Digital Systems IEEC 180A
Lecture 15
Bevan M. BaasTuesday, November 20, 2007
EEC 180A, Fall 2007, B. Baas 2
Counter Example
• 3-bit counter• Specification
– 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
0
36
reset
EEC 180A, Fall 2007, B. Baas 3
Counter ExampleUsing D FFs
000001010011100101110111
011xxxxxx110xxxxxx000xxx
000000000000000000000000
PresentStateABC
NextStateABC
NextStateABC
reset=0 reset=1• State Table• Two different values of reset treated as different Next States in this example
0
36
reset
EEC 180A, Fall 2007, B. Baas 4
Counter ExampleUsing D FFs
Combina-tionalLogic
resetoutput
(= state)
clk
XXXXXX0
“current” time
000001010011100101110111
reset
011xxxxxx110xxxxxx000xxx
000000000000000000000000
PresentStateABC
NextStateABC
NextStateABC
reset=0 reset=1
XXX
output XXX
DA,B,C
DA,B,C
0
36
reset
EEC 180A, Fall 2007, B. Baas 5
Counter ExampleUsing D FFs
Combina-tionalLogic
resetoutput
(= state)
clk
XXX0001
“current” time
000001010011100101110111
reset
011xxxxxx110xxxxxx000xxx
000000000000000000000000
PresentStateABC
NextStateABC
NextStateABC
reset=0 reset=1
XXX 000
output XXX XXX
DA,B,C
DA,B,C
0
36
reset
EEC 180A, Fall 2007, B. Baas 6
Counter ExampleUsing D FFs
Combina-tionalLogic
resetoutput
(= state)
clk
0000110
“current” time
000001010011100101110111
reset
011xxxxxx110xxxxxx000xxx
000000000000000000000000
PresentStateABC
NextStateABC
NextStateABC
reset=0 reset=1
XXX 000
output XXX XXX
DA,B,C
DA,B,C011
000
000
0
36
reset
EEC 180A, Fall 2007, B. Baas 7
Counter ExampleUsing D FFs
Combina-tionalLogic
resetoutput
(= state)
clk
0111100
“current” time
000001010011100101110111
reset
011xxxxxx110xxxxxx000xxx
000000000000000000000000
PresentStateABC
NextStateABC
NextStateABC
reset=0 reset=1
XXX 000
output XXX XXX
DA,B,C
DA,B,C011
000
110
011
0
36
reset
EEC 180A, Fall 2007, B. Baas 8
Counter ExampleUsing D FFs
Combina-tionalLogic
resetoutput
(= state)
clk
1100000
“current” time
000001010011100101110111
reset
011xxxxxx110xxxxxx000xxx
000000000000000000000000
PresentStateABC
NextStateABC
NextStateABC
reset=0 reset=1
XXX 000
output XXX XXX
DA,B,C
DA,B,C011
000
110
011
000
110
0
36
reset
EEC 180A, Fall 2007, B. Baas 9
Counter ExampleUsing D FFs
Combina-tionalLogic
resetoutput
(= state)
clk
0000110
“current” time
000001010011100101110111
reset
011xxxxxx110xxxxxx000xxx
000000000000000000000000
PresentStateABC
NextStateABC
NextStateABC
reset=0 reset=1
XXX 000
output XXX XXX
DA,B,C
DA,B,C011
000
110
011
000
110
011
000
0
36
reset
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