Sunday, July 15, 2012

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.


        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.                                                         

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