VHDL (VHSIC Hardware Description Language) is a powerful and versatile hardware description language used for the design, simulation, and synthesis of digital electronic systems. Originally developed for the U.S. Department of Defense's Very High Speed Integrated Circuits (VHSIC) program in the 1980s, VHDL allows engineers to create abstract models of hardware behavior and structure. Its capability to describe complex systems has made it essential in various industries, especially for FPGA (Field Programmable Gate Array) and ASIC (Application-Specific Integrated Circuit) designs.
VHDL was created in the early 1980s as part of an effort to standardize the design of digital circuits. The language was developed by a team led by Peter Ashenden at the U.S. Department of Defense to address the need for a consistent way to describe hardware. The initial intent was to facilitate the documentation of existing designs for the VHSIC program, but it evolved into a full-fledged language for hardware description.
In 1987, VHDL was standardized by the IEEE as IEEE 1076. Since then, it has undergone various updates, with IEEE 1076-1993 and IEEE 1076-2002 being significant revisions. The language has been extended to include constructs that support design verification, synthesis constraints, and simulation capabilities.
Today, VHDL is widely used in both academia and industry for designing complex electrical systems. Its syntax and behavior are influenced by programming languages such as Ada, Pascal, and C. Various tools for simulation and synthesis can interpret VHDL code, and it is often used in conjunction with other hardware description languages, most notably Verilog.
VHDL is a strongly typed language, meaning that every variable must be declared with a specific type before it is used. This prevents many common errors.
signal CLK : std_logic; -- Declaring a signal of type std_logic
VHDL supports concurrent execution of statements. Multiple processes can run simultaneously, which is essential for modeling hardware behavior.
process (CLK)
begin
if rising_edge(CLK) then
-- Task to be performed
end if;
end process;
VHDL uses an entity-architecture model where the entity describes the interface of a hardware component, while the architecture describes its internal behavior.
entity AND_Gate is
Port ( A, B : in std_logic;
Y : out std_logic);
end AND_Gate;
architecture Behavior of AND_Gate is
begin
Y <= A and B;
end Behavior;
VHDL has several built-in data types, including scalar (e.g., integer, bit), composite (e.g., array, record), and access types.
type MyArray is array (0 to 9) of integer; -- Array declaration
Signal assignments occur concurrently in VHDL. You can drive signals using assignments that get executed whenever an event occurs.
signal C : std_logic;
C <= A and B; -- C gets updated when A or B changes
VHDL supports packages, which allow users to define reusable components and types. Packages help in organizing code and promoting reuse.
package MyLib is
type MyType is array (0 to 7) of std_logic_vector(3 downto 0);
end MyLib;
VHDL allows configurations to be set at a higher level, enabling the selection of different architectures for entities.
configuration MyConfig of AND_Gate is
for Behavior
end for;
end MyConfig;
VHDL facilitates the creation of test benches for simulating hardware designs. A test bench contains instances of the design under test and applies stimulus.
entity TB_AND_Gate is
end TB_AND_Gate;
architecture sim of TB_AND_Gate is
signal A, B : std_logic;
signal Y : std_logic;
begin
uut: entity work.AND_Gate port map (A, B, Y);
-- Stimulus process here
end sim;
Assertions can be used to check properties during simulation and help in debugging designs.
assert (A = '1') report "A is not high" severity error;
Generics allow parameterization of designs, making them flexible and reusable without changing the core logic.
entity Counter is
generic (N : integer := 4);
port (CLK : in std_logic;
COUNT : out std_logic_vector(N-1 downto 0));
end Counter;
Several Integrated Development Environments (IDEs) and tools support VHDL development, including:
To build a VHDL project, designers typically write their VHDL code in .vhdl or .vhd files, create a test bench for simulation, and utilize an appropriate tool to synthesize the design into a target hardware. The process typically involves:
VHDL is predominantly used in the design of digital systems that require precise timing and functionality. Its applications include:
VHDL can be compared to several other programming languages, each having its unique features and intended applications:
C and C++ are general-purpose programming languages that allow procedural and object-oriented programming. In contrast, VHDL is explicitly designed for hardware description, focusing on concurrent execution rather than sequential.
Verilog is another hardware description language that shares similarities with VHDL but is generally considered less verbose. Verilog uses a more succinct syntax and is often favored for system-level design.
Python is a high-level scripting language that excels in rapid application development but is not designed for direct hardware description. However, libraries such as MyHDL allow Python code to be converted to VHDL.
MATLAB is used for numerical computing and algorithm development but can be integrated with VHDL for hardware implementation through its HDL Coder and Simulink.
Rust and Ada have features that promote memory safety and concurrent programming but are not specialized for hardware description. VHDL's emphasis on hardware abstraction is unique among these languages.
Translating source code from other languages into VHDL often requires special consideration of concurrency and timing that is inherent in hardware design.
Several source-to-source translation tools are available to assist in converting code:
In conclusion, VHDL serves as a critical tool in digital design, offering robust features, a rich history, and a variety of applications across electronics and computing. Its unique capabilities in hardware description set it apart from many widely-used programming languages.