Test Bench for 1x4 DeMultiplexer in VHDL Find out DeMultiplexer Code here. Library ieee; use ieee.stdlogic1164.all; entity dmux1x4seqtst is end dmux1x4seqtst. In VHDL behavioral code, i.e. When we write a VHDL code of a test bench in a pure behavioral model, the FOR-LOOP usage statement can be considered as a common SW implementation of a loop statement as in the other SW languages. In VHDL RTL the FOR-LOOP statement shall be used taking into account the final hardware implementation. VHDL Code for Round Robin Arbiter with Variable Ti. Test Bench for 1-Bit Full-Adder in VHDL; VHDL Code for 1-Bit Full Adder; Test Bench for 4x1 Multiplexer in VHDL; VHDL Code for 4x1 Multiplexer; Test Bench for 1x4 DeMultiplexer in VHDL; VHDL Code for 1x4 DeMultiplexer; Test Bench for 8x3 Encoder in VHDL; VHDL Code for 8x3 Encoder; VHDL Code.
- Vhdl Code For Demultiplexer Using Behavioural Modelling
- Vhdl Code For Demultiplexer Using Structural Modelling
In previous tutorials VHDL tutorial (#6), we built a circuit for D Morgan’s Theorems in VHDL and verified its output to prove D Morgan’s theorems.
(PDF) To implement the multiplexer and demultiplexer with. VLSI Assignment. USEFUL LINKS to VHDL CODES. Refer following as well as links mentioned on left side panel for useful VHDL codes. D Flipflop T Flipflop Read Write RAM 4X1 MUX 4 bit binary counter Radix4 Butterfly 16QAM Modulation 2bit Parallel to serial. RF and Wireless tutorials.
(If you are not following this VHDL tutorial series one by one, you are requested to go through all previous tutorials of these series before going ahead in this tutorial)
In this tutorial,
- We shall write a VHDL program to build all other gates (AND, OR, NOT, XOR, NOR, etc.) using only NAND gates
- Verify the output waveform of the program (digital circuit) with the truth table of AND, OR, NOT, XOR, NOR gates
Digital Circuit Dungeon masters 5e pdf.
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Truth Table
Now we shall write a VHDL program, compile it, simulate it, and get the output in the form of a waveform. Finely, we shall verify those output waveforms with a given truth table.
(Please go through step by step procedure given in VHDL-tutorial 3 to create a project, edit and compile the program, create waveform file, simulate the program, and generate output waveforms.)
VHDL Program:
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
entity nand_uni_gate is
Port ( a,b : in std_logic;
y_not,y_and, y_or, y_xor: out std_logic
);
end nand_uni_gate;
architecture nand_uni_gate_arch of nand_uni_gate is
begin
y_not <= a nand a;
y_and <= (a nand b) nand (a nand b);
y_or <= (a nand a) nand (b nand b);
y_nor <=((a nand a) nand (b nand b)) nand ((a nand a) nand (b nand b));
y_xor <= (a nand (a nand b)) nand (b nand (a nand b));
end nand_uni_gate_arch;
“entity” describes input-output connections of a digital circuit. As per our circuit given above, we have only two inputs ‘A’ and ‘B’ and five outputs for five circuits of different gates build using the NAND gate only.
“architecture” describes the operation of the circuit – means how the output is generated from a given input.
(To know more and get more details about VHDL program(s), please go through the first two tutorials VHDL tutorial 1 and VHDL tutorial 2 of these series.)
Next, compile above program – create waveform file with all inputs and outputs listed – simulate the project and you will get the following result
Simulation Waveform
From these output waveforms, we can easily say that the output of different gate circuits built using only NAND gates is the same as the output of a particular gate.
That means we can design all other gates using only the NAND gate, so the NAND gate is a universal gate.
In the next tutorial, we shall prove the NOR gate as a universal gate by designing AND, OR, NOT, NAND, and XNOR gates using only NOR gate.
Before reading the post, if you need the VHDL code example of the FOR-LOOP, just put your email in the box you find in the post. There is no need to post a comment asking me for the code 🙂
If you don’t receive the email, please check your SPAM folder, enjoy!
VHDL Iterative Statement
In VHDL the FOR-LOOP statement is a sequential statement that can be used inside a process statement as well as in subprograms.
The FOR-LOOP statement is used whenever an operation needs to be repeated.
In VHDL behavioral code, i.e. when we write a VHDL code of a test bench in a pure behavioral model, the FOR-LOOP usage statement can be considered as a common SW implementation of a loop statement as in the other SW languages.
In VHDL RTL the FOR-LOOP statement shall be used taking into account the final hardware implementation.
This consideration, of course, is always valid in any VHDL code implementation.
The FOR-LOOP statement is more difficult to visualize as a final result in HW implementation.
In the next section, we will learn how the FOR-LOOP statement is mapped into hardware logic using a couple of examples.
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The FOR-LOOP VHDL BNF syntax is:
The loop label is optional but is a good practice to use since the VHDL code became more readable even if the VHDL source file became larger (nowadays the space on the disk is cheaper than some years ago, so it is not an issue).
FOR-LOOP VHDL example
A typical example of FOR-LOOP statement is a parity checker.
A simple implementation of a parity checker on an 8-bit register can be implemented using an 8 input XOR gate as in Figure 1.
The only problem is that no all technologies offer an 8-input XOR gate.
A simple possible solution could be to cascade 7 XOR port where the output of the k-th port is the input of the (k+1) XOR port as in Figure 2
VHDL code for 8-bit parity checker using FOR-LOOP
Another example is represented by a VHDL code that implements an accumulator.
The architecture is very similar to a parity checker (that can be seen as an accumulator over GF(2), i.e Galois Field of order 2) the 8-bit values are added together using the temporary variable for accumulation and then assigned to the entity output.
A possible VHDL code is reported below:
VHDL code accumulator using FOR-LOOP with no optimization
A more optimized architecture of the adder for the 8 values is represented in Figure 4.
In this case, the VHDL code implements a cascade of balanced adder equalizing the delay over the adder structure. A possible VHDL code description using FOR-LOOP approach of the adder cascade is given below
VHDL code accumulator using FOR-LOOP with balanced adder tree optimization
Layout consideration on FOR LOOP
Some consideration should be done on the VHDL code above.
If we try to layout either the parity check VHDL code or the accumulator VHDL code the VHDL synthesize can optimize our code implementing the best hardware structure for the device we are using.
Altera/Intel Implementation
Altera Quartus II implements the different VHDL description of the adder in the same way. As clear from Figure 5, the RTL viewer reports different implementation for the VHDL code of cascaded adder or balanced tree adder.
In the first case, the adder tree is not balanced, in the second case the addition is performed using a balance adder tree.
In both cases, the Fitter and netlist optimizer implements on an Altera/Intel Cyclone II the same hardware mapping as clear from the fitter report and timing analysis in Figure 6.
Xilinx ISE Implementation
Xilinx ISE implements the different VHDL description of the adder in a slightly different way and the implementation depends on the VHDL RTL code. The FOR-LOOP implementation triggers different hardware architecture. A timing and area report for the two design is reported in
As clear, on a Xilinx Spartan 3 FPGA, equivalent to the Cyclone II Altera FPGA, the area and timing report for the two-different implementation differs both on area and timing. The maximum timing achievable is in the balanced VHDL code version and it is about 80 MHz in the cascaded adder implementation and about 107 MHz in the balanced tree adder implementation.
Xilinx vs Altera
In this particular example, Altera Quartus II optimization seems to work better than Xilinx ISE optimization.
It is always a good VHDL design approach to read the layout report such as mapper, fitter and static timing analysis in order to verify if the obtained results are in line with the expected ones.
Conclusion
In this post, we treated the implementation of FOR-LOOP in VHDL. After the VHDL syntax of the FOR-LOOP and its BNF, we tried to understand, in two different examples, the consideration to take into account when dealing with FOR-LOOP VHDL statement.
Last, but not least, the same VHDL code could be translated in different implementation by different synthesizer as we confirmed using Altera Quartus II and Xilinx ISE .
As a design rule, we should write a VHDL code that reflects the hardware architecture we want to realize in order to guide the VHDL synthesizer versus our hardware implementation.
Even if the synthesis software is becoming more and more powerful, we should always check the synthesis and mapping results to verify in our VHDL code has been translated as we expect.
Reference
[1] RTL HARDWARE DESIGN USING VHDL Coding for Efficiency, Portability, and Scalability
[2] VHDL Programming by Example 4th Ed Douglas – Perry
[3] The VHDL Cookbook
[4] Xilinx ISE
[5] Xilinx Spartan 3 FPGA
Vhdl Code For Demultiplexer Using Behavioural Modelling
[6] Altera/Intel Cyclone II
Vhdl Code For Demultiplexer Using Structural Modelling
[7] Altera Quartus II