Events - SystemVerilog - VLSI Verification

What Are Events?

An event is like a doorbell. One process rings it (triggers), and another process waiting for it wakes up. Events have no data - they just signal "something happened".

Basic Event Example

event done;  // Declare an event

initial begin
    fork
        // Process 1: Wait for event
        begin
            $display("[%0t] Waiting for event...", $time);
            @(done);  // Block until event triggers
            $display("[%0t] Event received!", $time);
        end
        
        // Process 2: Trigger event
        begin
            #10;
            $display("[%0t] Triggering event!", $time);
            -> done;  // Trigger the event
        end
    join
end

// Output:
// [0] Waiting for event...
// [10] Triggering event!
// [10] Event received!

Triggering Events

-> (Named Event Trigger)

The standard way to trigger an event:

Standard Trigger

event my_event;

initial begin
    -> my_event;  // Trigger immediately
end

->> (Non-blocking Event Trigger)

Schedules the trigger for a later time in the same time slot. Useful to avoid race conditions:

Non-blocking Trigger

event my_event;

initial begin
    ->> my_event;  // Schedule trigger (non-blocking)
    $display("This prints first");
end

Waiting for Events

@ operator

Blocks until the event triggers. Simple but can miss events:

Using @ Operator

@(my_event);  // Wait for next trigger
$display("Event happened!");

wait() with .triggered

Better approach - catches events triggered in the same time slot:

Using wait(.triggered)

event my_event;

initial begin
    fork
        begin
            wait(my_event.triggered);  // Catches same-timestep triggers
            $display("Event caught!");
        end
        begin
            -> my_event;  // Trigger immediately
        end
    join
end

Practical Example: Driver-Monitor Sync

Driver-Monitor Synchronization

class Testbench;
    event transaction_complete;
    event check_done;
    
    task run();
        fork
            driver();
            monitor();
        join
    endtask
    
    task driver();
        repeat(5) begin
            // Drive transaction
            $display("[%0t] Driving transaction", $time);
            #10;
            
            // Signal that transaction is complete
            -> transaction_complete;
            
            // Wait for checker to finish before next transaction
            @(check_done);
        end
    endtask
    
    task monitor();
        repeat(5) begin
            // Wait for transaction to complete
            @(transaction_complete);
            $display("[%0t] Checking transaction", $time);
            #5;  // Simulation of checking time
            
            // Signal checker is done
            -> check_done;
        end
    endtask
endclass

Event Variables and Assignment

Events can be assigned to each other, creating aliases:

Event Assignment

event e1, e2;

initial begin
    e2 = e1;  // e2 now points to same event as e1
    
    fork
        begin
            @(e2);  // Wait on e2
            $display("e2 triggered!");
        end
        begin
            #10;
            -> e1;  // Trigger e1 (also triggers e2!)
        end
    join
end

Quick Summary

event - Synchronization object with no data
-> - Trigger event (blocking)
->> - Trigger event (non-blocking)
@(event) - Wait for event trigger
wait(event.triggered) - Better wait, catches same-timestep
Events can be assigned (aliased) to each other

Events in SystemVerilog