Johnson or Twisting Ring or Switch Tail Counter

Johnson or Twisting Ring or Switch Tail Counter

• In a Johnson counter, the Q output of each stage of flip-flop is connected to the D input of the next stage.

• The single exception is that the complement output of the last flip-flop is connected back to the D-input of the first flip-flop as shown in Fig. 6.7.1.

Note : Johnson counter can be implemented with SR or JK flip-flops as well.

• As shown in Fig. 6.7.1 there is a feedback from the rightmost flip-flop complement output to the leftmost flip-flop input. This arrangement produces a unique sequence of states.

• Initially, the register (all flip-flops) is cleared. So all the outputs,

Q_A, Q_B, Q_C, Q_D ARE ZERO-

•  The output of last stage, Q_D is zero. Therefore complement output of last stage,  is one. This is connected back to the D input of first stage. So D_A is one.

• The first falling clock edge produces Q_A= 1 and Q_B = 0,

Q_C = 0, Q_D = 0 since D_B, D_C, D_D are zero.

• The next clock pulse produces Q_A = 1, Q_B = 1, Q_C = 0, Q_D = 0.

• The sequence of states is summarized in Table 6.7.1 

• After 8 states the same sequence is repeated.

• In this case, four-bit register is used. So the four-bit sequence has a total of eight states.

• Fig. 6.7.2 gives the timing sequence for a four-bit Johnson counter.

• If we design a counter of five-bit sequence, it has a total of ten states.

• An n-stage Johnson counter will produce a modulus of 2xn, where n is the number of stages (i.e. flip-flops) in the counter.

• Johnson counter requires only half the number of flip-flops compared to the standard ring counter. However, it requires more flip-flop than binary counter.

Ex. 6.7.1 Draw a 5 flip-flop shift (Johnson) counter, its truth table and waveforms. Explain its operation as a decade counter.

Sol. : The Fig. 6.7.3 shows the 5-bit shift (Johnson) counter. Since this counter goes through 10 states, the frequency at the output of last flip-flop is l/l0th of the clock frequency and hence it is a decade counter.

The Table 6.7.2 shows the truth table for the 5 flip-flop shift counter and illustrates its operation.

Waveforms :

Review Questions

1. Draw the 4-bit Johnson counter and explain the operation.

2. Design Johnson counter and state its advantages and disadvantages.

AU : CSE : Dec.-11, Marks 8