Class hierarchy was discussed in the previous session. Now, let's try to construct a testbench that will print out "Hello UVM". We'll keep the testbench simple with only two main components.

  • Test
  • Environment


You can download/clone uvm-101 from repository.

Testbench Setup

The testbench structure is setup as shown in the image below, with the environment instantiated inside the test and the DUT interacting with the test via the DUT interface.

simple UVM testbench

Let us talk about each component.


The DUT is a simple memory module that will accept data into the memory array upon a write transaction and provide data at the output ports for a read transaction. It has the following ports

module dut (   
               input    clk,                 // Clock at some freq
               input    rstn,                // Active Low  Sync Reset
               input    wr,                  // Active High Write
               input    en,                  // Module Enable
               input    wdata,               // Write Data
               input    addr,                // Address
               output   rdata                // Read Data


Let's put all the signals of the DUT inside an interface and use an interface handle to access the signals. The interface is defined as below.

interface dut_if (input clk);
   logic          rstn;
   logic [7:0]    wdata;
   logic [7:0]    rdata;
   logic [7:0]    addr;
   logic          wr;
   logic          en;

DUT Wrapper

It'll be easier to instantiate the DUT in the top level module if we put a wrapper around it.

module dut_wrapper (dut_if _if);
   // Instantiate the design module and connect interface signals to DUT
   dut   dsn0     (  .clk     (_if.clk),
                     .rstn    (_if.rstn),
                     .wr      (_if.wr),
                     .en      (_if.en),
                     .wdata   (_if.wdata),
                     .addr    (_if.addr),
                     .rdata   (_if.rdata));

With the DUT wrapper, instantiating at the top level reduces to a single line.

dut_wrapper    dut_wr0  (._if (dut_if1));

Watch how to install Modelsim and run Hello World !

The test is an object of uvm_test and contains the environment. The interface handle is obtained from the top level module and will pass it to all components inside the environment. The test will also decide on what sequence to run when simulation is started. This is very convenient because now you have the ability to create multiple tests that can choose from a library of sequences wihtout having to edit the code in the testbench. Let's break the class test and go through each line.

class base_test extends uvm_test;

We have defined a class called base_test that inherits all the properties and methods from uvm_test. Now, we have to register this component with the UVM factory for it to work properly by calling the macro `uvm_component_utils because test is a derivative of uvm_component.

  `uvm_component_utils (base_test)

Now, we'll create an object of the environment and a virtual interface handle that can be made to point to the correct interface.

    my_env   m_top_env;
    virtual  dut_if dut_vi;

Next, we'll code the constructor new () with arguments name and parent (default is set to null).

      function new (string name, uvm_component parent = null); (name, parent);
      endfunction : new

Then, the build phase is defined to instantiate an object of the environment, and collect the interface from top level module. Note that we have called the parent's build phase first using super keyword. The line that contains type_id is the preferred way to create objects of classes in UVM. This is because it uses factory methods to create the object that can be easily substituted/modified later on. Note that you can also create an object using the new () method, but that does not make object instantiation flexible enough and hence is not recommended. The line that contains uvm_config_db is the new way of setting and retrieving variable values in UVM. In C, you would have declared the variable to be global and it can be accessed from anywhere, in any header/C file. In UVM, variables can be put inside the database and make it available to selected components and only those components will be able to access the variable value. So, in this case we are trying to get the dut_if object from the database.

     virtual function void build_phase (uvm_phase phase);
         super.build_phase (phase);
         m_top_env  = my_env::type_id::create ("m_top_env", this);
         if (! uvm_config_db #(virtual dut_if) :: get (this, "", "dut_if", dut_vi)) begin
            `uvm_error (get_type_name (), "DUT Interface not found !")
      endfunction : build_phase

The function get () has arguments in the order ['context', 'instance_name', 'field_name', 'variable_to store_value'].

We can also print out the topology of the environment after the build and connect phases.

  virtual function void end_of_elaboration_phase (uvm_phase phase);
     uvm_top.print_topology ();

In the start_of_simulation_phase you can specify the sequence that a particular sequencer has to operate on.


All we do in the environment is to print out a message "Hello UVM !"

class my_env extends uvm_env ;
  `uvm_component_utils (my_env)
  function new (string name, uvm_component parent); (name, parent);
  endfunction : new
  function void build_phase (uvm_phase phase);
    super.build_phase (phase);
  endfunction : build_phase
  task run_phase (uvm_phase phase);
    set_report_verbosity_level (UVM_MEDIUM);
    uvm_report_info      (get_name(), $sformatf ("Hello UVM ! Simulation has started."), UVM_MEDIUM, `__FILE__, `__LINE__);
    `uvm_info   (get_name(), $sformatf("Finishing up with run_phase ... "), UVM_LOW)
  endtask : run_phase
endclass : my_env

Simulation Output
CDNS-UVM-1.1d (14.10-s004)
(C) 2007-2013 Mentor Graphics Corporation
(C) 2007-2013 Cadence Design Systems, Inc.
(C) 2006-2013 Synopsys, Inc.
(C) 2011-2013 Cypress Semiconductor Corp.
UVM_INFO @ 0: reporter [RNTST] Running test base_test...
UVM_INFO @ 0: reporter [UVMTOP] UVM testbench topology:
Name          Type       Size  Value
uvm_test_top  base_test  -     @2601
  m_top_env   my_env     -     @201

UVM_INFO ./tb/ @ 0: uvm_test_top.m_top_env [m_top_env] Hello UVM ! Simulation has started.
UVM_INFO ./tb/ @ 0: uvm_test_top.m_top_env [m_top_env] Finishing up with run_phase ...

--- UVM Report catcher Summary ---

Number of demoted UVM_FATAL reports  :    0
Number of demoted UVM_ERROR reports  :    0
Number of demoted UVM_WARNING reports:    0
Number of caught UVM_FATAL reports   :    0
Number of caught UVM_ERROR reports   :    0
Number of caught UVM_WARNING reports :    0

--- UVM Report Summary ---

** Report counts by severity
UVM_INFO :    4
UVM_ERROR :    0
UVM_FATAL :    0
** Report counts by id
[RNTST]     1
[UVMTOP]     1
[m_top_env]     2
Simulation complete via $finish(1) at time 0 FS + 179

Go to the next step Data and Driver

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