# sequential logic level indicator with D flip flops

#### george2525

Jan 30, 2015
170
Hello.

Ive been asked to do this realy simple idea using D type flip flops

a single input is to light up 3 LED's in sequence - 000 / 100 / 110 / 111 and then if any additional highs are inputted - remain at 111 until a low is input, following which it will go backwards from 111 to 110 etc...

im aware this is pretty basic but im quite stuck. I am thinking a shift register of some kind that can go right and left?

the spec is that I use D type flip flops and that there is only one input (and a clock)

If anyone has a min to throw me a few bones id realy appreciate it!

Thanks

#### Laplace

Apr 4, 2010
1,252
So the problem is to design a state machine that could be described as an up/down Johnson counter with a truncated ring. Start the sequential analysis with a next-state table where the next-state of Q1,Q2,Q3 is determined by the previous-state of Q1,Q2,Q3 & INPUT. Then recognize that the next state is controlled by D1,D2,D3. Create truth tables for D1 & D2 & D3 from the Q1 & Q2 & Q3 & INPUT signals, and minimize the logic with a Karnaugh map for each D. Note that each D does not depend on every Q. -- That is the process for becoming unstuck in logic design.

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