Uvm_no_action

5 min read Oct 04, 2024

Understanding uvm_no_action in UVM

The uvm_no_action is a special function in the Universal Verification Methodology (UVM) that plays a crucial role in handling asynchronous events and managing the flow of verification sequences.

But what exactly is uvm_no_action and why is it so important in UVM?

What is uvm_no_action?

uvm_no_action is a built-in function in UVM that serves as a placeholder for a sequence item or a sequence step. It indicates that no specific action needs to be taken at a particular point in the verification sequence. This function is especially useful when dealing with asynchronous events, where the timing of certain actions is unpredictable or dependent on external events.

When to use uvm_no_action?

Here are some common scenarios where using uvm_no_action is beneficial:

Waiting for an asynchronous event: If you need to wait for a specific event to occur before continuing your sequence, uvm_no_action can hold your sequence at that point until the event is triggered.
Handling timeouts: Sometimes, you might need to introduce a timeout mechanism in your verification sequence. If the event you're waiting for doesn't occur within a certain time frame, you can use uvm_no_action along with the wait function to introduce a timeout condition.
Skipping a step: If a particular step in your sequence is not relevant for a specific test case, you can use uvm_no_action to skip that step without affecting the rest of the sequence execution.

Example: Waiting for a signal

Let's say you have a sequence that needs to wait for a specific signal to become active before sending a command to the DUT (Device Under Test). You can use uvm_no_action in combination with the wait function to achieve this:

class my_sequence extends uvm_sequence #(my_sequence_item);
  
  `uvm_object_utils(my_sequence)

  function new(string name = "my_sequence");
    super.new(name);
  endfunction
  
  task body();
    // Create a sequence item
    my_sequence_item item = my_sequence_item::type_id::create("item");
    
    // Wait for the signal to become active
    wait(signal == 1'b1);

    // Send the command
    start_item(item);
  endtask

endclass

In this example, the wait function will hold the sequence execution until the signal becomes active. The uvm_no_action function is implicitly used within the wait function to handle the waiting time.

Benefits of using uvm_no_action:

Improved readability: Using uvm_no_action makes your sequences more readable and understandable, as it clearly indicates that no action is expected at a particular point.
Flexibility: It allows you to dynamically adjust the behavior of your sequences based on different test scenarios.
Simplified asynchronous handling: It simplifies the handling of asynchronous events within your verification environment.

Conclusion

uvm_no_action is a powerful tool in UVM that helps you effectively manage asynchronous events and improve the flexibility and readability of your verification sequences. By understanding its purpose and application, you can leverage this function to enhance your verification environment and ensure robust test coverage.