While Loop

While Loop

The while loop is used where the number of repetitions is not known from the start. A while loop continues to execute as long as a specified condition is true.

Syntax

The basic structure of a while loop is:

loop_label: while <condition> loop
    -- Going through...
    -- IMPORTANT: Must include logic to eventually make the condition false!
end loop loop_label;

• <condition> is a boolean expression that is checked before each iteration. If it's true, the loop body executes. If it's false, the loop terminates.

Warning: Infinite Loops

A while loop can create an infinite loop. This occurs if the loop's condition never becomes false. Unlike in C, Java, Python etc. where infinite loops are harmless, VHDL shouldn't have these.

• In a simulation, an infinite loop will cause the simulator to hang and never finish.
• In synthesis, it can be interpreted as a feedback path that creates a latch, or the synthesizer might fail with an error.

To avoid this, ensure that the logic inside the loop will eventually cause the condition to become false.

Example: Finding the First '1'

In this example, we'll search a vector from left to right (from the most significant bit) to find the position of the first bit that is a '1'.

Notice that unlike a for loop, we must manually declare and update our index variable (i).

-- Inside a process...
p_Find_First_One: process(a_vector)
    -- We must declare our own index variable for a while loop
    variable i : integer := a_vector'left; -- Start at the leftmost bit
    variable i_position : integer := -1; -- -1 if no 1 is found
begin
    -- Reset variables for each run of the process
    i_position := -1;
    i := a_vector'left;

    FIRST_ONE: while (i >= a_vector'right) loop

        if a_vector(i) = '1' then
            i_position := i;
            exit FIRST_ONE; -- Quit if found
        end if;

        -- Manually decrement the index to check the next bit
        i := i - 1;

    end loop SEARCH_LOOP;

    -- Assign the result to a signal
    found_position_signal <= i_position;

end process p_Find_First_One;

In this code, the loop continues as long as i is within the vector's bounds. If a '1' is found, the exit statement terminates the loop early. If no '1' is found, the loop completes naturally when i goes out of bounds.