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Digital Systems II

This example demonstrates a 3-bit counter using D flip-flops. The counter increments by 3 and wraps back to 0 at 6. It has a reset signal input that sets the counter value to 0 when reset=1.

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Digital Systems II

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  1. Digital Systems II EEC 180 Bevan M. 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 reset 0 6 3

  3. Counter ExampleUsing 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 xxx 110 xxx xxx 000 xxx 000 000 000 000 000 000 000 000 reset 0 6 3

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

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

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

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

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

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

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