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· 5 min read
Duong Van

Overview​

The case for the need of this access is inside a UVM Testbench, where the test would generate some randomization on the data and need to access the instantiated module (let say model in this case) in the top.

The workaround​

For the workaround demonstration, a simple UVM testbench generated with EasierUVM will be used.

Overview of the testbench​

// Update the graph later with the model’s module

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Implementation​

The DUT in this case is a simple ALU’s adder

module adder (
    input  logic [7:0] A,
    input  logic [7:0] B,
    output logic [8:0] F
        );
    always_comb
        F <= A+B;
endmodule

The module wanted to be accessed from the test is the input generator (this is for demonstration purpose only, for normal UVM testbench this should be implemented as UVM driver-driver BFM pair)

module input_model (
    input  logic clk,
    output logic [7:0] A,
    output logic [7:0] B
);

  logic [7:0] input_pool[*];
  int current_index = 0;
  int rand_index = 0;
  reg [7:0] reg_A;
  reg [7:0] reg_B;

  // Function to be called for adding randomized input
  function void insert_input(logic[7:0] value);
    input_pool[current_index++] = value;
    $display("INPUT MODEL: Currently inserting: 0x%8x at index %d", value, current_index-1);
  endfunction : insert_input

  always @(posedge clk) begin
    assert ( randomize(rand_index) with { rand_index inside {[0:current_index-1]}; }) else 
    begin
      $display("why the fuck can it not randomize properly?");
    end
    $display("Current rand_index: %d", rand_index);
    $display("Current current_index: %d", current_index);
    reg_A <= input_pool[rand_index];
    reg_B <= input_pool[rand_index];
  end

  assign A = reg_A;
  assign B = reg_B;

endmodule : input_model

The package containing the virtual class extending uvm_object → This will be used in other testbench’s class components

package backdoor_access_pkg;
  import uvm_pkg::*;

  virtual class input_model_backdoor extends uvm_pkg::uvm_object;
    function new(string name="input_model_backdoor");
      super.new(name);
    endfunction

    pure virtual function void insert_input(logic [7:0] value);

  endclass : input_model_backdoor

endpackage : backdoor_access_pkg

The wrapper class for the module that extends and implement all the virtual methods of the created virtual class

`include "uvm_macros.svh"
import uvm_pkg::*;
import backdoor_access_pkg::*;

class backdoor_im extends input_model_backdoor;
  
  function new(string name="backdoor_im");
    super.new(name);
  endfunction : new 

  function void insert(logic[7:0] value);
    **top_tb.th.model.insert_input(value);**
  endfunction : insert
            
endclass : backdoor_im

initial begin
  backdoor_im backdoor_im_i;
  backdoor_im_i = new();
  uvm_config_db #(uvm_object)::set(uvm_root::get(), "*", "IM_BACKDOOR_ACCESS", backdoor_im_i);  
end
  • Adding the handle to the uvm_config_db under uvm_root::get() as the parent, * as the scope, key string and the handle itself:
  • Instantiating it in the top.sv , (top_tb.sv in this example)
module top_tb;

  timeunit      1ns;
  timeprecision 1ps;

  `include "uvm_macros.svh"

  import uvm_pkg::*;

  import top_test_pkg::*;
  import top_pkg::top_config;

  // Configuration object for top-level environment
  top_config top_env_config;

  // Test harness
  top_th th();

  **`include "backdoor_input_model.sv"**

  initial
  begin
    // Create and populate top-level configuration object
    top_env_config = new("top_env_config");
    if ( !top_env_config.randomize() )
      `uvm_error("top_tb", "Failed to randomize top-level configuration object" )

    top_env_config.arith_vif             = th.arith_if_0;
    top_env_config.is_active_arith       = UVM_ACTIVE;   
    top_env_config.checks_enable_arith   = 1;            
    top_env_config.coverage_enable_arith = 1;            

    uvm_config_db #(top_config)::set(null, "uvm_test_top", "config", top_env_config);
    uvm_config_db #(top_config)::set(null, "uvm_test_top.m_env", "config", top_env_config);

    run_test();
  end

endmodule

Including the packages in the environment (in this case in the agent’s package)

package arith_pkg;

  `include "uvm_macros.svh"

  import uvm_pkg::*;
    import backdoor_access_pkg::*;

  `include "arith_trans.sv"
  `include "arith_config.sv"
  `include "arith_driver.sv"
  `include "arith_monitor.sv"
  `include "arith_sequencer.sv"
  `include "arith_coverage.sv"
  `include "arith_agent.sv"
  `include "arith_seq_lib.sv"

endpackage : arith_pkg

And the remaining is to access the module from the test using the virtual class, by fetching the handle from the uvm_config_db as an uvm_object and cast it to the correct type of the virtual class

`ifndef TOP_TEST_SV
`define TOP_TEST_SV

// You can insert code here by setting test_inc_before_class in file common.tpl

class top_test extends uvm_test;

  `uvm_component_utils(top_test)

  // Change made here
  top_env m_env;
  logic [7:0] rand_input = 0;
  // end : Change made here

  backdoor_access_pkg::input_model_backdoor backdoor_im_i;
  uvm_object backdoor_object_i;

  extern function new(string name, uvm_component parent);

  extern function void build_phase(uvm_phase phase);

  extern task run_phase(uvm_phase phase);

endclass : top_test

function top_test::new(string name, uvm_component parent);
  super.new(name, parent);
endfunction : new

function void top_test::build_phase(uvm_phase phase);

  m_env = top_env::type_id::create("m_env", this);

  // Fetching the handle from the uvm_config_db and cast it to the correct type
  **if(!uvm_config_db#(uvm_object)::get(uvm_root::get(), "", "IM_BACKDOOR_ACCESS", backdoor_object_i))   
  begin
    `uvm_fatal("TEST","Failed to get input model backdoor access object")
  end
  assert($cast(backdoor_im_i, backdoor_object_i)) else begin
    `uvm_fatal("ASSERT", "Dynamic casting backdoor_im_instance failed!")
  end**

endfunction : build_phase

task top_test::run_phase(uvm_phase phase);
  $display("Went in the run_phase of the test!");
  #10;
  for (int i = 0; i < 256; ++i) begin
    assert (randomize(rand_input));
    backdoor_im_i.insert(rand_input);
  end
  #1000;
endtask

`endif // TOP_TEST_SV

Also remember to compile the .sv file and packages properly. The compiling sequence should be included in the compile_questa.do script.

The result when running the simulation

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The result when dumping signals, by adding some signals on top_th.sv and a little addition on top_tb.sv file

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This result concludes the ability to manipulate the module that is instantiated on the top from a test component in UVM testbench

Full testbench can be found here.

Summary​

  • Create a virtual class extending uvm_object (for example VC)
  • Create a wrapper class for the module that extends and implement all the virtual methods of the created virtual class

    • Instantiating it in the top.sv

    • Adding the handle to the uvm_config_db under uvm_root::get() as the parent, * as the scope, key string and the handle itself:

      uvm_config_db #(uvm_object)::set(uvm_root::get(), "*", "<KEY>", <wrapper_handle>);

      This will ensure the scope of the statement used to call the top's instantiated module.

      Meaning the statement top.<module_name>.<method_name>() should be valid.

  • The virtual class VC mentioned above will be used for the lower level component in the environment hierarchy for accessing the top instantiated module’s methods.