The Johnson Ring Counter or "Twisted Ring Counters", is another shift register with feedback exactly the same as the standard Ring Counter, except that this time the inverted output Q of the last flip-flop is now connected back to the input D of the first flip-flop as shown below. The main advantage of this type of ring counter is that it only needs half the number of flip-flops compared to the standard ring counter then it’s modulo number is halved. So an "n-stage" Johnson counter will circulate a single data bit giving sequence of 2n different states and can therefore be considered as a "mod-2n counter".

This inversion of Q before it is fed back to input D causes the counter to "count" in a different way. Instead of counting through a fixed set of patterns like the normal ring counter such as for a 4-bit counter, "0001"(1), "0010"(2), "0100"(4), "1000"(8) and repeat, the Johnson counter counts up and then down as the initial logic "1" passes through it to the right replacing the preceding logic "0". A 4-bit Johnson ring counter passes blocks of four logic "0" and then four logic "1" thereby producing an 8-bit pattern. As the inverted output Q is connected to the input D this 8-bit pattern continually repeats. For example, "1000", "1100", "1110", "1111", "0111", "0011", "0001", "0000" and this is demonstrated in the following table .

Timing Diagram (Johnson Counter)

Start a Johnson counter by clearing all stages to 0s before the first clock. This is often done at power-up time. Referring to the diagram, the first clock shifts three 0s from ( QA QB QC) to the right into ( QB QCQD). The 1 at QD' (the complement of Q) is shifted back into QA. Thus, we start shifting 1s to the right, replacing the 0s. Where a ring counter recirculates a single 1, the 4-stage Johnson counter recirculates four 0s then four 1s for an 8-bit pattern, then repeats.

Application and Examples (Johnson Counter)

The Johnson counter generates 3-phase square waves, phased 60o apart with respect to (QA QB QC). However, we need 120o phased waveforms of power applications . Choosing P1=QA P2=QC P3=QB' yields the 120o phasing desired. See figure below. If these (P1 P2 P3) are low-pass filtered to sine waves and amplified, this could be the beginnings of a 3-phase power supply. For example, do you need to drive a small 3-phase 400 Hz aircraft motor? Then, feed 6x 400Hz to the above circuit CLOCK. Note that all these waveforms are 50% duty cycle.

The Johnson counter overcomes some of the limitations of the ring counter. Like a ring counter a Johnson counter is a shift register fed back on its' self. It requires half the stages of a comparable ring counter for a given division ratio. If the complement output of a ring counter is fed back to the input instead of the true output, a Johnson counter results. The difference between a ring counter and a Johnson counter is which output of the last stage is fed back (Q or Q'). Carefully compare the feedback connection below to the previous ring counter. This "reversed" feedback connection has a profound effect upon the behaviour of the otherwise similar circuits. Recirculating a single 1 around a ring counter divides the input clock by a factor equal to the number of stages. Whereas, a Johnson counter divides by a factor equal to twice the number of stages. For example, a 4-stage ring counter divides by 4. A 4-stage Johnson counter divides by 8. Start a Johnson counter by clearing all stages to 0s before the first clock. This is often done at power-up time. Referring to the figure below, the first clock shifts three 0s from ( QA QB QC) to the right into ( QB QCQD). The 1 at QD' (the complement of Q) is shifted back into QA. Thus, we start shifting 1s to the right, replacing the 0s. Where a ring counter recirculates a single 1, the 4-stage Johnson counter recirculates four0s then four 1s for an 8-bit pattern, and then repeats.