What's the difference between creating a class in build_phase and creating them on the spot in body()?

UVM
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1

What’s the difference between creating a class in build_phase and creating them on the spot in body()?
Look at this two examples:

Example 2: Creating class on the spot in body()


function new(string name = "pas4_otp_mem_dl_seq");
  super.new(name);
endfunction : new

virtual function void build_phase(uvm_phase phase);
  super.build_phase(phase);

  // Class created here
  con = pas4_norm_cont_seq::type_id::create("con");
  send_data_seq = pas4_send_otp_data_seq::type_id::create("send_data_seq");
  wait_data_seq = pas4_wait_for_req_addr_seq::type_id::create("wait_data_seq");
endfunction

virtual task body();
  int del;

  `uvm_info(get_type_name(),"Starting OTP DL Seq",UVM_LOW);

  con.start(m_sequencer);
    
  for(cnt = 0; cnt <8 ; cnt++) begin 
    for(i = 0; i<3; i++) begin
      wait_data_seq.start(m_sequencer);
      if(wait_data_seq.grant) begin
        send_data_seq.start(m_sequencer);
      end
    end
  end
    
  `uvm_info(get_type_name(),"OTP DL Seq Done", UVM_LOW);

endtask : body

Example 2: Creating class on the spot in body()


function new(string name = "pas4_otp_mem_dl_seq");
  super.new(name);
endfunction : new

virtual task body();
  int del;

  `uvm_info(get_type_name(),"Starting OTP DL Seq",UVM_LOW);
  con = pas4_norm_cont_seq::type_id::create("con");

  con.start(m_sequencer);
    
  for(cnt = 0; cnt <8 ; cnt++) begin 
    for(i = 0; i<3; i++) begin
      send_data_seq = pas4_send_otp_data_seq::type_id::create("send_data_seq");     // Class created on the spot
      wait_data_seq = pas4_wait_for_req_addr_seq::type_id::create("wait_data_seq"); // Class created on the spot
      wait_data_seq.start(m_sequencer);
      if(wait_data_seq.grant) begin
        send_data_seq.start(m_sequencer);
      end
    end
  end
    
  `uvm_info(get_type_name(),"OTP DL Seq Done", UVM_LOW);

endtask : body

I have a guess. Is it because that creating them on the spot will reset its values to their initialized values?

2

In reply to Reuben:

If config_db has anything to do with those class , it may lead to bad handle reference error (as most of get and set usually happens at build phase). And do objects have phases ?.

3

UVM Components can only be created in build_phase.
UVM Objects (including sequences) can be created any time.
I don’t think your first example will work, because uvm_components don’t have a body() method and uvm_objects don’t have a build_phase() method.
We recommend that sequences be created at run_time, usually from the test, and passed down via the config_db.

4

In reply to tfitz:

Hi,

Thanks for the info.
May I know why it is recommended to create sequences at run_time?

5

In reply to Reuben:

You only want to create sequences when you’re going to run them. At build_time, you don’t always know what the component hierarchy is going to look like, since it may depend on configuration parameters. At run_time, you always know what you have, so that’s the best time to decide which sequences to create and run.
If you want to create them before run_phase() begins, you can do this in start_of_simulation_phase() [which is a function] and then actuall start them in run_phase.