UVM Sequence Starting Methods

🔍 Introduction

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A UVM Sequence is a fundamental building block for generating stimulus in a testbench. Starting a sequence and controlling its flow is key to writing flexible and reusable testbenches. This page explains the uvm_do macro and alternative ways to start a sequence, including where to raise objections and how to manually implement the steps that uvm_do performs behind the scenes.

🚀 Sequence Starting Methods

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1️⃣ Using uvm_do Macro

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The most common and convenient way in UVM:

`uvm_do(seq_item)

This macro effectively does:

1. Calls create() to instantiate the sequence item.
2. Calls randomize() to apply constraints (if any).
3. Calls start_item() to begin the transaction.
4. Calls finish_item() to finalize it.

👉 In other words, uvm_do creates and sends the item in a single line.

2️⃣ Manual Sequence Starting (Step-by-Step)

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If you don’t want to use uvm_do or need finer control, you can manually write the steps:

my_item item;
item = my_item::type_id::create("item");

// Randomization (add constraints if needed)
if (!item.randomize())
  `uvm_error("MY_SEQ", "Randomization failed")

// Start the item
start_item(item);

// (Optional) Fill in the fields manually
item.data = 8'hFF;

// Finish the item
finish_item(item);

This is especially useful when you want to set certain fields manually or apply custom constraints.

3️⃣ Starting a Sequence via Sequencer

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You can also start a sequence by explicitly calling start() on the sequencer:

my_sequence seq;
seq = my_sequence::type_id::create("seq");
seq.start(my_sequencer);

This approach is typically used for nested sequences or more complex flows.

🛠️ Using uvm_send

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The uvm_send macro is used to send single sequence items inside a sequence:

`uvm_send(item)

Behind the scenes, this does:

• start_item(item)
• (optional randomization or manual field setting)
• finish_item(item)

all in one step.

📌 Objection Usage

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In UVM, it’s common to raise an objection to prevent the simulation from ending while a sequence is running. Typically done in:

• start_of_simulation_phase
• run_phase
• main_phase

Example:

phase.raise_objection(this, "My sequence started");
...
phase.drop_objection(this, "My sequence completed");

💡 Note: You can raise objections inside the sequence itself or inside the test. This controls when the simulation ends.

🎯 When to Use Each Method?

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📝 Conclusion

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UVM Sequence starting methods add flexibility and control to your verification environment. Macros like uvm_do are great for rapid development, while manual methods provide more customization. Choose the best method based on your testbench needs and flow control.