Implementation of Decoder using VHDL

An Overview

Decoders are used by digital systems to transform encoded data into a more intelligible format. In many different applications, including memory addressing, data routing, and control systems, these essential components are frequently used. The implementation of a decoder using VHDL (Very High-Speed Integrated Circuit Hardware Description Language) will be discussed in this article. Before physical implementation, designers can model and simulate digital circuits using the potent hardware description language known as VHDL. We'll approach comprehending and putting a decoder into use using VHDL step-by-step.

Decoder

A decoder is a combinational logic circuit that uses a particular decoding strategy to produce an output from an input code. It is a crucial element in digital systems that makes it possible to comprehend and retrieve data encoded in various formats.

A decoder's main job is to translate an encoded input signal into a corresponding output signal. A series of bits called a binary code are often used as the input to a decoder. The decoder examines the input code and, depending on the value, activates one or more particular output lines.

Applications where decoders are frequently employed include memory addressing, data routing, and control systems. Depending on the input code, they enable the selection and activation of particular memory regions, data pathways, or control signals.

The amount of input bits and output lines can affect the setup of a decoder. Typical varieties of decoders include −

• Two-to-four Decoder − A two-to-four decoder converts a two-bit input code into a four-bit output. Each 2-bit input combination corresponds to a different output line.

• Three-to-eight Decoder − A three-to-eight decoder converts a three-bit input code into an eight-bit output. Each 3-bit input combination chooses a certain output line.

• Four-to-Sixteen Decoder − A four-to-sixteen decoder converts a four-bit input code into a sixteen-bit output. A particular output line is activated by each combination of the 4-bit input.

Different technologies, including discrete logic devices (AND, OR, NOT gates) and programmable logic devices like Field-Programmable Gate Arrays (FPGAs) or Application-Specific Integrated Circuits (ASICs), can be used to create decoders. The complexity and particular requirements of the digital system determine how it will be implemented.

VHDL

A hardware description language called VHDL (Very High-Speed Integrated Circuit Hardware Description Language) is used to model and describe digital systems. It offers a standardised and organised method for describing the structure and behaviour of digital circuits at different levels of abstraction.

The U.S. Department of Defence created VHDL in the 1980s to replace the previous Verilog hardware description language. It is frequently used in the design and verification of digital systems, including sophisticated integrated circuits, field-programmable gate arrays (FPGAs), and application-specific integrated circuits (ASICs). Since that time, it has developed into an industry standard.

The main goal of VHDL is to give designers the ability to specify the functionality and organisation of digital systems in a way that is acceptable for automated tools to simulate, synthesise, and implement the designs on hardware platforms while also being human-readable. It enables designers to describe temporal restrictions, model component interactions, and capture the behaviour of circuits.

VHDL's definition of things and architectures makes it possible to construct digital systems hierarchically. An entity specifies the inputs, outputs, and other pertinent properties of a component's interface. A component's internal structure and behaviour, as well as how different subcomponents are connected, are defined by its architecture.

Both behavioural and structural modelling are supported by the language. Similar to a programming language, behavioural modelling enables designers to specify the operation of a component using concurrent and sequential statements. In order to construct a higher-level design, structural modelling entails defining the connection and instantiation of lower-level components.

Additionally, testbenches—separate VHDL code modules used to stimulate and confirm the behaviour of the design—can be created using the capabilities provided by VHDL. Test benches let designers replicate various situations and evaluate the accuracy and effectiveness of their creations.

The portability of VHDL is one of its key benefits. Because VHDL designs may be synthesised and executed on a variety of hardware platforms, designers can target alternative technologies without having to completely rewrite the design.

Implementation of Decoder Using VHDL

Step 1: Defining the Functionality of the Decoder

Clearly defining a decoder's functionality is the first step towards putting it into practise. A decoder generates a 2n output from an n-bit input. Each output is associated with a different set of input states. For instance, a 2-to-4 decoder converts a 2-bit input into a 4-bit output, where each output bit corresponds to a particular combination of the input bits.

Step 2: Creating the VHDL Entity

We may create the VHDL entity for the decoder once the functionality has been defined. The object gives information on the decoder module's input and output ports. The entity declaration for a 2-to-4 decoder would resemble this −

entity Decoder_2x4 is
   port (
      input : in std_logic_vector(1 downto 0);
      output : out std_logic_vector(3 downto 0)
   );
end entity Decoder_2x4;

Step 3: Putting the Decoder Architecture into Practise

We can move on to implement the decoder's architecture after specifying the entity. The internal reasoning and behaviour of the module are described by the architecture. This phase involves defining the decoder's truth table and mapping the input bit combinations to their corresponding output bits. Here is an illustration of a 2-to-4 decoder implementation −

architecture Behavioral of Decoder_2x4 is
begin
   process (input)
   begin
      case input is
         when "00" =>
            output <= "0001";
         when "01" =>
            output <= "0010";
         when "10" =>
            output <= "0100";
         when "11" =>
            output <= "1000";
         when others =>
            output <= (others => '0');
      end case;
   end process;
end architecture Behavioral;

Step 4: Testing and Simulating

To confirm the VHDL design's accuracy and usefulness, simulation is a crucial step. VHDL offers strong simulation capabilities that enable designers to validate the decoder's behaviour. Code for a test bench can be created to apply various input patterns and track the resulting outputs.

Step 5: Implementation and Synthesis

The design can be synthesised to create a netlist, which is a more abstract representation of the digital circuit, after it has been properly simulated and confirmed. The physical implementation of the decoder on a particular target device or FPGA can then be determined by further processing the synthesised netlist using tools like place-and-route.

Step 6: Physical Verification and Testing

The decoder can be tested and verified physically after being physically implemented. On the actual hardware, this entails checking for adequate functioning, timing restrictions, and other design specifications.

Conclusion

The functionality and layout of the digital circuit can be succinctly and effectively described by designers by utilising VHDL to create a decoder. One can effectively implement a decoder using VHDL, simulate its behaviour, synthesise it for physical implementation, and carry out extensive testing and verification by adhering to the above-mentioned processes. VHDL is a great alternative due to its adaptability and widespread use.