What is Universal Shift Register & Its Working

In digital electronics, shift registers are the sequential logic circuits that can store the data temporarily and provides the data transfer towards its output device for every clock pulse. These are capable of transferring/shifting the data either towards the right or left in serial and parallel modes. Based on the mode of input/output operations, shift registers can be used as a serial-in-parallel-out shift register, serial-in-serial-out shift register, parallel-in-parallel-out shift register, parallel-in-parallel-out shift register. Based on shifting the data, there are universal shift registers and bidirectional shift registers. Here is a complete description of the universal shift register.

What is a Universal Shift Register?

Definition: A register that can store the data and /shifts the data towards the right and left along with the parallel load capability is known as a universal shift register. It can be used to perform input/output operations in both serial and parallel modes. Unidirectional shift registers and bidirectional shift registers are combined together to get the design of the universal shift register. It is also known as a parallel-in-parallel-out shift register or shift register with the parallel load.


Universal shift registers are capable of performing 3 operations as listed below.

  • Parallel load operation – stores the data in parallel as well as the data in parallel
  • Shift left operation – stores the data and transfers the data shifting towards left in the serial path
  • Shift right operation – stores the data and transfers the data by shifting towards right in the serial path.

Hence, Universal shift registers can perform input/output operations with both serial and parallel loads.

Universal Shift Register Diagram

4-bit Universal shift register diagram is shown below.

Universal Shift Register Diagram
Universal Shift Register Diagram
  • Serial input for shift-right control enables the data transfer towards the right and all the serial input and output lines are connected to the shift-right mode. The input is given to the AND gate-1 of the flip-flop -1 as shown in the figure via serial input pin.
  • Serial input for shift-left enables the data transfer towards the left and all the serial input and output lines are connected to shift-left mode.
  • In parallel data transfer, all the parallel inputs and outputs lines are associated with the parallel load.
  • Clear pin clears the register and set to 0.
  • CLK pin provides clock pulses to synchronize all the operations.
  • In the control state, the information or data in the register would not change even though the clock pulse is applied.
  • If the register operates with a parallel load and shifts the data towards the right and left, then it acts as a universal shift register.

Design of Universal Shift Register

The design of a 4-bit universal shift register using multiplexers and flip-flops is shown below.

PCBWay

Universal Shift Register Design
Universal Shift Register Design
  • S0 and S1 are the selected pins that are used to select the mode of operation of this register. It may be shift left operation or shift right operation or parallel mode.
  • Pin-0 of first 4×1 Mux is fed to the output pin of the first flip-flop. Observe the connections as shown in the figure.
  • Pin-1 of the first 4X1 MUX is connected to serial input for shift right. In this mode, the register shifts the data towards the right.
  • Similarly, pin-2 of 4X1 MUX is connected to the serial input for shift-left. In this mode, the universal shift register shifts the data towards the left.
  • M1 is the parallel input data given to the pin-3 of the first 4×1 MUX to provide parallel mode operation and stores the data into the register.
  • Similarly, remaining individual parallel input data bits are given to the pin-3 of related 4X1MUX to provide parallel loading.
  • F1, F2, F3, and F4 are the parallel outputs of Flip-flops, which are associated with the 4×1 MUX.

Universal Shift Register Working

  • From the above figure, selected pins the mode of operation of the universal shift register. Serial input shifts the data towards the right and left and stores the data within the register.
  • Clear pin and CLK pin are connected to the flip-flop.
  • M0, M1, M2, M3 are the parallel inputs while F0, F1, F2, F3 are the parallel outputs of flip-flops
  • When the input pin is active HIGH, then the universal shift register loads / retrieve the data in parallel. In this case, the input pin is directly connected to 4×1 MUX
  • When the input pin (mode) is active LOW, then the universal shift register shifts the data. In this case, the input pin is connected to 4×1 MUX via NOT gate.
  • When the input pin (mode) is connected to GND (Ground), then the universal shift register acts as a Bi-directional shift register.
  • To perform the shift-right operation, the input pin is fed to the 1st AND gate of the 1st flip-flop via serial input for shit-right.
  • To perform the shift-left operation, the input pin is fed to the 8th AND gate of the last flip-flop via input M.
  • If the selected pins S0= 0 and S1 = 0, then this register doesn’t operate in any mode. That means it will be in a Locked state or no change state even though the clock pulses are applied.
  • If the selected pins S0 = 0 and S1 = 1, then this register transfers or shifts the data to left and stores the data.
  • If the selected pins S0 = 1 and S1 = 0, then this register shifts the data to right and hence performs the shift-right operation.
  • If the selected pins S0 = 1 and S1 = 1, then this register loads the data in parallel. Hence it performs the parallel loading operation and stores the data.

S0

S1

Mode of Operation

0

0Locked state (No change)

0

1Shift-Left
10

Shift-Right

11

Parallel Loading

From the above table, we can observe that this register operates in all modes with serial/parallel inputs using 4×1 multiplexers and flip-flops.

Advantages

The advantages of a universal shift register include the following.

  • This register can perform 3 operations such as shift-left, shift-right, and parallel loading.
  • Stores the data temporarily with in the register.
  • It can perform serial to parallel, parallel to serial, parallel to parallel and serial to serial operations.
  • It can perform input-output operations in both the modes serial and parallel.
  • A Combination of the unidirectional shift register and bidirectional shift register gives the universe shift register.
  • This register acts as an interface between one device to another device to transfer the data.

Applications

The applications of a universal shift register include the following.

  • Used in micro-controllers for I/O expansion
  • Used as a serial-to-serial converter
  • Used as a parallel-to-parallel data converter
  • Used as a serial-to-parallel data converter.
  • Used in serial – to – serial data transfer
  • Used in parallel data transfer.
  • Used as a memory element in digital electronics like computers.
  • Used in time delay applications
  • Used as frequency counters, binary counters, and Digital clocks
  • Used in data manipulation applications.

Thus, this is all about the universal shift register – definition, diagram, design, working, advantages, and disadvantages. There are different kinds of 4-bit registers are available in the form of IC 74291, IC 74395, and many more. Here is a question for you, “What is the working of the Bidirectional universal shift register?”

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