Draw the diagram of a 4 bit twisted Johnson Counter ,explaining its operation with the help of a timing diagram.
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Johnson Counter

  • A Johnson counter is a modified ring counter, where the inverted output from the last flip flop is connected to the input to the first. The register cycles through a sequence of bit-patterns. The MOD of the Johnson counter is 2n if n flip-flops are used. The main advantage of the Johnson counter counter is that it only needs half the number of flip-flops compared to the standard ring counter for the same MOD.

  • It can be implemented using D-type flip-flops (or JK-type flip-flops).

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  • Enable the flips flops by clicking on the RESET (Green) switch. The RESET switch is a on/off switch (similar to a room light switch)

  • Click on CLK (Red) switch and observe the changes in the outputs of the flip flops. The CLK switch is a momentary switch (similar to a door bell switch - normally off).

  • The D flip flop clock has a rising edge CLK input. For example Q1 behaves as follows:

Truth Table
State Q0 Q1 Q2
0 0 0 0
1 1 0 0
2 1 1 0
3 1 1 1
4 0 1 1
5 0 0 1
  • The D input value just before the CLK rising edge is noted (Q0). When CLK rising edge occurs, Q1 is assigned the previously noted D value (Q0).

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  • The MOD or number of unique states of this 3 flip flop johnson counter is 6.
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