# Shift Register Parallel In Serial Out Vhdl Code

So I've been using VHDL to make a register, where it loads in the input X if LOAD is '1' , and outputs the data in serial fashion , basically a parallel in serial out register. The input X is a 4 bit ( 3 downto 0 ) input , what I want to make the program do is constantly output 0 when the register has successfully output all the btis in the input.

## shift register parallel in serial out vhdl code

ENTITY shift_register ISPORT (clock IN; ser_in IN; ser_out OUT);END shift register;ARCHITECTURE rtl OF shift_register ISSIGNAL shiftreg STD_LOGIC_VECTOR(15 DOWNTO 0);BEGIN -- Architectureser_out

Shift register are the registers which are used to shift the stored bit in one or both directions. In this section, shift register is implemented which can be used for shifting data in both direction. Further it can be used as parallel to serial converter or serial to parallel converter. VHDL files required for this example are listed below,

In Listing 11.7, the parallel-counter-data is converted into serial data using Listing 11.5. Then received serial data is converted back to parallel data by Listing 11.6. The simulation results are shown in Fig. 11.4

There are many ways to create a shift register in VHDL, though not all of them are equal. You can dramatically reduce the number of consumed resources by choosing the right shift register implementation for your needs and FPGA architecture.

A shift register implements a FIFO of fixed length. Every time a new element enters the queue, it shifts the existing ones one place further away from the input. To understand the basics of the shift register, I recommend viewing the VHDL tutorial about the std_logic_vector.

While any shift register is suitable for creating generic, smaller buffers, there are methods of efficiently creating larger ones. Many FPGAs have logic elements that can double as specialized shift register primitives. You can improve performance in magnitudes by being mindful of how you write your VHDL code.

Even though the shift register should require 128 flip-flops (FFs), we see that the resource usage reported by Vivado and Quartus is far less. Instead of using expensive FFs, the synthesis tools have used special built-in features of the logic blocks.

The most straightforward way to create a shift register is to use vector slicing. Insert the new element at one end of the vector, while simultaneously shifting all of the others one place closer to the output side. Put the code in a clocked process and tap the last bit in the vector, and you have your shift register.

You should be cautious about adding reset values to the shift register vector or output. The problem is that it prevents the synthesis tool from packing the shift register into LUTs or BRAM. Consider the example below, which is the same as the first one in this article, but with synchronous reset added.

The synchronous reset has forced the synthesis tool to implement the shift register entirely in FFs. Therefore, you should ask yourself if you need to be able to reset the entire shift register at once.

The final example in this article is a shift register with generic width and depth, using synchronous reset. The code below shows the implementation which uses the reset counter that we discussed earlier in this article.

We can see from the listing below that the Xilinx FPGA needs eight additional regular LUTs and seven FFs for implementing the counter reset. Intel Quartus II still somehow reports the same resource usage as without reset. The Lattice FPGA consumes 24 more LUTs and 31 more FFs for implementing the counter, but the shift register still fits in one BRAM.

But you can override the automatic choice by using a synthesis attribute, also known as a pragma or compiler directive. The different FPGA vendors have their own sets of VHDL attributes. To specify a desired primitive type, you define the attribute in the architecture region of the VHDL file, referencing your shift register array or vector by name.

Setting the shreg_extract attribute to "no" disables all shift register optimization. This setting acts like a master switch, overriding other SRL synthesis settings. You can also assign "yes" to shreg_extract, but this is the default setting anyway.

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.

Serial in, serial out? How about: out i need a vhdl code for 16 bit serial-in, serial-out shift register computer_2(); Mon, 11 Apr 2005 11:34:27 GMT M Pedl#3 / 4 need vhdl code for 16 bit serial-in, serial-out shift register A 16 bit fifo?? Mon, 11 Apr 2005 23:55:31 GMT Niv#4 / 4 need vhdl code for 16 bit serial-in, serial-out shift register How about (but ignore syntax just a very quick stab at it); ENTITY shift_register IS PORT (clock IN; ser_in IN; ser_out OUT); END shift register; ARCHITECTURE rtl OF shift_register IS SIGNAL shiftreg STD_LOGIC_VECTOR(15 DOWNTO 0); BEGIN -- Architecture ser_out

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