VHDL Code for shift register can be categorised in serial in serial out shift register, serial in parallel out shift register, parallel in parallel out shift register and parallel in serial out shift register.
Generally, a register can be defined as a device used to store the binary data but if you want to store multiple data bits then a set of Flip flops are used which are connected in series. The data which is stored in the registers can be shifted by using shift registers on either the right side or left side by providing CLK pulses. Shift Register is a group of flip flops used to store multiple bits of data. Similarly, a shift register with n-bits can be formed by simply connecting n flip-flops wherever every flip-flop simply stores a single data bit. Once the register shifts the bits to the right side it is the right shift register whereas if it shifts to the left side then it is known as a left shift register. This article discusses an overview of one of the types of shift register namely serial in parallel out shift register or SIPO shift register.
4 bit serial in parallel out shift register verilog code for truth
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The shift register which allows serial input parallel output is known as the SIPO shift register. In the SIPO register, the term SIPO stands for serial input parallel output. In this type of shift register, the input data is given bit by bit serially. For each clock pulse, the input data at all the FFs can be shifted by a single position. The o/p at every flip-flop can be received parallel.
The SISO shift register circuit diagram is shown below. This circuit can be built with 4 D flip-flops which are connected as shown in the diagram where the CLR signal is given additionally to the CLK signal to all FFs o RESET them. In the above circuit, the first FF output is given to the second FFs input. All these four D flip-flops are connected with each other serially because the same CLK signal is given to every flip-flop.
The working of the SIPO shift register is; that it takes the serial data input from the first flip flop of the left side and generates a parallel data output. The 4-bit SIPO shift register circuit is shown below. The operation of this shift register is, first all the flip flops from the circuit from FF1 to FF4 have to RESET so that all the outputs of FFs like QA to QD will be at logic zero level so there is no parallel data output.
A 74HC595 IC is an 8-bit serial in parallel out shift register, so it uses inputs serially and provides parallel outputs. This IC includes 16-pins and is available in different packages like SOIC, DIP, TSSOP & SSOP.
Shift Registers are used for data storage or for the movement of data and are therefore commonly used inside calculators or computers to store data such as two binary numbers before they are added together, or to convert the data from either a serial to parallel or parallel to serial format. The individual data latches that make up a single shift register are all driven by a common clock ( Clk ) signal making them synchronous devices.
This shift register is very similar to the SIPO above, except were before the data was read directly in a parallel form from the outputs QA to QD, this time the data is allowed to flow straight through the register and out of the other end. Since there is only one output, the DATA leaves the shift register one bit at a time in a serial pattern, hence the name Serial-in to Serial-Out Shift Register or SISO.
The SISO shift register is one of the simplest of the four configurations as it has only three connections, the serial input (SI) which determines what enters the left hand flip-flop, the serial output (SO) which is taken from the output of the right hand flip-flop and the sequencing clock signal (Clk). The logic circuit diagram below shows a generalized serial-in serial-out shift register.
The Parallel-in to Serial-out shift register acts in the opposite way to the serial-in to parallel-out one above. The data is loaded into the register in a parallel format in which all the data bits enter their inputs simultaneously, to the parallel input pins PA to PD of the register. The data is then read out sequentially in the normal shift-right mode from the register at Q representing the data present at PA to PD.
This data is outputted one bit at a time on each clock cycle in a serial format. It is important to note that with this type of data register a clock pulse is not required to parallel load the register as it is already present, but four clock pulses are required to unload the data.
The final mode of operation is the Parallel-in to Parallel-out Shift Register. This type of shift register also acts as a temporary storage device or as a time delay device similar to the SISO configuration above. The data is presented in a parallel format to the parallel input pins PA to PD and then transferred together directly to their respective output pins QA to QD by the same clock pulse. Then one clock pulse loads and unloads the register. This arrangement for parallel loading and unloading is shown below.
The PIPO shift register is the simplest of the four configurations as it has only three connections, the parallel input (PI) which determines what enters the flip-flop, the parallel output (PO) and the sequencing clock signal (Clk).
Similar to the Serial-in to Serial-out shift register, this type of register also acts as a temporary storage device or as a time delay device, with the amount of time delay being varied by the frequency of the clock pulses. Also, in this type of register there are no interconnections between the individual flip-flops since no serial shifting of the data is required.
These universal shift registers can perform any combination of parallel and serial input to output operations but require additional inputs to specify desired function and to pre-load and reset the device. A commonly used universal shift register is the TTL 74LS194 as shown below.
Universal shift registers are very useful digital devices. They can be configured to respond to operations that require some form of temporary memory storage or for the delay of information such as the SISO or PIPO configuration modes or transfer data from one point to another in either a serial or parallel format. Universal shift registers are frequently used in arithmetic operations to shift data to the left or right for multiplication or division.
The shift register, which allows serial input (one bit after the other through a single data line) and produces a serial output is known as Serial-In Serial-Out shift register. Since there is only one output, the data leaves the shift register one bit at a time in a serial pattern, thus the name Serial-In Serial-Out Shift Register.
The logic circuit given below shows a serial-in serial-out shift register. The circuit consists of four D flip-flops which are connected in a serial manner. All these flip-flops are synchronous with each other since the same clock signal is applied to each flip flop.
The logic circuit given below shows a serial-in-parallel-out shift register. The circuit consists of four D flip-flops which are connected. The clear (CLR) signal is connected in addition to the clock signal to all the 4 flip flops in order to RESET them. The output of the first flip flop is connected to the input of the next flip flop and so on. All these flip-flops are synchronous with each other since the same clock signal is applied to each flip flop.
The above circuit is an example of shift right register, taking the serial data input from the left side of the flip flop and producing a parallel output. They are used in communication lines where demultiplexing of a data line into several parallel lines is required because the main use of the SIPO register is to convert serial data into parallel data.
The shift register, which allows parallel input (data is given separately to each flip flop and in a simultaneous manner) and produces a serial output is known as Parallel-In Serial-Out shift register.
The logic circuit given below shows a parallel-in-serial-out shift register. The circuit consists of four D flip-flops which are connected. The clock input is directly connected to all the flip flops but the input data is connected individually to each flip flop through a multiplexer at the input of every flip flop. The output of the previous flip flop and parallel data input are connected to the input of the MUX and the output of MUX is connected to the next flip flop. All these flip-flops are synchronous with each other since the same clock signal is applied to each flip flop.
The shift register, which allows parallel input (data is given separately to each flip flop and in a simultaneous manner) and also produces a parallel output is known as Parallel-In parallel-Out shift register.
The logic circuit given below shows a parallel-in-parallel-out shift register. The circuit consists of four D flip-flops which are connected. The clear (CLR) signal and clock signals are connected to all the 4 flip flops. In this type of register, there are no interconnections between the individual flip-flops since no serial shifting of the data is required. Data is given as input separately for each flip flop and in the same way, output also collected individually from each flip flop.
When the shift/load bar line is low (1), the AND gates 2, 4 and 6 become inactive.Hence, the parallel loading of the data becomes impossible.But the AND gates 1,3 and 5 become active.Therefore the shifting of data takes place from left-to-right bit-by-bit on the application of clock pulses.Thus, the parallel-in serial-out operation takes place.
The following image shows an n-bit bidirectional shift register with serial data loading and retrieval capacity. Initially, all the flip flops in the register are reset by driving their clear pins high.
The logic circuit given below shows a parallel-in-parallel-out shift register. The circuit consists of four D flip-flops which are connected. The clear (CLR) signal and clock signals are connected to all the 4 flip flops. 2ff7e9595c
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