The objective of this lab manual is to introduce students to VHDL (VHSIC Hardware Description Language) programming. This manual will cover fundamental language constructs, syntax, data types, modeling styles, and implementation of basic digital circuits using VHDL.
VHDL is a hardware description language used for modeling and simulating digital systems. It enables engineers to design and describe the behavior of circuits before implementing them in hardware.
Traditional programming languages like C or C++ are not well-suited for designing and simulating hardware because they are sequential in nature, whereas digital circuits are inherently parallel. VHDL allows designers to describe concurrent operations, which is crucial for accurate hardware modeling and simulation.
Concurrency Handling: Unlike C or C++, VHDL naturally supports parallel processing, which aligns with the real behavior of digital circuits.
Hardware-Specific Constructs: VHDL includes constructs like signals, processes, and wait statements that directly model digital circuits.
Timing and Delays: VHDL allows explicit modeling of delays and clock cycles, which is essential for circuit simulation.
Testbenches for Verification: VHDL makes it easier to create testbenches for verifying digital circuits.
Synthesis Capability: VHDL descriptions can be synthesized into real hardware, whereas C/C++ is only suitable for software applications.
Before starting with VHDL programming, students should have a basic understanding of:
Digital Logic Design
Boolean Algebra
Logic Gates
Flip-Flops and Sequential Circuits
Xilinx Vivado or ISE or GHDL
ModelSim or GTKWave for simulation
Notepad++ or any text editor for writing VHDL code
Define the problem and design specifications.
Write the VHDL code.
Compile and synthesize the code.
Simulate the design.
Implement the design on FPGA (if required).
Verify the hardware implementation.
LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
ENTITY expression IS
PORT ( A, B, C : IN STD_LOGIC;
Y : OUT STD_LOGIC);
END expression;
ARCHITECTURE behavior OF expression IS
SIGNAL temp : STD_LOGIC;
BEGIN
temp <= B AND C;
Y <= A OR temp;
END behavior;
LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
ENTITY testbench IS
END testbench;
ARCHITECTURE tb OF testbench IS
SIGNAL A, B, C, Y : STD_LOGIC;
COMPONENT expression
PORT(A, B, C : IN STD_LOGIC;
Y : OUT STD_LOGIC);
END COMPONENT;
BEGIN
UUT: expression PORT MAP(A => A, B => B, C => C, Y => Y);
PROCESS
BEGIN
FOR i IN 0 TO 7 LOOP
A <= STD_LOGIC'VAL((i/4) MOD 2);
B <= STD_LOGIC'VAL((i/2) MOD 2);
C <= STD_LOGIC'VAL(i MOD 2);
WAIT FOR 10 ns;
END LOOP;
WAIT;
END PROCESS;
END tb;
Compile the expression.vhd and testbench.vhd files in ModelSim or any other VHDL simulator.
Run the simulation and generate the .vcd file.
Open GTKWave and load the .vcd file.
Add signals (A, B, C, and Y) to the waveform view.
Observe and analyze the waveform for all 8 input combinations.
This lab introduced students to VHDL, its constructs, and different modeling styles. Additionally, the implementation of A + BC using VHDL was demonstrated, along with simulation and waveform generation steps.