SystemVerilog Events

Table of Contents

Events are static objects useful for synchronization between the process. Events operations are of two staged processes in which one process will trigger the event, and the other processes will wait for an event to be triggered.

Events are triggered using -> operator or ->> operator
wait for an event to be triggered using @ operator or wait() construct

SystemVerilog events act as handles to synchronization queues. thus, they can be passed as arguments to tasks, and they can be assigned to one another or compared.

Event triggering

-> operator

Named events are triggered via the -> operator. Triggering an event unblocks all processes currently waiting on that event.

->> operator

Non-blocking events are triggered using the ->> operator.

Waiting for event trigger

@ operator

wait for an event to be triggered is via the event control operator, @.

@(event_name.triggered);

The @ operator blocks the calling process until the given event is triggered.
For a trigger to unblock a process waiting on an event, the waiting process must execute the @ statement before the triggering process executes the trigger operator, ->

Note: If the trigger executes first, then the waiting process remains blocked.

wait operator

If the event triggering and waiting for event trigger with @ operator happens at the same time, @ operator may miss detecting the event trigger.

Whereas wait(); construct will detect the event triggering.

wait(event_name.triggered);

wait_order();

The wait_order construct is blocking the process until all of the specified events are triggered in the given order (left to right). event trigger with out of order will not unblock the process.

Example:

wait_order(a,b,c);

Blocks the process until events a, b, and c trigger in the order a –> b –> c. If the events trigger out of order, a run-time error is generated.

Example:

wait_order( a, b, c ) else $display( "Error: events out of order" );

In this example, the fail statement specifies that upon failure of the construct, a user message is displayed, but without an error being generated.

Example:

bit success;
wait_order( a, b, c ) success = 1; else success = 0;

In this example, the completion status is stored in the variable success, without an error being generated.

Merging events

When one event variable is assigned to another, the two become merged. Thus, executing -> on either event variable affects processes waiting on either event variable.

Event examples

the event waiting with @ operator

The below example shows the event triggering and waiting for the event trigger.

module events_ex;
  event ev_1; //declaring event ev_1

  initial begin
    fork
      //process-1, triggers the event
      begin
        #40;
        $display($time,"\tTriggering The Event");
        ->ev_1;
      end
    
      //process-2, wait for the event to trigger
      begin
        $display($time,"\tWaiting for the Event to trigger");
        @(ev_1.triggered);
        $display($time,"\tEvent triggered");
      end
    join
  end
endmodule

Simulator Output

0 Waiting for the Event to trigger
40 Triggering The Event
40 Event triggered

Click to execute on

trigger first and then waiting for a trigger

In the below example,
event triggering happens first and then the waiting for trigger happens. As the waiting happens later, it will be blocking, so the statements after the wait for the trigger will not be executed.

module events_ex;

  event ev_1; //declaring event ev_1

  initial begin
    fork
      //process-1, triggers the event
      begin
        #40;
        $display($time,"\tTriggering The Event");
        ->ev_1;
      end
    
      //process-2, wait for the event to trigger
      begin
        $display($time,"\tWaiting for the Event to trigger");
        #60;
        @(ev_1.triggered);
        $display($time,"\tEvent triggered");
      end
    join
  end
  initial begin
    #100;
    $display($time,"\tEnding the Simulation");
    $finish;
  end
endmodule

Simulator Output

0  Waiting for the Event to trigger
40  Triggering The Event
100 Ending the Simulation

Click to execute on

event examples are continued in the next page

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