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JOHNSON COUNTER

Form The Book Practical Electronics For Inventors by Paul Scherz

 

The Johnson shift counter is similar to the ring counter except that its last flip-flop feeds data back to the first flip-flop from its inverted output (Q). In the simple 4-bit Johnson shift counter shown below, you start out by applying a low to the START line, which sets presets Q0 high and Q1, Q2, and

Q3 lowóQ3 high. In other words, you load the register with the binary word 1000, as you did with the ring counter. Now, when you bring START line low, data will shift through the register. However, unlike the ring counter, the first bit sent back to the D0 input of the first flip-flop will be high because feedback is from Q3 not Q3. At the next clock edge, another high is fed back to D0; at the next clock edge, another high is fed back; at the next edge, another high is fed back. Only after the fourth clock edge does a low get fed back (the 1 has shifted down to the last flip-flop and Q3 goes high). At this point, the shift register is full of 1s. As more clock pulses arrive, the feedback loop supplies lows to D0 for the next four clock pulses. After that, the Q outputs of all the flip-flops are low while Q3 goes high. This high from Q3 is fed back to D0 during the next positive clock edge, and the cycle repeats. As you can see, the 4-bit Johnson shift counter has 8 output stages (which require 8 clock pulses to recycle), not 4, as was the case with the ring counter. Johnson counter using positive edge-triggered D flip-flops

            

 

 

THIS APPLICATION CAN USE AS LAMP DECORATION OR ADVERTISER. YOU CAN MAKE MORE OUTPUT BY THOSE CONNECTION USE MORE D-FLIP FLOP

 

 

 

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