Special Shift Registers

Bidirectional Shift Register

If we shift a binary number to the left by one position, it is equivalent to multiplying the number by 2 and if we shift a binary number to the right by one position, it is equivalent to dividing the number by 2.To perform these operations we need a register which can shift the data in either direction.

Bidirectional shift registers are the registers which are capable of shifting the data either right or left depending on the mode selected. If the mode selected is 1(high), the data will be shifted towards the right direction and if the mode selected is 0(low), the data will be shifted towards the left direction.

The logic circuit given below shows a Bidirectional shift register. The circuit consists of four D flip-flops which are connected. The input data is connected at two ends of the circuit and depending on the mode selected only one and gate is in the active state.

Ring Counter

A ring counter is basically a shift register counter in which the output of the first flip flop is connected to the next flip flop and so on and the output of the last flip flop is again fed back to the input of the first flip flop, thus the name ring counter. The data pattern within the shift register will circulate as long as clock pulses are applied.

The logic circuit given below shows a Ring Counter. The circuit consists of four D flip-flops which are connected. Since the circuit consists of four flip flops the data pattern will repeat after every four clock pulses as shown in the truth table below:

Johnson Counter

A Johnson counter is basically a shift register counter in which the output of the first flip flop is connected to the next flip flop and so on and the inverted output of the last flip flop is again fed back to the input of the first flip flop. They are also known as twisted ring counters.

The logic circuit given below shows a Johnson Counter. The circuit consists of four D flip-flops which are connected. An n-stage Johnson counter yields a count sequence of 2n different states, thus also known as a mod-2n counter. Since the circuit consists of four flip flops the data pattern will repeat every eight clock pulses as shown in the truth table below: