What is Process in VHDL: Explanation and Example

In VHDL, a process is a block of code that executes sequentially and models behavior that happens over time. It allows you to describe how signals change in response to events, like clock edges or input changes, making it essential for designing hardware logic.

How It Works

A process in VHDL works like a small program inside your hardware description that runs step-by-step whenever something it watches changes. Imagine it as a recipe that only starts cooking when you add an ingredient or when a timer rings. Inside the process, you write instructions that happen one after another, unlike the rest of VHDL which runs in parallel.

This lets you describe complex behaviors like counting, memory, or reacting to signals changing at specific times. The process waits for events on signals listed in its sensitivity list, and when one changes, it runs all the instructions inside once, then waits again. This way, you can model how real circuits react to inputs and clocks.

Example

This example shows a simple process that toggles an output signal every time a clock signal rises. It models a flip-flop behavior.

vhdl

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity ToggleFlipFlop is
    Port ( clk : in STD_LOGIC;
           reset : in STD_LOGIC;
           q : out STD_LOGIC);
end ToggleFlipFlop;

architecture Behavioral of ToggleFlipFlop is
    signal q_internal : STD_LOGIC := '0';
begin
    process(clk, reset)
    begin
        if reset = '1' then
            q_internal <= '0';
        elsif rising_edge(clk) then
            q_internal <= not q_internal;
        end if;
    end process;

    q <= q_internal;
end Behavioral;

Output

On each rising edge of clk, q toggles between '0' and '1'; if reset is '1', q resets to '0'.

When to Use

Use a process when you need to describe behavior that depends on time or events, such as clocked logic, state machines, or sequential operations. It is perfect for modeling registers, counters, or any logic that changes step-by-step rather than all at once.

For example, if you want to create a circuit that reacts only when a button is pressed or a clock ticks, a process lets you write that behavior clearly and precisely.

Key Points

• A process runs sequential code triggered by signal changes.
• It uses a sensitivity list to know when to run.
• Inside a process, you can use if, case, and loops to describe behavior.
• Processes are essential for modeling time-dependent hardware behavior.

Key Takeaways

A process in VHDL models sequential behavior triggered by signal changes.

Use processes to describe clocked logic and event-driven hardware actions.

Processes run all their statements in order when triggered by signals in their sensitivity list.

They are fundamental for designing registers, counters, and state machines.