Parallel In Serial Out (PISO) Shift Register

Digital circuits play a vital role in processing and manipulating data efficiently. One important component of these circuits is a shift register, which allows data to be shifted in and out in a sequential manner. Among the various types of shift registers, the Parallel-In Serial-Out (PISO) shift register offers unique functionality that is valuable in many applications.

What is PISO Shift Register?

A PISO shift register is a digital circuit that can accept parallel data and output serial data. It is made up of a succession of flip-flops, with each flip-flop capable of storing one bit of data. Unlike PIPO shift registers, which offer parallel input and output, a PISO shift register accepts data in parallel and outputs it sequentially, or serially.

Some Key Terminologies For PISO Shift Registers

1. Shift Register: A digital circuit that allows sequential shifting of data bits. It consists of a chain of flip-flops where data moves from one flip-flop to the next during each clock cycle.
2. Parallel-In Serial-Out (PISO): A type of shift register that accepts parallel input data and produces a sequential output. It loads data in parallel and outputs it in a serial manner.
3. Flip-Flops: Storage elements within a shift register that can store one bit of data. In a PISO shift register, each flip-flop represents a stage through which data passes during the shifting process.
4. Parallel Input: The process of loading data into the shift register simultaneously through multiple input lines. Parallel input allows for fast and efficient data transfer into the shift register.
5. Serial Output: The sequential output of data from the shift register, bit by bit, in a serial manner. The output represents the data that has been shifted through the register.
6. Clock Signal: A timing signal that controls the shifting operation in the shift register. Each clock pulse triggers the movement of data from one flip-flop to the next, enabling the sequential shifting process.
7. Most Significant Bit (MSB): The leftmost bit of the parallel input or serial output in a binary representation. It represents the highest value or the most significant position within the data.
8. Least Significant Bit (LSB): The rightmost bit of the parallel input or serial output in a binary representation. It represents the lowest value or the least significant position within the data.
9. Data Transmission: The process of sending data from one device to another. PISO shift registers are commonly used in data transmission applications, converting parallel data into a serial format for efficient transmission over serial communication channels.
10. Serial-to-Parallel Conversion: The process of converting serial data into parallel format. PISO shift registers can be used to load serial data and then output it in parallel, enabling the interfacing between serial and parallel systems.

How Does PISO Shift Register Work?

To understand the operation of a PISO shift register, let’s consider a basic example with four flip-flops labeled D0, D1, D2, and D3. Each flip-flop can store one bit of data. The parallel data is loaded into the flip-flops simultaneously through the parallel input lines. Once the data is loaded, it can be shifted out in a sequential manner through the serial output.

4-bit PISO Shift Register Circuit Diagram:

To shift the data out, a clock signal is applied to the shift register. Each clock pulse triggers the movement of data from one flip-flop to the next in a cascading fashion. The most significant bit (MSB) is usually the first to be shifted out, followed by the remaining bits. The serial output provides the bits one at a time, in the order they were loaded.

Example:

Let us consider, We have a PISO shift register with four flip-flops (D0, D1, D2, and D3) and a 2-to-1 multiplexer. Initially, all flip-flops are cleared and contain the value ‘0’. We want to load the binary value ‘1010‘ into the shift register using parallel input and then shift the data out serially.

Solution:

Step 1: Parallel Input To load the value ‘1010’ into the shift register, we apply the parallel input as follows:

Data: 1 0 1 0

Step 2: Multiplexer Selection To facilitate the serial output, we use a 2-to-1 multiplexer. The inputs to the multiplexer are the outputs of the flip-flops (Q0, Q1, Q2, and Q3), and the select line of the multiplexer is controlled by the clock signal.

Initially, with the clock selecting the line at logic ‘0’, the multiplexer selects the first input (Q0) as the output.

Step 3: Clock Cycle (Shift Out) Now, we activate the clock signal to shift the data out serially. Each clock pulse triggers the movement of data from one flip-flop to the next, and the output of the multiplexer represents the serial data.

Clock Cycle 1:

Data Out: 1 (Q0)

Clock Cycle 2:

Data Out: 0 (Q1)

Clock Cycle 3:

Data Out: 1 (Q2)

Clock Cycle 4:

Data Out: 0 (Q3)

After four clock cycles, the binary value ‘1010‘ has been shifted out serially, with the MSB (Most Significant Bit) appearing first and the LSB (Least Significant Bit) appearing last.

In this numerical example, we demonstrated the operation of a 4-bit PISO shift register with the use of a multiplexer. We loaded the binary value ‘1010’ in parallel, and by applying clock cycles, we obtained the serial output ‘1010’ through the multiplexer. The combination of a PISO shift register and a multiplexer enables the efficient conversion of parallel data into a sequential, serial output, facilitating various applications such as data transmission, sensor systems, and more.

Application of PISO Registers

1. Serial Data Transmission: PISO shift registers are commonly used in applications where parallel data needs to be transmitted over a serial communication channel. By converting parallel data into a serial stream, PISO shift registers facilitate efficient data transmission and enable compatibility with serial protocols.
2. Sensor Data Acquisition: In sensor systems where data from multiple sensors is collected simultaneously, a PISO shift register can be used to acquire and output the data in a sequential manner. This allows for efficient processing and analysis of the sensor readings.
3. Serial-to-Parallel Conversion: PISO shift registers can also be used to convert serial data into parallel format. By loading the serial data into the shift register and then outputting it in parallel, PISO shift registers enable interfacing between serial and parallel systems.
4. Data Logging: PISO shift registers can serve as data logging elements where data from different sources is sequentially stored and later accessed for analysis or further processing.