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).

RESET must be 1!
  • Enable the flip-flops by clicking 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:
    • 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).
    Johnson Counter Waveform
  • The MOD or number of unique states of this 3 flip flop johnson counter is 6.
  • 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
  • Simulate and Breadboard the Johnson Counter circuit.

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