Sections 1 and 2

Name:__________________________________
Number:___________________
Spring 2010
CPE231 Digital Logic
Section 1
Quiz 3-A
Problem 1
Design a sequential circuit with two D flip-flops A and B and one input X. When X=0, the
state of the circuit remains the same. When X=1, the circuit goes through the state
transitions from 00 to 10 to 11 to 01, back to 00, and then repeats. You do not have to
draw the logic diagram.
1 mark
1 mark
2 marks
2 marks
4 marks
Page 1 of 1

Name:__________________________________
Number:___________________
Spring 2010
CPE231 Digital Logic
Section 1
Quiz 3-B
Problem 1
Design a sequential circuit with two D flip-flops A and B and one input X. When X=1, the
state of the circuit remains the same. When X=0, the circuit goes through the state
transitions from 00 to 11 to 01 to 10, back to 00, and then repeats. You do not have to
draw the logic diagram.
Present state Input Next state
A B X A B
0 0 0 1 1
0 0 1 0 0
0 1 0 1 0
0 1 1 0 1
1 0 0 0 0
1 0 1 1 0
1 1 0 0 1
1 1 1 1 1
4 marks
DA
A
B
1 1
1 1
X
1 mark
DA=AX+A'X'
2 marks
DB
A
B
1 1
1 1
X
1 mark
DB=AB+BX+A'B'X'
2 marks
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Name:__________________________________
Number:___________________
Spring 2010
CPE231 Digital Logic
Section 2 ; Quiz 3-A
A serial leading 0’s detector is to be designed. A binary integer of arbitrary length is presented to the
serial leading 0’s detector, most significant bit first, on input X. When a given bit is presented on input X,
the corresponding output bit is to appear during the same clock cycle on output Z. As long as the bits
applied to X are 1, Z = 1. When the first 0 is applied to X, Z = 0. For all bit values applied to X after the
first 0 is applied, Z = 1. To indicate that a sequence is complete and that the circuit is to be initialized to
receive another sequence, input Y becomes 1 for one clock cycle. Otherwise, Y is 0.
(a) Find the state diagram for the serial leading 0’s detector.
(b) Find the state table for the serial leading 0’s detector.
a.
b.
A(present
state)
X Y A(next
state)
0 0 0 1 0
0 0 1 0 0
0 1 0 0 1
0 1 1 0 1
1 0 0 1 1
1 0 1 0 1
1 1 0 1 1
1 1 1 0 1
Z
Page 1 of 1

Name:__________________________________
Number:___________________
Spring 2010
CPE231 Digital Logic
Section 2 ; Quiz 3-B
Problem
A communication network link requires a circuit that produces the sequence 01111110. You are to
design a synchronous sequential circuit that starts producing this sequence for input E = 1. Once the
sequence starts, it completes. If E = 1, during the last output in the sequence, the sequence repeats.
Otherwise, if E 0, the output remains constant at 0.
(a) Draw the Moore state diagram for the circuit.
(b) Find the state table and make a state assignment.
a.
b.
D2D1D0 E=0 E=1 Z
000 001 001 0
001 010 010 1
010 011 011 1
011 100 100 1
100 101 101 1
101 110 110 1
110 111 111 1
111 111 000 0
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