uvm_sequence_base Methods

The randomized sequence items can be driven to the driver using pre-defined methods call present in the uvm_sequence_base class. There are two approaches with a set of uvm_sequence_base methods by which sequence can send sequence items and retrieve response from the driver (if applicable).

Approach A: Using wait_for_grant(), send_request(), wait_for_item_done() etc

Approach B: Using start_item/finish_item methods

A: Using wait_for_grant(), send_request(), wait_for_item_done() etc

The below methods are defined in the uvm_sequence_base class which is derived from the uvm_sequence_item class.

Type	Methods	Description
function	create_item	This creates and initializes request items or sequences and initializes using the factory.
task	wait_for_grant	This task issues a request to the current sequencer and it returns when the sequencer has granted the sequence. Hence, it is a blocking method that waits for a grant from the sequencer.
function	send_request(uvm_sequence_item request, bit rerandomize = 0)	Send request items to the driver via the sequencer. If rerandomize = 1, the item will be randomized before sent to the driver.
task	wait_for_item_done()	This task will block until the driver calls put or item_done. It is an optional call.
	get_response (RSP)	This is optional in case the driver sends back any response

Steps:

Create a seq_tem using create() method.
wait_for_grant
Randomize seq_item
send_request(req)
wait_for_item_done()
get_response (rsp)

class my_sequence extends uvm_sequencer #(seq_item);
  `uvm_object_utils(my_sequence)
  
   function new (string name = "my_sequence")
     super.new(name);
   endfunction

   task body();
     req = seq_item::type_id::create("req");
     wait_for_grant();
     assert(req.randomize());
     send_request(req);
     wait_for_item_done();
     get_respose(rsp);
   endtask
endclass

B: Using start_item/finish_item methods

The start_item and finish_item tasks are also defined in the uvm_sequence_base class. Both methods initiate the operation of sequence items.

Note: There should be no simulation time consumed between start_item and finish_item call.

Methods	Description
start_item(req)	It blocks until the sequencer grants the sequence and the sequence_item access to the driver.
finish_item(req)	It blocks the driver until it finishes the transfer protocol for the sequence item.

Steps:

Create a seq_tem using create() method.
start_item(req)
Randomize seq_item
finish_item(req)

class my_sequence extends uvm_sequencer #(seq_item);
  `uvm_object_utils(my_sequence)
  
   function new (string name = "my_sequence")
     super.new(name);
   endfunction

   task body();
     req = seq_item::type_id::create("req");
     start_item(req);
     assert(req.randomize());
     finish_item(req);
   endtask
endclass

Important Note:

Approach B is the same as approach A. The start_item(req) method internally calls wait_for_grant() method to get grant from the sequencer. The finish_item(req) method internally calls send_request() and wait_for_item_done() methods. Hence, start_item and finish_item methods act as wrapper methods over them.

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