Force and Release in VHDL 2008: Explanation and Example

force in VHDL 2008 is a way to override a signal's value temporarily, ignoring other drivers. release removes this override, allowing normal signal assignment to resume. These commands help in testbenches to control signals directly.

How It Works

Imagine you have a remote control car that usually follows commands from its driver. Using force is like taking the remote control yourself and making the car move exactly how you want, ignoring the driver's commands. This means you temporarily take full control of the signal's value.

When you use release, it's like handing the remote control back to the driver, so the car goes back to following the original commands. In VHDL, force sets a signal to a specific value no matter what other parts of the design say, and release stops this override so the signal behaves normally again.

This is especially useful in testbenches where you want to test how your design reacts to certain signal values without changing the actual design code.

Example

This example shows how to force a signal to '1' and then release it to let normal assignments take over.

vhdl

library ieee;
use ieee.std_logic_1164.all;

entity force_release_example is
end entity;

architecture testbench of force_release_example is
  signal sig : std_logic := '0';
begin
  process
  begin
    -- Normal assignment
    sig <= '0';
    wait for 10 ns;

    -- Force signal to '1'
    force sig <= '1';
    wait for 10 ns;

    -- Release the forced value
    release sig;
    wait for 10 ns;

    -- Normal assignment resumes
    sig <= '0';
    wait;
  end process;
end architecture;

Output

At 0 ns: sig = '0'
At 10 ns: sig forced to '1'
At 20 ns: sig released, normal assignment resumes
At 30 ns: sig = '0'

When to Use

Use force and release mainly in testbenches to simulate conditions that are hard to create with normal signal assignments. For example, you can force a signal to a stuck-at fault value to test error handling or force inputs to specific values to check design responses.

They are not meant for use inside synthesizable design code because forcing signals breaks normal signal resolution and can cause confusion in hardware behavior.

Key Points

• force overrides a signal's value temporarily.
• release removes the override and restores normal signal behavior.
• Primarily used in testbenches for controlled testing.
• Not suitable for synthesizable hardware code.
• Helps simulate special conditions like faults or fixed inputs.

Key Takeaways

force sets a signal to a fixed value ignoring other drivers.

release stops the forced value and returns control to normal assignments.

These commands are useful in testbenches for simulating special conditions.

Avoid using force and release in synthesizable design code.

They help test how designs react to unusual or fixed signal values.