What is a Hardware Description Language (HDL)?

Imagine we’re about to build a magnificent Lego castle. Before we start, you need a blueprint—a detailed plan that tells us which pieces to use and how to put them together. In the world of computers, creating intricate designs for hardware—like processors, memory, and other electronic components—requires a similar blueprint. This is where Hardware Description Language (HDL) comes into play.

HDL is a specialized language used to model digital electronic circuits. It allows us to express complex hardware designs in a concise and systematic manner, making it easier to understand, simulate, and ultimately implement in physical hardware.

Difference between HDL and a programming language

One may wonder how HDL is any different than the programming languages. In a standard programming language, the commands we provide eventually run on the hardware (processor), and it does the job just fine. So, what’s the role of an HDL in this whole picture? Well, while both programming languages and HDL involve writing instructions for a computer to follow, there are some key differences between an HDL and a standard programming language like Python or Java.

Programming language

While using programming languages, we have a specific set of hardware inside the CPU. When we write the code to tell a computer what to do, focusing on tasks like calculations, data manipulation, and decision-making, it repeatedly uses that specific set of hardware to achieve the final goal. Furthermore, these instructions are executed sequentially by a processor, which follows the program’s logic step by step, line by line.

HDL

HDL is about describing the structure and/or the behavior of hardware components. Instead of using a specific set of hardware to use repeatedly, we’re essentially telling the machine what hardware to build to accomplish the required task. HDL allows us to define components like logic gates, flip-flops, and registers, and how they need to interact with each other to carry out a specific functionality.

But we can't alter the hardware of the computers we use just like that. We need to buy processors, RAM sticks, etc. So, what good is an HDL?

Uses of HDL

The circuits designed using HDL are not meant to interact with our CPUs. These circuits are either converted to Application-specific Integrated Circuits (ASICs) after passing through the fabrication process or are temporarily deployed on Field-programmable Gate Arrays (FPGAs).

ASICs: These are custom-made integrated circuits tailored for specific applications. HDL plays a crucial role in designing and optimizing ASICs for tasks ranging from signal processing to artificial intelligence. With HDL, we can create highly efficient and specialized hardware solutions optimized for their intended purpose.
FPGA Prototyping: FPGAs are programmable hardware devices that can be configured to implement custom logic circuits. A circuit on an FPGA is temporary and can be replaced if there is a mistake or if one needs to make improvements to an existing circuitry, the old circuitry can be replaced with a new one. Hence, HDL with FPGAs is suitable for prototyping new hardware designs, enabling rapid iteration and experimentation without the effort of going through the costly fabrication process.

But how is an HDL code translated to a circuitry?

From HDL to physical circuit

Once you’ve written your HDL code to describe a hardware design, the next step is to convert it into a format that can be implemented on physical hardware. This process typically involves several steps:

Synthesis: The HDL code is analyzed and translated into a netlist—a detailed description of the logical components and their connections in the design. This step optimizes the design for efficiency and performance.
Implementation: The netlist is further processed to map the logical components onto physical resources, such as ASICs or programmable logic devices (PLDs) on FPGAs. This step involves tasks like placement, routing, and timing analysis to ensure the design meets the required specifications.
Configuration/Fabrication: Once the design has been mapped onto physical hardware, the next step for it is to take a physical shape. In the case of programmable devices like FPGAs, the circuitry needs to be configured on the FPGA chip. This typically involves generating a configuration bitstream (.bit file) that can be loaded onto the device to program its internal logic. For ASICs, the design can be sent for fabrication.

By following these steps, one can turn their HDL designs into tangible hardware implementations ready for testing and deployment.

Popular HDLs available

While there are a bunch of HDLs available, there are a couple of key players in the HDL world.

Verilog: Its syntax is similar to C programming language, making it easier for software engineers to learn and understand.
VHDL: Its syntax is more verbose and resembles Ada programming language, which can be challenging for newcomers, but at the same time, designs made with VHDL are more readable and robust.

Designing a 2-to-1 multiplexer

To compare the two, let’s have a look at the code of a 2-to-1 multiplexer implemented in both of these languages. A multiplexer is a device that has two standard binary inputs and one standard binary output. Furthermore, it also has a special kind of binary input known as a "selector". Based on what value is provided by the selector, the multiplexer outputs one of the two standard inputs.

// Define a module named mux_2to1 that represents a 2-to-1 multiplexer
module mux_2to1(
    input in1,  // First input to the multiplexer
    input in2,  // Second input to the multiplexer
    input sel,  // Select signal to choose between in1 and in2
    output out  // Output of the multiplexer
);
// Use the assign statement to set the value of out based on the value of sel
// If sel is 1, out will be in2; otherwise, out will be in1
assign out = sel ? in2 : in1;
endmodule

Verilog example: 2-to-1 Multiplexer

-- Include the IEEE standard logic library
library IEEE;
use IEEE.std_logic_1164.all;
-- Define the entity for the 2-to-1 multiplexer
entity mux_2to1 is
    port(
        in1, in2, sel: in std_logic;  -- Declare inputs in1, in2, and sel of type std_logic
        output: out std_logic         -- Declare output of type std_logic
    );
end mux_2to1;
-- Define the architecture for the 2-to-1 multiplexer architecture Behavioral of mux_2to1 is
begin
    -- Define a process that will execute when in1, in2, or sel change
    process(in1, in2, sel)
    begin
        -- If sel is '1', output in2; otherwise, output in1
        if sel = '1' then
            output <= in2;
        else
            output <= in1;
        end if;
    end process;
end Behavioral;

VHDL example: 2-to-1 Multiplexer

While the functionality is the same for both Verilog and VHDL, we can observe the syntactical differences between the two languages. Verilog tends to be more concise, while VHDL is more verbose.

Conclusion

HDLs play a crucial role in the design, modeling, and implementation of digital electronic circuits. By providing a systematic way to describe hardware components, HDL enables engineers to create complex digital systems. Whether it's designing microprocessors, testing new algorithms, or prototyping custom hardware solutions, HDL lets us bring ideas to life in the world of digital electronics.

Free Resources