Johnson counter-Working Animation

Johnson Counter:

A Johnson counter (or switchtail ring counter, twisted-ring counter, walking-ring counter, or Moebius counter) is a modified ring counter, where the output from the last stage is inverted and fed back as input to the first stage. The register cycles through a sequence of bit-patterns, whose length is equal to twice the length of the shift register, continuing indefinitely. These counters find specialist applications, including those similar to the decade counter, digital-to-analog conversion, etc. They can be implemented easily using D- or JK-type flip-flops.

Working:

        circuit diagram represents a 3 bit Johnson counter using 7474 D flip flop. You can easily extent this     circuit up to 4 bit, 5 bit, etc. by adding flip flops after 3rd flips flop.

• A single 7474 IC consist of 2 flip flops so you need two 7474 ICs for implementing Johnson counter.

• Initially all the flip flops are cleared, so the time inverted output (Q’) of 3rd flip flop is high or logic 1.

• This logic 1 is appears at the input of 1st flip flop. During the first clock pulse this logic 1 is transferred to the output of 1st flip flop. Thus the total output of Johnson counter is 100.

• Then input of 1st and 2nd flip flop is logic 1 and after the second clock pulse these inputs appear at the outputs of 1st and 2nd flip flop. So the total output is 110.

• Similarly for the next clock pulse, the output will be 111.

• During this state (111) the time inverted output (Q’) is logic 0. This 0 is fed to the 1st flip flop. Then the 0 will circulate through the flip flops as 011,001,000.