Mastering Conditional Logic in VHDL: If-Then-Elsif-Else Explained

In the previous tutorial we used a conditional expression with the Wait Until statement to trigger a process only when two counter signals matched. What if we need the program to take different actions based on varying inputs?

The If-Then-Elsif-Else construct lets us branch our logic. Depending on a variable’s value or the result of an expression, the program can follow distinct paths.

This article is part of the Basic VHDL Tutorials series.

Basic Syntax

The canonical form is:

if <condition> then
elsif <condition> then
else
end if;

The elsif and else clauses are optional, and elsif can appear multiple times. A <condition> may be a boolean literal or an expression that evaluates to true or false.

Example: MyCounter < 10 is true when MyCounter is less than ten.

Relational Operators

Logical Operators

Exercise

Below is a complete VHDL example that demonstrates the If-Then-Elsif-Else construct in action.

Final code:

entity T08_IfTb is
end entity;

architecture sim of T08_IfTb is

    signal CountUp   : integer := 0;
    signal CountDown : integer := 10;

begin

    process is
    begin

        CountUp   <= CountUp + 1;
        CountDown <= CountDown - 1;
        wait for 10 ns;

    end process;

    process is
    begin

        if CountUp > CountDown then
            report "CountUp is larger";
        elsif CountUp < CountDown then
            report "CountDown is larger";
        else
            report "They are equal";
        end if;

        wait on CountUp, CountDown;

    end process;

end architecture;

Simulator console output when running the model in ModelSim:

VSIM 2> run
# ** Note: CountDown is larger
#    Time: 0 ns  Iteration: 0  Instance: /t08_iftb
# ** Note: CountDown is larger
#    Time: 0 ns  Iteration: 1  Instance: /t08_iftb
# ** Note: CountDown is larger
#    Time: 10 ns  Iteration: 1  Instance: /t08_iftb
# ** Note: CountDown is larger
#    Time: 20 ns  Iteration: 1  Instance: /t08_iftb
# ** Note: CountDown is larger
#    Time: 30 ns  Iteration: 1  Instance: /t08_iftb
# ** Note: They are equal
#    Time: 40 ns  Iteration: 1  Instance: /t08_iftb
# ** Note: Countup is larger
#    Time: 50 ns  Iteration: 1  Instance: /t08_iftb
# ** Note: Countup is larger
#    Time: 60 ns  Iteration: 1  Instance: /t08_iftb
...

Analysis

We initialized CountDown to 10 and CountUp to 0. Both counters are updated simultaneously every 10 ns in the first process. The second process waits on these signals, so it triggers each time they change. The If-Then-Elsif-Else block then selects one of three branches: CountUp > CountDown, CountUp < CountDown, or CountUp = CountDown. Although an explicit else is not mandatory, including it is a best practice, ensuring all edge cases are handled.

The console trace confirms that the second process evaluates the condition after each update, printing the appropriate message.

Takeaway

• Use If-Then alone or combine with Elsif and Else to cover all possible scenarios.
• Conditions can include relational, logical, and arithmetic expressions.

Continue to the next tutorial »