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.

• Fig. 7.4.1 shows the 4–bit universal shift register. It has all the capabilities listed above. 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. Table 7.4.1 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. (Refer Fig. 7.4.1)

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.