Concurrent Statement in VHDL: Definition and Usage

In VHDL, a concurrent statement is a command that runs simultaneously with others, modeling hardware behavior that happens at the same time. Unlike sequential statements, concurrent statements describe circuits where multiple operations occur in parallel.

How It Works

Think of concurrent statements in VHDL like multiple people working side by side on different tasks at the same time. Each statement represents a piece of hardware logic that operates independently but simultaneously with others.

In hardware, many signals change together, so VHDL uses concurrent statements to describe this parallel activity naturally. This is different from sequential statements, which run one after another like steps in a recipe.

Example

This example shows a simple 2-input AND gate using a concurrent statement. The output Y is always the AND of inputs A and B, running in parallel with other logic.

vhdl

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

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

architecture Behavioral of AndGate is
begin
    Y <= A and B;  -- concurrent statement
end Behavioral;

When to Use

Use concurrent statements when you want to describe hardware components that work at the same time, such as gates, multiplexers, or signal assignments that happen in parallel. They are essential for modeling real circuits where many signals change simultaneously.

For example, in designing a digital circuit like an adder or a flip-flop, concurrent statements let you specify how outputs depend on inputs continuously without waiting for other operations to finish.

Key Points

• Concurrent statements run in parallel, reflecting real hardware behavior.
• They are used outside processes in VHDL architectures.
• Common concurrent statements include signal assignments and component instantiations.
• They differ from sequential statements, which run step-by-step inside processes.

Key Takeaways

Concurrent statements model hardware operations that happen at the same time.

They are written outside processes and run in parallel in VHDL architectures.

Use them to describe continuous signal assignments and parallel logic.

They differ from sequential statements, which execute in order inside processes.