Understanding SystemVerilog's Event Data Type

The event data type is a powerful feature in SystemVerilog that can be used to signal other processes or threads that a certain condition has occurred. In this post, we will explore the event data type, its implementation, and its usage.

What is the SystemVerilog Event Data Type?

The event data type is a special type of data in SystemVerilog that provides a handle to a synchronization object. It can be used to create a persistent triggered state that is triggered when a specified condition is met. The condition is set by an event trigger, which is defined as an event name.

Events can be used to signal other processes or threads that a certain condition has occurred. The event name is used to trigger the event and can be set using the -> operator. For example, if you have defined an event called eventName, you can trigger it using ->eventName.

Implementing the SystemVerilog Event Data Type

To implement the event data type in SystemVerilog, you first need to define the event. This is done using the event keyword, followed by the event name. Here is an example:

event eventName;

This creates an event called eventName that can be triggered using the -> operator. You can also set the event as sensitive to certain conditions using the @ operator. For example:

event eventName;
always @ (posedge clk) -> eventName;

This sets the eventName event to trigger on the positive edge of the clk signal.

Once you have defined the event, you can use it in other parts of your code. For example, you can use the event to signal other processes that a certain condition has been met:

event eventName;
always @ (posedge clk) begin
  if (condition) -> eventName;
end

always @(eventName) begin
  // Do something when eventName is triggered
end

In this example, the eventName event is triggered when the condition is met. The second always block is sensitive to the eventName event and will execute whenever the event is triggered.

Waiting for an Event

You can wait for an event to trigger in SystemVerilog by using the built-in triggered() method. The triggered() method returns a boolean value that is true if the specified event has been triggered. Here is an example:

event eventName;
wait (eventName.triggered());

In this example, the code waits until the eventName event is triggered using the wait() statement.

You can also use the wait_order() statement to specify the order in which events should be triggered. The wait_order() statement waits for the events to be triggered in the specified order. Here is an example:

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

In this example, the code waits for the a, b, and c events to be triggered in that order. If the events are triggered out of order, an error message is displayed.

Comparing SystemVerilog Events

In SystemVerilog, events can be compared using various operators. The following table shows the comparison operators that can be used with events in SystemVerilog:

The == and != operators can be used to compare an event with another event or with null. The === and !== operators can also be used for event comparison and are equivalent to == and !=, respectively.

Conclusion

In summary, the event data type in SystemVerilog is a powerful feature that can be used to signal other processes or threads that a certain condition has occurred. You can use the event to trigger other processes and to create a persistent triggered state. To implement the event data type, you need to define the event using the event keyword and set the event trigger using the -> operator. You can also compare events using various operators and test for the presence of an event by using it in an expression. Finally, you can use the wait_order() statement to specify the order in which events should be triggered. With this knowledge, you can start using events in your circuit design to improve its functionality and efficiency.