Digital Systems I EEC 180A Lecture 15 Bevan M. Baas Tuesday,
November 20, 2007
Slide 2
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
Slide 3
EEC 180A, Fall 2007, B. Baas 3 Counter Example Using D FFs 000
001 010 011 100 101 110 111 011 xxx 110 xxx 000 xxx 000 Present
State ABC Next State ABC Next State ABC reset=0 reset=1 State Table
Two different values of reset treated as different Next States in
this example 0 36 reset
Slide 4
EEC 180A, Fall 2007, B. Baas 4 Counter Example Using D FFs
Combina- tional Logic reset output (= state) clk XXX 0 current time
000 001 010 011 100 101 110 111 reset 011 xxx 110 xxx 000 xxx 000
Present State ABC Next State ABC Next State ABC reset=0 reset=1 XXX
output XXX D A,B,C 0 36 reset
Slide 5
EEC 180A, Fall 2007, B. Baas 5 Counter Example Using D FFs
Combina- tional Logic reset output (= state) clk XXX000 1 current
time 000 001 010 011 100 101 110 111 reset 011 xxx 110 xxx 000 xxx
000 Present State ABC Next State ABC Next State ABC reset=0 reset=1
XXX 000 output XXX D A,B,C 0 36 reset
Slide 6
EEC 180A, Fall 2007, B. Baas 6 Counter Example Using D FFs
Combina- tional Logic reset output (= state) clk 000011 0 current
time 000 001 010 011 100 101 110 111 reset 011 xxx 110 xxx 000 xxx
000 Present State ABC Next State ABC Next State ABC reset=0 reset=1
XXX 000 output XXX D A,B,C 011 000 0 36 reset
Slide 7
EEC 180A, Fall 2007, B. Baas 7 Counter Example Using D FFs
Combina- tional Logic reset output (= state) clk 011110 0 current
time 000 001 010 011 100 101 110 111 reset 011 xxx 110 xxx 000 xxx
000 Present State ABC Next State ABC Next State ABC reset=0 reset=1
XXX 000 output XXX D A,B,C 011 000 110 011 0 36 reset
Slide 8
EEC 180A, Fall 2007, B. Baas 8 Counter Example Using D FFs
Combina- tional Logic reset output (= state) clk 110000 0 current
time 000 001 010 011 100 101 110 111 reset 011 xxx 110 xxx 000 xxx
000 Present State ABC Next State ABC Next State ABC reset=0 reset=1
XXX 000 output XXX D A,B,C 011 000 110 011 000 110 0 36 reset
Slide 9
EEC 180A, Fall 2007, B. Baas 9 Counter Example Using D FFs
Combina- tional Logic reset output (= state) clk 000011 0 current
time 000 001 010 011 100 101 110 111 reset 011 xxx 110 xxx 000 xxx
000 Present State ABC Next State ABC Next State ABC reset=0 reset=1
XXX 000 output XXX D A,B,C 011 000 110 011 000 110 011 000 0 36
reset
