uvm_sequence_base

The uvm_sequence_base class provides the interfaces needed to create streams of sequence items and/or other sequences.

A sequence is executed by calling its start method, either directly or invocation of any of the `uvm_do_* macros.

Executing sequences via start

A sequence’s start method has a parent_sequence argument that controls whether pre_do, mid_do, and post_do are called in the parent sequence.  It also has a call_pre_post argument that controls whether its pre_body and post_body methods are called.  In all cases, its pre_start and post_start methods are always called.

When start is called directly, you can provide the appropriate arguments according to your application.

The sequence execution flow looks like this

User code

sub_seq.randomize(...); // optional
sub_seq.start(seqr, parent_seq, priority, call_pre_post)

The following methods are called, in order

sub_seq.pre_start()        (task)
sub_seq.pre_body()         (task)  if call_pre_post==1
  parent_seq.pre_do(0)     (task)  if parent_sequence!=null
  parent_seq.mid_do(this)  (func)  if parent_sequence!=null
sub_seq.body               (task)  YOUR STIMULUS CODE
  parent_seq.post_do(this) (func)  if parent_sequence!=null
sub_seq.post_body()        (task)  if call_pre_post==1
sub_seq.post_start()       (task)

Executing sub-sequences via `uvm_do macros

A sequence can also be indirectly started as a child in the body of a parent sequence.  The child sequence’s start method is called indirectly by invoking any of the `uvm_do macros.  In thise cases, start is called with call_pre_post set to 0, preventing the started sequence’s pre_body and post_body methods from being called.  During execution of the child sequence, the parent’s pre_do, mid_do, and post_do methods are called.

The sub-sequence execution flow looks like

User code

`uvm_do_with_prior(seq_seq, { constraints }, priority)

The following methods are called, in order

sub_seq.pre_start()         (task)
parent_seq.pre_do(0)        (task)
parent_req.mid_do(sub_seq)  (func)
  sub_seq.body()            (task)
parent_seq.post_do(sub_seq) (func)
sub_seq.post_start()        (task)

Remember, it is the parent sequence’s pre|mid|post_do that are called, not the sequence being executed.

Executing sequence items via start_item/finish_item or `uvm_do macros

Items are started in the body of a parent sequence via calls to start_item/finish_item or invocations of any of the `uvm_do macros.  The pre_do, mid_do, and post_do methods of the parent sequence will be called as the item is executed.

The sequence-item execution flow looks like

User code

parent_seq.start_item(item, priority);
item.randomize(...) [with {constraints}];
parent_seq.finish_item(item);

or

`uvm_do_with_prior(item, constraints, priority)

The following methods are called, in order

sequencer.wait_for_grant(prior) (task) \ start_item  \
parent_seq.pre_do(1)            (task) /              \
                                                   `uvm_do* macros
parent_seq.mid_do(item)         (func) \              /
sequencer.send_request(item)    (func)  \finish_item /
sequencer.wait_for_item_done()  (task)  /
parent_seq.post_do(item)        (func) /

Attempting to execute a sequence via start_item/finish_item will produce a run-time error.

Summary
uvm_sequence_base
The uvm_sequence_base class provides the interfaces needed to create streams of sequence items and/or other sequences.
Class Hierarchy
Class Declaration
class uvm_sequence_base extends uvm_sequence_item
newThe constructor for uvm_sequence_base.
is_itemReturns 1 on items and 0 on sequences.
get_sequence_stateReturns the sequence state as an enumerated value.
wait_for_sequence_stateWaits until the sequence reaches the given state.
Sequence Execution
startExecutes this sequence, returning when the sequence has completed.
pre_startThis task is a user-definable callback that is called before the optional execution of pre_body.
pre_bodyThis task is a user-definable callback that is called before the execution of body only when the sequence is started with start.
pre_doThis task is a user-definable callback task that is called on the parent sequence, if any.the sequence has issued a wait_for_grant() call and after the sequencer has selected this sequence, and before the item is randomized.
mid_doThis function is a user-definable callback function that is called after the sequence item has been randomized, and just before the item is sent to the driver.
bodyThis is the user-defined task where the main sequence code resides.
post_doThis function is a user-definable callback function that is called after the driver has indicated that it has completed the item, using either this item_done or put methods.
post_bodyThis task is a user-definable callback task that is called after the execution of body only when the sequence is started with start.
post_startThis task is a user-definable callback that is called after the optional execution of post_body.
starting_phaseIf non-null, specifies the phase in which this sequence was started.
Sequence Control
set_priorityThe priority of a sequence may be changed at any point in time.
get_priorityThis function returns the current priority of the sequence.
is_relevantThe default is_relevant implementation returns 1, indicating that the sequence is always relevant.
wait_for_relevantThis method is called by the sequencer when all available sequences are not relevant.
lockRequests a lock on the specified sequencer.
grabRequests a lock on the specified sequencer.
unlockRemoves any locks or grabs obtained by this sequence on the specified sequencer.
ungrabRemoves any locks or grabs obtained by this sequence on the specified sequencer.
is_blockedReturns a bit indicating whether this sequence is currently prevented from running due to another lock or grab.
has_lockReturns 1 if this sequence has a lock, 0 otherwise.
killThis function will kill the sequence, and cause all current locks and requests in the sequence’s default sequencer to be removed.
do_killThis function is a user hook that is called whenever a sequence is terminated by using either sequence.kill() or sequencer.stop_sequences() (which effectively calls sequence.kill()).
Sequence Item Execution
create_itemCreate_item will create and initialize a sequence_item or sequence using the factory.
start_itemstart_item and finish_item together will initiate operation of a sequence item.
finish_itemfinish_item, together with start_item together will initiate operation of a sequence_item.
wait_for_grantThis task issues a request to the current sequencer.
send_requestThe send_request function may only be called after a wait_for_grant call.
wait_for_item_doneA sequence may optionally call wait_for_item_done.
Response API
use_response_handlerWhen called with enable set to 1, responses will be sent to the response handler.
get_use_response_handlerReturns the state of the use_response_handler bit.
response_handlerWhen the use_reponse_handler bit is set to 1, this virtual task is called by the sequencer for each response that arrives for this sequence.
set_response_queue_error_report_disabledBy default, if the response_queue overflows, an error is reported.
get_response_queue_error_report_disabledWhen this bit is 0 (default value), error reports are generated when the response queue overflows.
set_response_queue_depthThe default maximum depth of the response queue is 8.
get_response_queue_depthReturns the current depth setting for the response queue.
clear_response_queueEmpties the response queue for this sequence.

new

function new (string name =  "uvm_sequence")

The constructor for uvm_sequence_base.

is_item

virtual function bit is_item()

Returns 1 on items and 0 on sequences.  As this object is a sequence, is_item will always return 0.

get_sequence_state

function uvm_sequence_state_enum get_sequence_state()

Returns the sequence state as an enumerated value.  Can use to wait on the sequence reaching or changing from one or more states.

wait(get_sequence_state() & (STOPPED|FINISHED));

wait_for_sequence_state

task wait_for_sequence_state(uvm_sequence_state_enum state)

Waits until the sequence reaches the given state.  If the sequence is already in this state, this method returns immediately.  Convenience for wait ( get_sequence_state == state );

start

virtual task start (uvm_sequencer_base sequencer,  
uvm_sequence_base parent_sequence =  null,
int this_priority =  -1,
bit call_pre_post =  1)

Executes this sequence, returning when the sequence has completed.

The sequencer argument specifies the sequencer on which to run this sequence.  The sequencer must be compatible with the sequence.

If parent_sequence is null, then this sequence is a root parent, otherwise it is a child of parent_sequence.  The parent_sequence’s pre_do, mid_do, and post_do methods will be called during the execution of this sequence.

By default, the priority of a sequence is the priority of its parent sequence.  If it is a root sequence, its default priority is 100.  A different priority may be specified by this_priority.  Higher numbers indicate higher priority.

If call_pre_post is set to 1 (default), then the pre_body and post_body tasks will be called before and after the sequence body is called.

pre_start

virtual task pre_start()

This task is a user-definable callback that is called before the optional execution of pre_body.  This method should not be called directly by the user.

pre_body

virtual task pre_body()

This task is a user-definable callback that is called before the execution of body only when the sequence is started with start.  If start is called with call_pre_post set to 0, pre_body is not called.  This method should not be called directly by the user.

pre_do

virtual task pre_do(bit is_item)

This task is a user-definable callback task that is called on the parent sequence, if any.the sequence has issued a wait_for_grant() call and after the sequencer has selected this sequence, and before the item is randomized.

Although pre_do is a task, consuming simulation cycles may result in unexpected behavior on the driver.

This method should not be called directly by the user.

mid_do

virtual function void mid_do(uvm_sequence_item this_item)

This function is a user-definable callback function that is called after the sequence item has been randomized, and just before the item is sent to the driver.  This mehod should not be called directly by the user.

body

virtual task body()

This is the user-defined task where the main sequence code resides.  This method should not be called directly by the user.

post_do

virtual function void post_do(uvm_sequence_item this_item)

This function is a user-definable callback function that is called after the driver has indicated that it has completed the item, using either this item_done or put methods.  This method should not be called directly by the user.

post_body

virtual task post_body()

This task is a user-definable callback task that is called after the execution of body only when the sequence is started with start.  If start is called with call_pre_post set to 0, post_body is not called.  This task is a user-definable callback task that is called after the execution of the body, unless the sequence is started with call_pre_post=0.  This method should not be called directly by the user.

post_start

virtual task post_start()

This task is a user-definable callback that is called after the optional execution of post_body.  This method should not be called directly by the user.

starting_phase

uvm_phase starting_phase

If non-null, specifies the phase in which this sequence was started.  The starting_phase is set automatically when this sequence is started as the default sequence.  See uvm_sequencer_base::start_phase_sequence.

virtual task user_sequence::body();
   if (starting_phase != null)
      starting_phase.raise_objection(this,"user_seq not finished");
   ...
   if (starting_phase != null)
      starting_phase.drop_objection(this,"user_seq finished");
endtask

set_priority

function void set_priority (int value)

The priority of a sequence may be changed at any point in time.  When the priority of a sequence is changed, the new priority will be used by the sequencer the next time that it arbitrates between sequences.

The default priority value for a sequence is 100.  Higher values result in higher priorities.

get_priority

function int get_priority()

This function returns the current priority of the sequence.

is_relevant

virtual function bit is_relevant()

The default is_relevant implementation returns 1, indicating that the sequence is always relevant.

Users may choose to override with their own virtual function to indicate to the sequencer that the sequence is not currently relevant after a request has been made.

When the sequencer arbitrates, it will call is_relevant on each requesting, unblocked sequence to see if it is relevant.  If a 0 is returned, then the sequence will not be chosen.

If all requesting sequences are not relevant, then the sequencer will call wait_for_relevant on all sequences and re-arbitrate upon its return.

Any sequence that implements is_relevant must also implement wait_for_relevant so that the sequencer has a way to wait for a sequence to become relevant.

wait_for_relevant

virtual task wait_for_relevant()

This method is called by the sequencer when all available sequences are not relevant.  When wait_for_relevant returns the sequencer attempt to re-arbitrate.

Returning from this call does not guarantee a sequence is relevant, although that would be the ideal.  The method provide some delay to prevent an infinite loop.

If a sequence defines is_relevant so that it is not always relevant (by default, a sequence is always relevant), then the sequence must also supply a wait_for_relevant method.

lock

task lock(uvm_sequencer_base sequencer =  null)

Requests a lock on the specified sequencer.  If sequencer is null, the lock will be requested on the current default sequencer.

A lock request will be arbitrated the same as any other request.  A lock is granted after all earlier requests are completed and no other locks or grabs are blocking this sequence.

The lock call will return when the lock has been granted.

grab

task grab(uvm_sequencer_base sequencer =  null)

Requests a lock on the specified sequencer.  If no argument is supplied, the lock will be requested on the current default sequencer.

A grab equest is put in front of the arbitration queue.  It will be arbitrated before any other requests.  A grab is granted when no other grabs or locks are blocking this sequence.

The grab call will return when the grab has been granted.

unlock

function void unlock(uvm_sequencer_base sequencer =  null)

Removes any locks or grabs obtained by this sequence on the specified sequencer.  If sequencer is null, then the unlock will be done on the current default sequencer.

ungrab

function void ungrab(uvm_sequencer_base sequencer =  null)

Removes any locks or grabs obtained by this sequence on the specified sequencer.  If sequencer is null, then the unlock will be done on the current default sequencer.

is_blocked

function bit is_blocked()

Returns a bit indicating whether this sequence is currently prevented from running due to another lock or grab.  A 1 is returned if the sequence is currently blocked.  A 0 is returned if no lock or grab prevents this sequence from executing.  Note that even if a sequence is not blocked, it is possible for another sequence to issue a lock or grab before this sequence can issue a request.

has_lock

function bit has_lock()

Returns 1 if this sequence has a lock, 0 otherwise.

Note that even if this sequence has a lock, a child sequence may also have a lock, in which case the sequence is still blocked from issuing operations on the sequencer.

kill

function void kill()

This function will kill the sequence, and cause all current locks and requests in the sequence’s default sequencer to be removed.  The sequence state will change to STOPPED, and its post_body() method, if will not b

If a sequence has issued locks, grabs, or requests on sequencers other than the default sequencer, then care must be taken to unregister the sequence with the other sequencer(s) using the sequencer unregister_sequence() method.

do_kill

virtual function void do_kill()

This function is a user hook that is called whenever a sequence is terminated by using either sequence.kill() or sequencer.stop_sequences() (which effectively calls sequence.kill()).

create_item

protected function uvm_sequence_item create_item(
   uvm_object_wrapper type_var,
   uvm_sequencer_base l_sequencer,
   string name
)

Create_item will create and initialize a sequence_item or sequence using the factory.  The sequence_item or sequence will be initialized to communicate with the specified sequencer.

start_item

virtual task start_item (uvm_sequence_item item,  
int set_priority =  -1,
uvm_sequencer_base sequencer = null)

start_item and finish_item together will initiate operation of a sequence item.  If the item has not already been initialized using create_item, then it will be initialized here to use the default sequencer specified by m_sequencer.  Randomization may be done between start_item and finish_item to ensure late generation

finish_item

virtual task finish_item (uvm_sequence_item item,  
int set_priority =  -1)

finish_item, together with start_item together will initiate operation of a sequence_item.  Finish_item must be called after start_item with no delays or delta-cycles.  Randomization, or other functions may be called between the start_item and finish_item calls.

wait_for_grant

virtual task wait_for_grant(int item_priority =  -1,
bit lock_request =  0)

This task issues a request to the current sequencer.  If item_priority is not specified, then the current sequence priority will be used by the arbiter.  If a lock_request is made, then the sequencer will issue a lock immediately before granting the sequence.  (Note that the lock may be granted without the sequence being granted if is_relevant is not asserted).

When this method returns, the sequencer has granted the sequence, and the sequence must call send_request without inserting any simulation delay other than delta cycles.  The driver is currently waiting for the next item to be sent via the send_request call.

send_request

virtual function void send_request(uvm_sequence_item request,  
bit rerandomize =  0)

The send_request function may only be called after a wait_for_grant call.  This call will send the request item to the sequencer, which will forward it to the driver.  If the rerandomize bit is set, the item will be randomized before being sent to the driver.

wait_for_item_done

virtual task wait_for_item_done(int transaction_id =  -1)

A sequence may optionally call wait_for_item_done.  This task will block until the driver calls item_done or put.  If no transaction_id parameter is specified, then the call will return the next time that the driver calls item_done or put.  If a specific transaction_id is specified, then the call will return when the driver indicates completion of that specific item.

Note that if a specific transaction_id has been specified, and the driver has already issued an item_done or put for that transaction, then the call will hang, having missed the earlier notification.

use_response_handler

function void use_response_handler(bit enable)

When called with enable set to 1, responses will be sent to the response handler.  Otherwise, responses must be retrieved using get_response.

By default, responses from the driver are retrieved in the sequence by calling get_response.

An alternative method is for the sequencer to call the response_handler function with each response.

get_use_response_handler

function bit get_use_response_handler()

Returns the state of the use_response_handler bit.

response_handler

virtual function void response_handler(uvm_sequence_item response)

When the use_reponse_handler bit is set to 1, this virtual task is called by the sequencer for each response that arrives for this sequence.

set_response_queue_error_report_disabled

function void set_response_queue_error_report_disabled(bit value)

By default, if the response_queue overflows, an error is reported.  The response_queue will overflow if more responses are sent to this sequence from the driver than get_response calls are made.  Setting value to 0 disables these errors, while setting it to 1 enables them.

get_response_queue_error_report_disabled

function bit get_response_queue_error_report_disabled()

When this bit is 0 (default value), error reports are generated when the response queue overflows.  When this bit is 1, no such error reports are generated.

set_response_queue_depth

function void set_response_queue_depth(int value)

The default maximum depth of the response queue is 8.  These method is used to examine or change the maximum depth of the response queue.

Setting the response_queue_depth to -1 indicates an arbitrarily deep response queue.  No checking is done.

get_response_queue_depth

function int get_response_queue_depth()

Returns the current depth setting for the response queue.

clear_response_queue

virtual function void clear_response_queue()

Empties the response queue for this sequence.

The uvm_void class is the base class for all UVM classes.
virtual class uvm_object extends uvm_void
The uvm_object class is the base class for all UVM data and hierarchical classes.
virtual class uvm_transaction extends uvm_object
The uvm_transaction class is the root base class for UVM transactions.
class uvm_sequence_item extends uvm_transaction
The base class for user-defined sequence items and also the base class for the uvm_sequence class.
class uvm_sequence_base extends uvm_sequence_item
The uvm_sequence_base class provides the interfaces needed to create streams of sequence items and/or other sequences.
function new (string name =  "uvm_sequence")
The constructor for uvm_sequence_base.
virtual function bit is_item()
Returns 1 on items and 0 on sequences.
function uvm_sequence_state_enum get_sequence_state()
Returns the sequence state as an enumerated value.
task wait_for_sequence_state(uvm_sequence_state_enum state)
Waits until the sequence reaches the given state.
virtual task start (uvm_sequencer_base sequencer,  
uvm_sequence_base parent_sequence =  null,
int this_priority =  -1,
bit call_pre_post =  1)
Executes this sequence, returning when the sequence has completed.
virtual task pre_start()
This task is a user-definable callback that is called before the optional execution of pre_body.
virtual task pre_body()
This task is a user-definable callback that is called before the execution of body only when the sequence is started with start.
virtual task body()
This is the user-defined task where the main sequence code resides.
virtual task pre_do(bit is_item)
This task is a user-definable callback task that is called on the parent sequence, if any.the sequence has issued a wait_for_grant() call and after the sequencer has selected this sequence, and before the item is randomized.
virtual function void mid_do(uvm_sequence_item this_item)
This function is a user-definable callback function that is called after the sequence item has been randomized, and just before the item is sent to the driver.
virtual function void post_do(uvm_sequence_item this_item)
This function is a user-definable callback function that is called after the driver has indicated that it has completed the item, using either this item_done or put methods.
virtual task post_body()
This task is a user-definable callback task that is called after the execution of body only when the sequence is started with start.
virtual task post_start()
This task is a user-definable callback that is called after the optional execution of post_body.
uvm_phase starting_phase
If non-null, specifies the phase in which this sequence was started.
function void set_priority (int value)
The priority of a sequence may be changed at any point in time.
function int get_priority()
This function returns the current priority of the sequence.
virtual function bit is_relevant()
The default is_relevant implementation returns 1, indicating that the sequence is always relevant.
virtual task wait_for_relevant()
This method is called by the sequencer when all available sequences are not relevant.
task lock(uvm_sequencer_base sequencer =  null)
Requests a lock on the specified sequencer.
task grab(uvm_sequencer_base sequencer =  null)
Requests a lock on the specified sequencer.
function void unlock(uvm_sequencer_base sequencer =  null)
Removes any locks or grabs obtained by this sequence on the specified sequencer.
function void ungrab(uvm_sequencer_base sequencer =  null)
Removes any locks or grabs obtained by this sequence on the specified sequencer.
function bit is_blocked()
Returns a bit indicating whether this sequence is currently prevented from running due to another lock or grab.
function bit has_lock()
Returns 1 if this sequence has a lock, 0 otherwise.
function void kill()
This function will kill the sequence, and cause all current locks and requests in the sequence’s default sequencer to be removed.
virtual function void do_kill()
This function is a user hook that is called whenever a sequence is terminated by using either sequence.kill() or sequencer.stop_sequences() (which effectively calls sequence.kill()).
protected function uvm_sequence_item create_item(
   uvm_object_wrapper type_var,
   uvm_sequencer_base l_sequencer,
   string name
)
Create_item will create and initialize a sequence_item or sequence using the factory.
virtual task start_item (uvm_sequence_item item,  
int set_priority =  -1,
uvm_sequencer_base sequencer = null)
start_item and finish_item together will initiate operation of a sequence item.
virtual task finish_item (uvm_sequence_item item,  
int set_priority =  -1)
finish_item, together with start_item together will initiate operation of a sequence_item.
virtual task wait_for_grant(int item_priority =  -1,
bit lock_request =  0)
This task issues a request to the current sequencer.
virtual function void send_request(uvm_sequence_item request,  
bit rerandomize =  0)
The send_request function may only be called after a wait_for_grant call.
virtual task wait_for_item_done(int transaction_id =  -1)
A sequence may optionally call wait_for_item_done.
function void use_response_handler(bit enable)
When called with enable set to 1, responses will be sent to the response handler.
function bit get_use_response_handler()
Returns the state of the use_response_handler bit.
virtual function void response_handler(uvm_sequence_item response)
When the use_reponse_handler bit is set to 1, this virtual task is called by the sequencer for each response that arrives for this sequence.
function void set_response_queue_error_report_disabled(bit value)
By default, if the response_queue overflows, an error is reported.
function bit get_response_queue_error_report_disabled()
When this bit is 0 (default value), error reports are generated when the response queue overflows.
function void set_response_queue_depth(int value)
The default maximum depth of the response queue is 8.
function int get_response_queue_depth()
Returns the current depth setting for the response queue.
virtual function void clear_response_queue()
Empties the response queue for this sequence.
virtual function void start_phase_sequence(uvm_phase phase)
Start the default sequence for this phase, if any.