock period. Figure 1. 2. Introduction The circuit counts from 1 to 3 so that the counter uses three states. Thus, two flip-flops are required and one of the four possible states will not be used. Add all digits of your ID number and divide the result by 4. The remainder (0, 1, 2 or 3) in the binary form will give you the unused state. Output Z can be taken as one of the flip-flop outputs, i.e. in this state machine the output logic gates are not required. The circuit structure is shown in Figure 2. To achieve the goal you have to perform the following steps 1) make the state assignment 2) determine the next state values 3) find the flip-flop input equations and obtain the circuit diagram 4) check the unused state for the obtained solution, i.e. find the next state values if the system enters the unused state at initialization 5) calculate the maximum clock frequency for the obtained solution Div-by-3 X = COUNT/WAIT CLOCK Z CLOCK time
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This note was uploaded on 04/12/2009 for the course ESE 218 taught by Professor Donetsky during the Spring '08 term at SUNY Stony Brook.

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lab9 - State University of New York at Stony Brook...

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