UVM - Sequences and sequencers

The first step in verifying a RTL design is defining what kind of data should be sent to the DUT. While the driver deals with signal activities at the bit level, it doesn’t make sense to keep this level of abstraction as we move away from the DUT, so the concept of transaction was created.

A transaction is a class object, usually extended from uvm_transaction or uvm_sequence_item classes, which includes the information needed to model the communication between two or more components.

Transactions are the smallest data transfers that can be executed in a verification model. They can include variables, constraints and even methods for operating on themselves. Due to their high abstraction level, they aren’t aware of the communication protocol between the components, so they can be reused and extended for different kind of tests if correctly programmed.

An example of a transaction could be an object that would model the communication bus of a master-slave topology. It could include two variables: the address of the device and the data to be transmitted to that device. The transaction would randomize these two variables and the verification environment would make sure that the variables would assume all possible and valid values to cover all combinations.

In order to drive a stimulus into the DUT, a driver component converts transactions into pin wiggles, while a monitor component performs the reverse operation, converting pin wiggles into transactions.

After a basic transaction has been specified, the verification environment will need to generate a collection of them and get them ready to be sent to the driver. This is a job for the sequence. Sequences are an ordered collection of transactions, they shape transactions to our needs and generate as many as we want. This means if we want to test just a specific set of addresses in a master-slave communication topology, we could restrict the randomization to that set of values instead of wasting simulation time in invalid values.

Sequences are extended from uvm_sequence and their main job is generating multiple transactions. After generating those transactions, there is another class that takes them to the driver: the sequencer. The code for the sequencer is usually very simple and in simple environments, the default class from UVM is enough to cover most of the cases.

A representation of this operation is shown in Figure 4.1.


Figure 4.1 - Relation between a sequence, a sequencer and a driver

The sequence englobes a group of transactions and the sequencer takes a transaction from the sequence and takes it to the driver.

To test our DUT we are going to define a simple transaction, extended fromuvm_sequence_item. It will include the following variables:

rand bit[1:0] ina
rand bit[1:0] inb
bit[2:0] out

The variables ina and inb are going to be random values to be driven to the inputs of the DUT and the variable out is going to store the result. The code for the transaction is represented in Code 4.1.

class simpleadder_transaction extends uvm_sequence_item;
     rand bit[1:0] ina;
     rand bit[1:0] inb;
     bit[2:0] out;
     function new(string name = "");
     endfunction: new
     `uvm_field_int(ina, UVM_ALL_ON)
     `uvm_field_int(inb, UVM_ALL_ON)
     `uvm_field_int(out, UVM_ALL_ON)
endclass: simpleadder_transaction
Code 4.1 – Transaction for the simpleadder

An explanation of the code will follow:

  • Lines 2 and 3 declare the variables for both inputs. The rand keyword asks the compiler to generate and store random values in these variables.
  • Lines 6 to 8 include the typical class constructor.
  • Lines 10 to 14 include the typical UVM macros.

These few lines of code define the information that is going to be exchanged between the DUT and the testbench.

To demonstrate the reuse capabilities of UVM, let’s imagine a situation where we would want to test a similar adder with a third input, a port named inc.

Instead of rewriting a different transaction to include a variable for this port, it would be easier just to extend the previous class to support the new input.

It’s possible to see an example in Code 5.2.

class simpleadder_transaction_3inputs extends simpleadder_transaction;
     rand bit[1:0] inc;
     function new(string name = "");
     endfunction: new
     `uvm_field_int(inc, UVM_ALL_ON)
endclass: simpleadder_transaction_3inputs
Code 5.2 – Extension of the previous transaction

As a result of the class simpleadder_transaction_3inputs being an extension of  simpleadder_transaction, we didn’t need to declare again the other variables. While in small examples, like this one, this might not look like something useful, for bigger verification environments, it might save a lot of work.


Now that we have a transaction, the next step is to create a sequence.

The code for the sequencer can be found in Code 5.3

class simpleadder_sequence extends uvm_sequence#(simpleadder_transaction);
     function new(string name = "");
     endfunction: new
     task body();
          simpleadder_transaction sa_tx;
          repeat(15) begin
               sa_tx = simpleadder_transaction::type_id::create(...
     endtask: body
endclass: simpleadder_sequence
Code 5.3 - Code for the sequencer

An explanation of the code will follow:

  • Line 8 starts the task body(), which is the main task of a sequence
  • Line 11 starts a cycle in order to generate 15 transactions
  • Line 12 initializes a blank transaction
  • Line 14 is a call that blocks until the driver accesses the transaction being created
  • Line 15 triggers the rand keyword of the transaction and randomizes the variables of the transaction to be sent to the driver
  • Line 16 is another blocking call which blocks until the driver has completed the operation for the current transaction


The only thing missing is the sequencer. The sequence will be extended from the class uvm_sequencer and it will be responsible for sending the sequences to the driver. The sequencer gets extended from uvm_sequencer. The code can be seen on Code 5.4.

typedef uvm_sequencer#(simpleadder_transaction) simpleadder_sequencer;
Code 5.4 – Extension of the previous transaction

The code for the sequencer is very simple, this line will tell UVM to create a basic sequencer with the default API because we don’t need to add anything else.

So, right now our environment has the following structure:



Figure 4.2 – State of the verification environment after the sequencer

You might have noticed two things missing:

  • How does the sequence connects to the sequencer?
  • How does the sequencer connects to the driver

The connection between the sequence and the sequencer is made by the test block, we will come to this later on chapter 10, and the connection between the sequencer and the driver will be explained on chapter 7.

For more information about transactions and sequences, you can consult:

UVM–Top Block

In a normal project, the development of the DUT is done separately from the development of the testbench, so there are two components that connects both of them:

  • The top block of the testbench
  • A virtual interface

The top block will create instances of the DUT and of the testbench and the virtual interface will act as a bridge between them.

The interface is a module that holds all the signals of the DUT. The monitor, the driver and the DUT are all going to be connected to this module.

The code for the interface can be seen in Code 3.1.

interface simpleadder_if;
     logic    sig_clock;
     logic    sig_ina;
     logic    sig_inb;
     logic    sig_en_i;
     logic    sig_out;
     logic    sig_en_o;
endinterface: simpleadder_ifCode 3.1: Interface module – simpleadder_if.sv

After we have an interface, we will need the top block. This block will be a normal SystemVerilog module and it will be responsible for:

  • Connecting the DUT to the test class, using the interface defined before.
  • Generating the clock for the DUT.
  • Registering the interface in the UVM factory. This is necessary in order to pass this interface to all other classes that will be instantiated in the testbench. It will be registered in the UVM factory by using the uvm_resource_db method and every block that will use the same interface, will need to get it by calling the same method. It might start to look complex, but for now we won’t need to worry about it too much.
  • Running the test.
    The source for the top block is represented in Code 3.2.

`include "simpleadder_pkg.sv"
`include "simpleadder.v"
`include "simpleadder_if.sv"
module simpleadder_tb_top;
     import uvm_pkg::*;
     //Interface declaration
     simpleadder_if vif();
     //Connects the Interface to the DUT
     simpleadder dut(vif.sig_clock,
     initial begin
          //Registers the Interface in the configuration block
          //so that other blocks can use it
          uvm_resource_db#(virtual simpleadder_if)::set(.scope("ifs"), .name("simpleadder_if"), .val(vif));
          //Executes the test
     //Variable initialization
     initial begin
          vif.sig_clock = 1'b1;
     //Clock generation
          #5 vif.sig_clock = ~vif.sig_clock;

Code 3.2: Top block – simepladder_tb_top.sv

A brief explanation of the code will follow:

  • The lines 2 and 3 include the DUT and the interface into the top block, the line 5 imports the UVM library, lines 11 to 16 connect the interface signals to the DUT.
  • Line 21 registers the interface in the factory database with the name simpleadder_if.
  • Line 24 runs one of the test classes defined at compilation runtime. This name is specified in the Makefile.
  • Line 34 generates the clock with a period of 10 time units. The time unit is also defined in the Makefile.

For more information about interfaces, you can consult the book “SystemVerilog for Verification: A Guide to Learning the TestBench Language Features“, chapter 5.3.

UVM - Defining The Verification Environment

Before understanding UVM, we need to understand verification.

Right now, we have a DUT and we will have to interact with it in order to test its functionality, so we need to stimulate it. To achieve this, we will need a block that generates sequences of bits to be transmitted to the DUT, this block is going to be named sequencer.

Usually sequencers are unaware of the communication bus, they are responsible for generating generic sequences of data and they pass that data to another block that takes care of the communication with the DUT. This block will be the driver.

While the driver maintains activity with the DUT by feeding it data generated from the sequencers, it doesn’t do any validation of the responses to the stimuli. We need another block that listens to the communication between the driver and the DUT and evaluates the responses from the DUT. This block is the monitor.

Monitors sample the inputs and the outputs of the DUT, they try to make a prediction of the expected result and send the prediction and result of the DUT to another block, the scoreboard, in order to be compared and evaluated.

All these blocks constitute a typical system used for verification and it’s the same structure used for UVM testbenches.

You can find a representation of a similar environment in Figure 2.1.


Figure 2.1: Typical UVM testbench

Usually, sequencers, drivers and monitors compose an agent. An agent and a scoreboard compose an environment. All these blocks are controlled by a greater block denominated of test. The test block controls all the blocks and sub blocks of the testbench. This means that just by changing a few lines of code, we could add, remove and override blocks in our testbench and build different environments without rewriting the whole test.

To illustrate the advantage of this feature, let’s imagine a situation where we are testing a another DUT that uses SPI for communication. If, by any chance, we want to test a similar DUT but with I2C instead, we would just need to add a monitor and a driver for I2C and override the existing SPI blocks, the sequencer and the scoreboard could reused just fine.

UVM Classes

The previous example demonstrates one of the great advantages of UVM. It’s very easy to replace components without having to modify the entire testbench, but it’s also due to the concept of classes and objects from SystemVerilog.

In UVM, all the mentioned blocks are represented as objects that are derived from the already existent classes.

A class tree of the most important UVM classes can be seen in Figure 2.2.


Figure 2.2: Partial UVM class tree

The data that travels to and from our DUT will stored in a class derived either from uvm_sequence_item or uvm_sequence. The sequencer will be derived from uvm_sequencer, the driver from uvm_driver, and so on.

Every each of these classes already have some useful methods implemented, so that the designer can only focus on the important part, which is the functional part of the class that will verify the design. These methods are going to addressed further ahead.

For more information about UVM classes, you can consult the document Accellera’s UVM 1.1 Class Reference.

UVM Phases

All these classes have simulation phases. Phases are ordered steps of execution implemented as methods. When we derive a new class, the simulation of our testbench will go through these different steps in order to construct, configure and connect the testbench component hierarchy.

The most important phases are represented in Figure 2.3.


Figure 2.3: Partial list of UVM phases

A brief explanation of each phase will follow:

  • The build phase is used to construct components of the hierarchy. For example, the build phase of the agent class will construct the classes for the monitor, for the sequencer and for the driver.
  • The connect is used to connect the different sub components of a class. Using the same example, the connect phase of the agent would connect the driver to the sequencer and it would connect the monitor to an external port.
  • The run phase is the main phase of the execution, this is where the actual code of a simulation will execute.
  • And at last, the report phase is the phase used to display the results of the simulation.

There are many more phases but none of them are mandatory. If we don’t need to have one in a particular class, we can just omit it and UVM will ignore it.

More information about UVM phasing can be consulted in Verification Academy’s UVM Cookbook, page 48.

UVM Macros

Another important aspect of UVM are the macros. These macros implement some useful methods in classes and in variables. they are optional, but recommended.

The most common ones are:

  • `uvm_component_utils – This macro registers the new class type. It’s usually used when deriving new classes like a new agent, driver, monitor and so on.
  • `uvm_field_int – This macro registers a variable in the UVM factory and implements some functions like copy(), compare() and print().
  • `uvm_info – This a very useful macro to print messages from the UVM environment during simulation time.

This guide will not go into much detail about macros, their usage is always the same for every class, so it’s not worth to put much thought into it for now.

More information can be found in Accellera’s UVM 1.1 Class Reference, page 405.


SimpleAdder UVM Testbench

After a brief overview of a UVM testbench, it’s time to start developing one. By the end of this guide, we will have the verification environment from the Figure 2.4.


Figure 2.4: SimpleAdder Final Testbench

This guide will begin to approach the top block and the interface (chapter 3), then it will explain what data will be generated with the sequences and sequencers on chapter 4.

Following the sequencers, it will explain how to drive the signals into the DUT and how to observe the response in chapters 5 and 6 respectively.

Subsequently, it will explain how to connect the sequencer to the driver and the monitor to the scoreboard in chapter 7. Then it will show to build a simple scoreboard in chapter 8.

And finally, the test will be executed and analyzed.

The testbench can be run with the execution of a Makefile provided in the repository. As I mentioned previously, this Makefile uses Synopsys VCS but it should be easily modifiable to be executed with any HDL simulator.

UVM Tutorial - The DUT

This training guide will focus on showing how we can build a basic UVM environment, so the device under test was kept very simple in order to emphasize the explanation of UVM itself.

The DUT used is a simple ALU, limited to a single operation: the add operation. The inputs and outputs are represented in Figure 1.1.


Figure 1.1: Representation of the DUT’s inputs/outputs

This DUT takes two values of 2 bits each, ina and inb, sums them and sends the result to the output out. The inputs are sampled to the signal of en_i and the output is sent at the same time en_o is signalled.

The operation of the DUT is represented as a timing diagram and as a state machine in Figure 1.2.



Figure 1.2: Operation of the DUT

Below is the code for sample DUT

UVM Introduction

As digital systems grow in complexity, verification methodologies get progressively more essential. While in the early beginnings, digital designs were verified by looking at waveforms and performing manual checks, the complexity we have today don’t allow for that kind of verification anymore and, as a result, designers have been trying to find the best way to automate this process.

The SystemVerilog language came to aid many verification engineers. The language featured some mechanisms, like classes, covergroups and constraints, that eased some aspects of verifying a digital design and then, verification methodologies started to appear.

UVM is one of the methodologies that were created from the need to automate verification. The Universal Verification Methodology is a collection of API and proven verification guidelines written for SystemVerilog that help an engineer to create an efficient verification environment. It’s an open-source standard maintained by Accellera and can be freely acquired in their website.

By mandating a universal convention in verification techniques, engineers started to develop generic verification components that were portable from one project to another, this promoted the cooperation and the sharing of techniques among the user base. It also encouraged the development of verification components generic enough to be easily extended and improved without modifying the original code.

All these aspects contributed for a reduced effort in developing new verification environments, as designers can just reuse testbenches from previous projects and easily modify the components to their needs.

These series of webpages will provide a training guide for verifying a basic adder block using UVM. The guide will assume that you have some basic knowledge of SystemVerilog and will require accompaniment of the following resources:

This guide will be divided in 3 different parts:

  • The first part, starting on chapter 1, will explain the operation of the device under test (DUT): the inputs, the outputs and the communication bus
  • The second part, starting on chapter 2, will give a brief overview of a generic verification environment and the approach into verifying the DUT
  • The third part, starting on chapter 3, will start to describe a possible UVM testbench to be used with our DUT with code examples. It’s important to consult to the external material in order to better understand the mechanism behind the testbench.

Transient Materials - Electronics that melt away

transient_materialsImagine tossing your old phone in the toilet, watching it dissolve and then flushing it down, instead of having it wind up in a landfill. Scientists are working on electronic devices that can be triggered to disappear when they are no longer needed.

The technology is years away, but Assistant Professor Reza Montazami and his research team in the mechanical engineering labs at Iowa State University have published a report that shows progress is being made. In the two years they've been working on the project, they have created a fully dissolvable and working antenna.

"You can actually send a signal to your passport via satellite that causes the passport to physically degrade, so no one can use it," Montazami said.

The electronics, made with special "transient materials," could have far-ranging possibilities. Dissolvable electronics could be used in medicine for localizing treatment and delivering vaccines inside the body. They also could eliminate extra surgeries to remove temporarily implanted devices.The military could design information-gathering gadgets that could complete their mission and dissolve without leaving a trace.

The researchers have developed and tested transient resistors and capacitors. They’re working on transient LED and transistor technology, said Montazami, who started the research as a way to connect his background in solid-state physics and materials science with applied work in mechanical engineering.

As the technology develops, Montazami sees more and more potential for the commercial application of transient materials.

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