UVM
Free Interview Questions & Answers
What is UVM?
UVM is a Standard Verification Methodology that uses System Verilog constructs based on which a fully functional testbench can be built to verify the functional correctness of the Design Under Test(DUT). It is an IEEE standard/methodology.UVM provides a framework to build testbench architecture that is reusable, scalable, and configurable.
Why is UVM important?
UVM is an important open-source code that provides:
- A library of base classes for building testbench components (Agent, Sequencer, Driver, Monitor, Scoreboards, Environment class, etc)
- A factory for constructing objects and substituting objects
- Verification phases for synchronizing concurrent processes
- A reporting mechanism for a consistent way of printing and logging results
- Transaction Level Modeling (TLM) for communication between verification components
- Macros to semi-automate generation of required UVM code.
How to prepare for the UVM Interviews?
UVM is a methodology that is developed using SystemVerilog. For UVM interviews, you should be strong at SV concepts and should have a complete understanding of UVM testbench architecture, UVM phases, TLM, configuration methods, and UVM sequence.
A monitor is a component that observes pin-level activity and converts its observations into transactions or sequence_items. It also sends these transactions to analysis components through an analysis port. A scoreboard is an analysis component that checks if the DUT is behaving correctly by comparing the data received from the monitor with reference data.
When verifying a DUT that handles packets flowing back and forth, you must create a verification environment that supports the appropriate abstraction level. In UVM, the verification components communicate at the transaction level. The transaction is a class that includes the information specific to the protocol that is required for the communication between two components which is at a higher level of abstraction. UVM provides a set of transaction-level communication interfaces and channels that you can use to connect components at the transaction level. The use of TLM interfaces isolates each component from changes in other components throughout the environment.
In Traditional Directed Testbenches, we used $finish after the required steps like reset, configuration, data transfer, and self-checking are completed. However, in UVM objection mechanism is used. The objection mechanism in UVM is to allow hierarchical status communication among components which is helpful in deciding the end of the test. Each participating member can raise or drop the objections which in turn increments or decrements the counter value. raise_objection() and drop_objection() are the methods to be used to do that. Once the value of the shared counter reaches to zero from a non-zero value – we can say that the “all dropped” condition is achieved. Once all the objections are dropped simulation is terminated.
Some of the benefits of using UVM are: Modularity and Reusability – The methodology is designed as modular components (Driver, Sequencer, Agents, env, etc) which enables reusing components across the unit level to multi-unit or chip level verification as well as across projects. Separating Tests from Test benches – Tests in terms of stimulus/sequencers are kept separate from the actual test bench hierarchy and hence there can be the reuse of stimulus across different units or across projects. Simulator independent – The base class library and the methodology is supported by all simulators and hence there is no dependence on any specific simulator. Better control on Stimulus generation – Sequence methodology gives good control over stimulus generation. There are several ways in which sequences can be developed which include randomization, layered sequences, virtual sequences, etc which provide good control and rich stimulus generation capability. Easy configuration – Config mechanisms simplify the configuration of objects with a deep hierarchy. The configuration mechanism helps in easily configuring different test bench components based on which verification environment uses it and without worrying about how deep any component is in the test bench hierarchy. Factory mechanism – Factory mechanisms simplify the modification of components easily. Creating each component using a factory enables them to be overridden in different tests or environments without changing the underlying code base.
In a verification context, a register model (or register abstraction layer) is a set of classes that model the memory-mapped behavior of registers and memories in the DUT in order to facilitate stimulus generation and functional checking (and optionally some aspects of functional coverage). The UVM provides a set of base classes that can be extended to implement comprehensive register modeling capabilities.
UVM Factory is used to manufacture (create) UVM objects and components. Apart from creating the UVM objects and components, the factory concept essentially means that you can modify or substitute the nature of the components created by the factory without making changes to the test bench. For example, if we want to substitute an existing driver class in the environment with the extended driver, we can achieve this easily with the help of a factory. For this, we have to register both drivers with the factory, you can ask the factory to substitute the existing driver in the environment with the other type. The code needed to achieve this is minimal and can be written in the test.
The get_next_item() is a blocking call to get the sequence item from the sequencer FIFO for processing by the driver. Once the sequence item is processed by a driver, it needs to call item_done() to complete the handshake before a new item is requested using get_next_item(). The get() is also a blocking call that gets the sequence item from sequencer FIFO for processing by the driver. However, while using get(), there is no need to explicitly call item_done() as the get() method completes the handshake implicitly.
Conclusion
UVM consists of a defined methodology in terms of architecting testbenches and test cases, and also comes with a library of classes that helps in building efficient constrained random testbenches easily. Anyone who wants to pursue a career in Verification must learn and master UVM by practicing the UVM interview questions to augment the productivity as well as the quality of the designs and make the Verification fly. These UVM interview questions and answers will help you prepare well for the Verification job roles and bag a good VLSI job opportunity.
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