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