1. EEC 180B Lecture 14 Bevan M. Baas Tuesday, May 22, 2018
Digital Systems II
EEC 180B
Lecture 14
Bevan M. Baas
Tuesday, May 22, 2018

2. Counter Example 3-bit counter Specification Has a reset signal input
Starting at zero, it increments by 3
at 6, it wraps back to 0
Has a reset signal input
When reset=1, the next counter value is 0
Uses D Flip-flops
reset 6 3
B. Baas

3. Counter Example Using D FFs
State Table
Two different values of reset treated as different Next States in this example
reset=0 reset=1
Present
State
ABC
Next
State
ABC
Next
State
ABC
000
001
010
011
100
101
110
111
011
xxx
110
000
000
reset 6 3
B. Baas

4. Counter Example Using D FFs
reset=0 reset=1
Present
State
ABC
Next
State
ABC
Next
State
ABC
XXX
XXX
Combina-
tional Logic
output
(= state)
000
001
010
011
100
101
110
111
011
xxx
110
000
000
reset
DA,B,C
clk
reset
DA,B,C
XXX
output
XXX
reset
“current” time
B. Baas 6 3

5. Counter Example Using D FFs
reset=0 reset=1
Present
State
ABC
Next
State
ABC
Next
State
ABC
000
XXX 1
Combina-
tional Logic
output
(= state)
000
001
010
011
100
101
110
111
011
xxx
110
000
000
reset
DA,B,C
clk
reset
DA,B,C
XXX
000
output
XXX
XXX
reset
“current” time
B. Baas 6 3

6. Counter Example Using D FFs
reset=0 reset=1
Present
State
ABC
Next
State
ABC
Next
State
ABC
011
000
000
Combina-
tional Logic
output
(= state)
000
001
010
011
100
101
110
111
011
xxx
110
000
000
reset
DA,B,C
clk
reset
DA,B,C
XXX
000
011
output
XXX
XXX
000
reset
“current” time
B. Baas 6 3

7. Counter Example Using D FFs
reset=0 reset=1
Present
State
ABC
Next
State
ABC
Next
State
ABC
110
011
Combina-
tional Logic
output
(= state)
000
001
010
011
100
101
110
111
011
xxx
110
000
000
reset
DA,B,C
clk
reset
DA,B,C
XXX
000
011
110
output
XXX
XXX
000
011
reset
“current” time
B. Baas 6 3

8. Counter Example Using D FFs
reset=0 reset=1
Present
State
ABC
Next
State
ABC
Next
State
ABC
000
110
Combina-
tional Logic
output
(= state)
000
001
010
011
100
101
110
111
011
xxx
110
000
000
reset
DA,B,C
clk
reset
DA,B,C
XXX
000
011
110
000
output
XXX
XXX
000
011
110
reset
“current” time
B. Baas 6 3

9. Counter Example Using D FFs
reset=0 reset=1
Present
State
ABC
Next
State
ABC
Next
State
ABC
011
000
Combina-
tional Logic
output
(= state)
000
001
010
011
100
101
110
111
011
xxx
110
000
000
reset
DA,B,C
clk
reset
DA,B,C
XXX
000
011
110
000
011
output
XXX
XXX
000
011
110
000
reset
“current” time
B. Baas 6 3