Universal Shift Register

Universal Shift Register

• A register capable of shifting in one direction only is a unidirectional shift register.

• A register capable of shifting in both directions is a bidirectional shift register.

• If the register has both shifts (right shift and left shift) and parallel load capabilities, it is referred to as universal shift register.

• The Fig. 6.4.1 shows the 4-bit universal shift register.

• It consists of four flip-flops and four multiplexers.

• The four multiplexers have two common selection inputs S₁ and S₀, and they select appropriate input for D flip-flop.

• The Table 6.4.1 shows the register operation depending on the selection inputs of multiplexers.

• When S₁S₀ = 00, input 0 is selected and the present value of the register is applied to the D inputs of the flip-flops. This results no change in the register value.

• When S₁S₀ = 01, input 1 is selected and circuit connections are such that it operates as a right shift register.

• When S₁S₀ = 10, input 2 is selected and circuit connections are such that it operates as a left shift register.

• Finally, when S₁S₀ = 11, the binary information on the parallel input lines is transferred into the register simultaneously and it is a parallel load operation.

Ex. 6.4.1 Draw the logic diagram of a 4-bit shift register with four D flip-flops and four 4x1 multiplexer with mode selection inputs Sx and S0. The register operates as follows.

AU : CSE : Dec.-08, Marks 12

Sol. : The Fig. 6.4.2 shows the register that does the given operations. 

Review Questions

1. What is meant by universal shift register? Explain the principle of operation of 4-bit universal shift register.

2. Explain the operation of universal shift register with neat block diagram.