UVM Sequence

The UVM sequence is a core component of the Universal Verification Methodology (UVM) and plays a crucial role in generating and delivering stimulus to the DUT (Device Under Test). As a user-defined class extending the uvm_sequence base class, the UVM sequence defines the behavior of transactions and interacts with the sequencer to generate stimuli in a controlled and organized manner. A UVM sequence is a transaction generator. It defines the flow of transactions that are sent to the driver via the sequencer. Sequences can call other sequences, enabling hierarchical and modular test creation.

A user-defined sequence class is extended from uvm_sequence. It should be parameterized with request(REQ) and response(RSP) sequence_item types. Response is optional. uvm_sequence is inherited by uvm_object.

uvm_sequence base class:

virtual class uvm_seqence #( type REQ = uvm_sequence_item, type RSP = REQ) extends uvm_sequence_base

UVM Sequence Anatomy: The body Method

The body method is the heart of a UVM sequence. It contains the procedural code that defines the flow and behavior of the sequence. The body method is automatically executed when the sequence starts.

Key Points About the body Method:

  1. Automatic Execution: When a sequence is started, the UVM framework automatically calls its body method.
  2. Transaction Generation: The body method is where transactions are created, randomized, and sent to the sequencer.
  3. Control Flow: Loops, conditionals, and synchronization constructs can be used to control the behavior of the sequence.

Example: Implementing a Simple UVM Sequence

Let’s consider a scenario where we need to drive an APB transaction to the DUT. Below is a step-by-step example of how to create and use a UVM sequence with the body method.

class pt_sequence extends uvm_sequence#(p_transaction);
  `uvm_object_utils(pt_sequence)
  
  p_transaction req;
  
  function new(string name = "pt_sequence");
    super.new(name);
  endfunction
  
  virtual task body();
    req=p_transaction::type_id::create("req");
    for (int i=4;i<21;i=i+4) begin
    start_item(req);
          req.paddr= i;
          req.pwdata = i+4;
          req.pwrite = 1;
      finish_item(req);
    end
    
    for (int i=4;i<21;i=i+4) begin
    start_item(req);
          req.paddr= i;
         // req.pwdata = i+4;
          req.pwrite = 0;
      finish_item(req);
    end
  endtask

  endclass

Explanation of the body Method

  1. Transaction Creation:
    The body method creates an instance of the apb_transaction class (req) in a loop.
  2. Sequencer Communication:
    • start_item(trans) initiates a handshake with the sequencer, notifying it that a new transaction is ready.
    • finish_item(trans) finalizes the transaction and sends it to the sequencer.
  3. Control Flow:
    The loop ensures that 5 transactions are created and sent sequentially.

Key Takeaways:

  1. The body method is automatically executed when a sequence is started.
  2. Use start_item and finish_item to send transactions to the sequencer.
  3. Leverage randomization and constraints to generate diverse stimuli.
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