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
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
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
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
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
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
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
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
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