UVM Callbacks

Callbacks let you inject custom behavior into existing components without modifying their source code. They're a lightweight alternative to factory overrides.

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The Idea

Think of callbacks as hooks in a component. The component says: "At this point, I'll call any registered callbacks." You provide the callback implementation separately.

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Step 1: Define the Callback Interface

systemverilog Copy
class driver_callback extends uvm_callback;
    `uvm_object_utils(driver_callback)

    function new(string name = "driver_callback");
        super.new(name);
    endfunction

    // Hook: called before driving a transaction
    virtual task pre_drive(my_driver drv, my_txn txn);
        // Default: do nothing
    endtask

    // Hook: called after driving a transaction
    virtual task post_drive(my_driver drv, my_txn txn);
        // Default: do nothing
    endtask
endclass

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Step 2: Add Hooks to the Component

systemverilog Copy
class my_driver extends uvm_driver #(my_txn);
    `uvm_component_utils(my_driver)
    `uvm_register_cb(my_driver, driver_callback)

    // ... build_phase, etc. ...

    task run_phase(uvm_phase phase);
        my_txn req;

        forever begin
            seq_item_port.get_next_item(req);

            // ── Fire pre-drive callbacks ──
            `uvm_do_callbacks(my_driver, driver_callback,
                pre_drive(this, req))

            drive_item(req);

            // ── Fire post-drive callbacks ──
            `uvm_do_callbacks(my_driver, driver_callback,
                post_drive(this, req))

            seq_item_port.item_done();
        end
    endtask
endclass

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Step 3: Implement and Register a Callback

systemverilog Copy
class error_inject_cb extends driver_callback;
    `uvm_object_utils(error_inject_cb)

    int error_count = 0;

    function new(string name = "error_inject_cb");
        super.new(name);
    endfunction

    // Flip a random bit with 5% probability
    task pre_drive(my_driver drv, my_txn txn);
        if ($urandom_range(100) < 5) begin
            txn.data[0] = ~txn.data[0];
            error_count++;
            `uvm_info("ERR_CB", $sformatf(
                "Injected error #%0d on data=0x%h",
                error_count, txn.data
            ), UVM_LOW)
        end
    endtask
endclass

// In the test:
class error_test extends base_test;
    `uvm_component_utils(error_test)

    function new(string name, uvm_component parent);
        super.new(name, parent);
    endfunction

    function void build_phase(uvm_phase phase);
        super.build_phase(phase);
        error_inject_cb err_cb;
        err_cb = error_inject_cb::type_id::create("err_cb");
        uvm_callbacks#(my_driver, driver_callback)::add(
            env.agent.driver, err_cb
        );
    endfunction
endclass

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Callbacks vs Factory Overrides

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Common Use Cases

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Best Practices

1. Define callbacks with virtual empty methods — safe defaults.
2. Place hooks at natural boundaries — pre/post drive, pre/post compare.
3. Keep callbacks lightweight — no complex logic.
4. Multiple callbacks execute in registration order.
5. Use \uvm_register_cb to declare which callbacks a component supports.