Bidirectional Shift Register in Digital Electronics

In digital electronic systems, a register is a basic memory element used to store and manipulate data in binary form. A register is basically a group of flip-flops, where a flip-flop is a one-bit storage device. The storage capacity of a register depends on the number of flip-flops used within the register.

• Registers are commonly used in different parts of a digital electronic system like processor, arithmetic unit, memory unit, etc.

• Based on the application and functionality, registers are classified into various types, such as data registers, shift registers, memory registers, etc.

In this article, we will explore bidirectional shift register. But before that let us first know a bit about shift register.

What is a Shift Register?

In digital electronic systems, a shift register is a type of register that can shift binary data stored from one location to another. The construction of a shift register consists of a series of flip-flops connected in a cascade fashion. In this cascaded group of flip-flops, the output of one flip-flop is connected to input of the next flip-flop. This arrangement allows for the shifting or repositioning of data within the register by moving the stored bits either to the left or right or both.

Based on their characteristics, the shift registers can be classified into the following types −

• Serial-In Serial-Out (SISO) Shift Register

• Parallel-In Parallel-Out (PIPO) Shift Register

• Serial-In Parallel-Out (SIPO) Shift Register

• Parallel-In Serial-Out (PISO) Shift Register

• Bidirectional Shift Register

The shift registers are used in various digital electronic system, such as data storage units, processors, data converters, signal processing systems, and more.

After getting insights into the basics of digital register and shift register. Let us now discuss the bidirectional shift register.

What is a Bidirectional Shift Register?

A type of shift register that allows for shifting of data to both the left and right directions is referred to as a bidirectional shift register. Therefore, the bidirectional shift registers offer greater flexibility to shift and manipulate data within the register.

In the bidirectional shift register, the direction of shifting of the data is controlled by a control signal and it depends on the desired operation. For this purpose, an additional control circuit is provided within the register.

A bidirectional shift register is typically constructed by cascading a series of flip-flops. In this series, the flip-flops are connected such that the output of each flip-flop is connected to the input of the next flip-flop. It is important to note that the input of the first flip-flop is connected to the data input line.

The control/clock signal generated by the control circuit determines the direction of data shift. These control signals are generally named as shift-left (to shift data towards the left side) and shift-right (to shift data towards the right side).

Additionally, the bidirectional register also has a control signal, referred to as parallel-load (PL), that allows the register to accept data in parallel and load them into the flip-flops at the same time.

Circuit Diagram and Working of Bidirectional Shift Register

The circuit diagram of a 4-bit bidirectional shift register is depicted in the following figure −

It consists of four D-flip-flops cascaded together along with a control circuit consisting of two AND gates and one OR gate for each flip-flop. It has a control input signal R/L' that controls the direction of the shift.

From the control signal R/L', it is clear that R is an active high signal, while L' is an active low signal. When R/L' signal is HIGH, the shift register acts as a shift-right shift register. When the R/L' is LOW, then the shift register acts as a shift-left shift register.

How Do Bidirectional Shift Registers Operate?

Now, let us understand the operation of this bidirectional shift resister.

Case 1 – Shift-Right Operation

When the control signal R/L' is HIGH, then the AND gates 1, 3, 5, and 7 are enabled, and the AND gates 2, 4, 6, and 8 are disabled. The output of the flip-flop A is connected to the input of the flip-flop B, the output of the flip-flop B is connected to the input of the flip-flop C, and the output of the flip-flop C is connected to the input of the flip-flop D. Hence, when a clock signal occurs, the data bits are shifted one place to the right.

Case 2 – Shift-Left Operation

When the control signal R/L' is LOW, then the AND gates 2, 4, 6, and 8 are enabled, and the AND gates 1, 3, 5, and 7 are disabled. The output Q of each flip-flop connected to the D input of the following flip-flop. Thus, when a clock signal occurs, the data bits are shifted one place to the left.

This is how a bidirectional shift-register is constructed and its operation takes place.

Advantages of Bidirectional Shift Register

The use of bidirectional shift registers provides several benefits in digital electronics circuits. Some key advantages of bidirectional shift registers are listed below −

• Bidirectional shift registers allow to shift data in both left and right directions. This improves the flexibility of the digital circuit.

• Bidirectional shift registers reduce the circuit complexity by integrating both right-shift and left-shift operations in a single unit.

• Bidirectional shift registers also optimize the memory utilization.

Applications of Bidirectional Shift Register

The following some major applications of bidirectional shift registers −

• Bidirectional shift registers are widely used in microprocessors for faster data processing.

• Bidirectional shift registers are used in arithmetic circuits.

• They are also used in memory units to store and manipulate data.

• Bidirectional shift registers find application in the field of image processing to perform tasks like image scanning, rotation, etc.

• Bidirectional shift registers are also employed in data compression and encryption applications.

• They are also used in digital signal processing to reduce noise, analyze signals, filtering operation, etc.

Conclusion

Bidirectional shift registers are flexible in shifting data bits in both left and right directionals using a control signal. They offer several advantages like reduced circuit complexity, lower cost, higher efficiency, etc. Due to all these advantages, they are crucial components in various digital electronic circuits.