Why combinational design is the VHDL foundation - See It in Action

Why combinational design is the VHDL foundation

📖 Scenario: You are learning VHDL, a language used to describe digital circuits. Combinational design is the base for many digital systems because it defines outputs based only on current inputs, without memory.

🎯 Goal: Build a simple combinational logic circuit in VHDL that outputs the AND of two inputs. This will show why combinational design is the foundation of VHDL.

📋 What You'll Learn

Create a VHDL entity with two inputs and one output

Define a signal that represents the AND of the inputs

Use a concurrent assignment to implement combinational logic

Print the output signal in a testbench

💡 Why This Matters

🌍 Real World

Combinational logic is used in digital circuits like adders, multiplexers, and control units.

💼 Career

Understanding combinational design is essential for hardware engineers and FPGA developers working with VHDL.

1

Create the VHDL entity with inputs and output

Write a VHDL entity called AndGate with two inputs A and B of type std_logic, and one output Y of type std_logic.

VHDL

entity AndGate is
    Port (
        A : in std_logic;
        B : in std_logic;
        Y : out std_logic
    );
end AndGate;

-- Your code here

Need a hint?

Define the entity with inputs and output ports exactly as named.

2

Add the architecture with a signal for AND operation

Write an architecture called Behavioral for AndGate. Inside it, declare a signal and_result of type std_logic.

VHDL

entity AndGate is
    Port (
        A : in std_logic;
        B : in std_logic;
        Y : out std_logic
    );
end AndGate;

architecture Behavioral of AndGate is
    signal and_result : std_logic;
begin
-- Your code here

Need a hint?

Declare the architecture and the signal inside it.

3

Implement combinational logic with concurrent assignment

Inside the Behavioral architecture, assign and_result to the AND of A and B using a concurrent signal assignment. Then assign Y to and_result.

VHDL

entity AndGate is
    Port (
        A : in std_logic;
        B : in std_logic;
        Y : out std_logic
    );
end AndGate;

architecture Behavioral of AndGate is
    signal and_result : std_logic;
begin
    -- Your code here

Need a hint?

Use concurrent signal assignments to implement the AND logic.

4

Create a testbench to display the output

Write a simple testbench entity and architecture that instantiates AndGate. Set inputs A and B to '1' and '0' respectively, then print the output Y using report statement.

VHDL

entity AndGate is
    Port (
        A : in std_logic;
        B : in std_logic;
        Y : out std_logic
    );
end AndGate;

architecture Behavioral of AndGate is
    signal and_result : std_logic;
begin
    and_result <= A and B;
    Y <= and_result;
end Behavioral;

-- Your testbench code here

Need a hint?

Instantiate the AndGate and use a process to print the output after setting inputs.