4 Bit Serial In Serial Out Shift Register Vhdl Code For A Jk

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.

Also, the directional movement of the data through a shift register can be either to the left, (left shifting) to the right, (right shifting) left-in but right-out, (rotation) or both left and right shifting within the same register thereby making it bidirectional. In this tutorial it is assumed that all the data shifts to the right, (right shifting).

The effect of each clock pulse is to shift the data contents of each stage one place to the right, and this is shown in the following table until the complete data value of 0-0-0-1 is stored in the register. This data value can now be read directly from the outputs of QA to QD.

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.

In the next tutorial about Sequential Logic Circuits, we will look at what happens when the output of the last flip-flop in a shift register is connected directly back to the input of the first flip-flop producing a closed loop circuit that constantly recirculates the data around the loop. This then produces another type of sequential logic circuit called a Ring Counter that are used as decade counters and dividers.

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.

Serial-in, serial-out shift registers delay data by one clock time for eachstage. They will store a bit of data for each register. A serial-in,serial-out shift register may be one to 64 bits in length, longer ifregisters or packages are cascaded.

The main usage for a shift register is for converting from a serial datainput stream to a parallel data output or vice versa. For a serial toparallel data conversion, the bits are shifted into the register at eachclock cycle, and when all the bits (usually eight bits) are shifted in, the8-bit register can be read to produce the eight bit parallel output. For aparallel to serial conversion, the 8-bit register is first loaded with theinput data. The bits are then individually shifted out, one bit per clockcycle, on the serial output line. In general a shift register ischaracterized by the following control and data signals:

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Shift registers consist of D flip-flops as shown in the figure below. This is a four bit shift register and therefore consists of four D flip-flops. This shift register is configured to shift data from the left to the right.

This example creates a shift register using a VHDL signal called shift_reg shown in the code listing below. This register is initialized with the value of 00h so that when power is switched on to the CPLD board, the register will be cleared. The shift_reg register is 8 bits wide and the VHDL code connects each bit in the register to an LED, so that 8 LEDs show the value in each bit of the register.

In the above code, the shifting is done by moving seven bits of data in a single line of code. Bits 7 to 1 (the upper seven bits) are moved to bits 6 to 0 all in one go. In other words the upper seven bits are moved right by one bit position.

A shift register is a type of digital circuit using a cascade of flip-flops where the output of one flip-flop is connected to the input of the next. They share a single clock signal, which causes the data stored in the system to shift from one location to the next. By connecting the last flip-flop back to the first, the data can cycle within the shifters for extended periods, and in this form they were used as a form of computer memory. In this role they are very similar to the delay-line memory systems and were widely used in the late 1960s and early 1970s to replace that form of memory. 2b1af7f3a8