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Table of contents
1.
Introduction
2.
Most Asked Verilog Interview Questions
2.1.
1. Explain Verilog
2.2.
2. What is the complete form of VHDL in VLSI?
2.3.
3. What are the main applications for VHDL?       
2.4.
4. Are Verilog and VHDL the same thing?
2.5.
5. How do VHDL and Verilog differ from one another?
2.6.
6. Describe HDL simulators.
2.7.
7. Differentiate between blocking systems in Verilog from non-blocking systems?
2.8.
8. What is PLI? Mention its uses.
2.9.
9. What do you mean by sensitivity list?
2.10.
10. What do you know about Verilog parallel and full case statements?
2.11.
11. What is the distinction between == and === in Verilog?
2.12.
12. What do you mean by $monitor, $display, and $strobe?
2.13.
13. What are the primary differences between Verilog's Task and Function?
2.14.
14. How are blocking and non-blocking assignments executed?
2.15.
15. What do you mean by continuous assignment?
2.16.
16. What do you mean by inertial delay and transport delay?
2.17.
17. What does a Verilog repeat loop do?
2.18.
18. What are the key differences between Reg and Wire?
2.19.
 
2.20.
19. How do you write FSM code in Verilog?
2.21.
20. Which will be updated first between the variable and the signal?
2.22.
21. Is it required to list every input in the sensitivity disc for a pure combinational circuit? If so, why then?
2.23.
22. Explain the Verilog concepts of freeze, deposit, force, and drive.
2.24.
23. What distinguishes a $monitor from a $display?
2.25.
24. What does the Verilog code timeframe 1 Ns/ 1 Ps mean?
2.26.
25. In Verilog, what do the casex and casez statements mean?
2.27.
26. Write a Verilog program to switch the contents of two registers with and without a temporary register.
2.28.
27. How can a sine wave be produced using Verilog coding?
2.29.
28. What is the difference between blocking and non-blocking assignments?
2.30.
29. When should you use tasks instead of functions?
2.31.
30. Can race conditions occur in Verilog?
3.
Frequently Asked Questions
3.1.
What are the interview questions for Verilog?
3.2.
What Verilog is used for?
3.3.
Which is updated first in Verilog HDL?
3.4.
What are the 3 types of coding in Verilog?
4.
Conclusion
Last Updated: Mar 27, 2024
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Top 25+ Verilog Interview Questions and Answers for 2024

Author Ishita mishra
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12 Jun, 2024 @ 01:30 PM

Introduction

Read on if you are planning to give a Verilog interview but are unsure how to prepare or what the interviewer is most likely to ask. Well, you've come to the right place. We have compiled the most frequently asked Verilog interview questions for both freshers and professionals that may be asked in many Verilog interviews.

To help you prepare for the Verilog interview, we have curated a list of the Top Frequently Asked Verilog Interview Questions and Answers. Now let's take a look at them—

Verilog Interview Questions

Most Asked Verilog Interview Questions

1. Explain Verilog

Verilog is a text-based hardware description language used to describe electronic systems and circuits. Verilog is designed to be used in electronic design for timing analysis, test analysis (fault grading and testability analysis), logic synthesis, and verification through simulation.

The following three levels of abstraction are supported by Verilog most frequently in a design:

  • Behavioral level
     
  • Register-transfer level
     
  • Gate level

2. What is the complete form of VHDL in VLSI?

The acronym VHDL stands for Very High-Speed Integrated Circuit Hardware Description Language. It is a programming language used to represent the digital system using modeling approaches, including dataflow, behavioral, and structural, and to describe the circuits in digital systems.

3. What are the main applications for VHDL?       

The principal applications of VHDL are as follows.

  • The behavior of electronic circuits; typically, digital circuits are described using the hardware description language, or VHDL.
     
  • It is mainly used to design hardware and test entities to check how it behaves.
     
  • It is utilized as an entry format for designs by different EDA tools.

 

4. Are Verilog and VHDL the same thing?

VHDL and Verilog are not interchangeable. They are distinct from one another, with the primary distinction being that Verilog is based on the C language, whereas VHDL is based on the Ada and Pascal languages.

5. How do VHDL and Verilog differ from one another?

This Verilog interview question is one of the most frequently asked. Let's compare them to identify their significant differences

VHDL

Verilog

It was established in 1980, making it an older language.

It was formed between late 1983 and early 1984.

It uses the base languages, Ada and Pascal

The base language is C.

The VHDL language is more compact.

It contains comparatively more LOCs or lines of code.

As a language that encompasses more extensive capabilities than just modeling systems, VHDL is more reliable and developed.

Considered to be of considerably lower level as a language mainly used for modeling,

VHDL is not case sensitive

Verilog is case sensitive

VHDL analyses and simulates the behavior of the digital system in addition to describing its structure.

They are only applied to describe digital systems.

6. Describe HDL simulators.

Simulating expressions written in one of the Hardware Description Languages, such as Verilog, VHDL, or SystemVerilog, is done using software programs known as HDL simulators.

7. Differentiate between blocking systems in Verilog from non-blocking systems?

This is just another of the general Verilog interview questions. In Verilog, blocking and non-blocking procedural assignment statements are two different sorts. They differ in the assignment operators they use, which are denoted by the symbols = and <=.

Blocking System

Non-Blocking System

Blocking is the simple term for when one register assignment prevents another assignment from being executed simultaneously. 

Non-blocking allows assignments to be executed concurrently regardless of the sequence of dependencies.

Assignments will only be blocked in order until the one before them is finished. In contrast, it doesn't stop when running in parallel.

Non-blocking will plan the execution of sequential assignments so that concurrent execution of tasks is possible.

8. What is PLI? Mention its uses.

It is one of the most commonly asked Verilog interview questions. Programming Language Interface is known by the acronym PLI. It is a method that makes it easier for C and Verilog applications to interface with one another. Additionally, it offers a way for the C program to access the simulator's internal databases. The users can use PLI to implement challenging system calls using Verilog syntax. Additionally, it offers the benefits of Verilog's parallel and hardware-related features and the sequential development of a C program.

9. What do you mean by sensitivity list?

The sensitivity list defines that when changes are made to any of the list's elements, a begin-end statement must be placed inside that element.

10. What do you know about Verilog parallel and full case statements?

In Verilog, there are two different kinds of case statements.

  • Verilog parallel case statements: A statement that matches just one case item in a case expression is referred to as a parallel case statement. The matching case items are known as "overlapping case items," The case statement would not be parallel if you could find a case expression that would fit more than one case item.
     
  • Verilog full case statements: Every case expression in a Verilog full case statement must have binary patterns that match either a case item or the default. The case statement would not be regarded as complete if it did not include a case default and was likely to come across a binary case expression that did not match any of the specified case elements.

11. What is the distinction between == and === in Verilog?

The primary differences between == and === in Verilog are listed below

==

===

The key distinction between == and === in Verilog is that the result of == can either be 1, 0 or X.

The output of ===, on the other hand, can only be either 0 or 1.

If two numbers are compared using == and one or both numbers have one or more bits set to X, the result will be X.

The result of a === comparison between two numbers would be either 0 or 1.

Since it cannot compare Xs, == can only be used to compare 1s and 0s.

Xs can also be compared using ===.

12. What do you mean by $monitor, $display, and $strobe?

The $monitor, $display, and $strobe commands all have a similar syntax and display text on the screen while a simulation is ongoing. Compared to waveform tools like cwaves, these commands are often less user-friendly. 

  • $monitor: 

Syntax: $Monitor(<format_string>,<parameter>,<parameter>)

When $Monitor is used, it will execute and display the parameters each time one of the parameters is modified, that is, each time a new value for the parameter is displayed. Signals can be expressions, variables, or strings, and signal values can be changed using the $Monitor command.

  • $display:

Syntax: $display(<list of arguments>);

Another task for arguments is $display, which also aids debugging.

$display will always append a newline to the end of the string while showing the arguments.

  • $strobe:

Syntax: $display(<list of arguments>);

In terms of the format of presenting its arguments, $strobe is quite similar to $display; however, it will only display all of its arguments after the current time unit.

13. What are the primary differences between Verilog's Task and Function?

Another most frequently asked Verilog interview question. The key distinctions between a Task and a Function in Verilog are as follows:

Task

Function

In Verilog, tasks can activate both additional tasks and functions.

A function in Verilog can enable other functions but cannot allow a task.

 

A task is permitted to contain any event, delay, or timing control statement.

Event, delay, or timing control statements are not allowed. Hence they cannot be contained in a function.

The simulation for tasks is also zero. However, they can also be carried out in a simulation time that is not zero if necessary.

Because the program time is not increased during the function routine, a function can do its required task in zero simulation time.

A task may employ zero or more arguments of the output or input types arguments.

A function must receive at least one parameter when it is invoked.

Using the output and input statements, a task can pass multiple values but cannot return a value.

Functions cannot employ either output or input statements and only return a single value.

14. How are blocking and non-blocking assignments executed?

Executing blocking and non-blocking assignments is a simple process. We must follow a straightforward process of evaluating the right-side equation and updating the left-side expression without interference from another Verilog statement to carry out blocking assignments. On the other hand, there are two processes involved in non-blocking assignments:

  • At the beginning of the time step, evaluate the right-hand side of every non-blocking statement.
     
  • After the time step, update the left side of all non-blocking statements.

15. What do you mean by continuous assignment?

Combinational logic is modeled using continuous assignment statements in Verilog. Combinational logic refers to the digital logic that Boolean circuits implement. The sole pure function of the inputs in combinational logic is the output. The present information depends on the present input and the past inputs in sequential logic but not in combinational logic. The assigned statement or a wire declaration is used to implement the continuous assignment statements.

16. What do you mean by inertial delay and transport delay?

  • Inertial delay: The time it takes for a gate to change its output is known as the inertial delay.
     
  • Transport delay: One sort of delay brought on by the wires connecting to the gates is transport delay. The signal is delayed by the resistance and inductance of the wire.

17. What does a Verilog repeat loop do?

As its name suggests, the repeat loop behaves like loops used in popular programming languages like the for a loop. A repeat loop will execute a piece of code as often as it is accessed. The repeating piece of code will not make any reference to the loop cycle.

Syntax: 

repeat(<no. of times the loop will run>) <statement to be repeated>

Repetition in a line of code can be avoided with a repeat loop.

18. What are the key differences between Reg and Wire?

Reg and wire are often an interviewer’s favorite in the Verilog interview questions, so it is important to prepare them thoroughly. Essential differences between Reg and Wire are:

Reg

Wire

Reg is employed to store value.

The wire is employed to determine value.

Reg can get the output without a driver.

The wire needs drivers to obtain output values.

The reg components can be used for both sequential and combinational logic.

The only type of logic that the wire elements can model is combinational logic.

Reg cannot be used on an assigned statement's left-hand side.

Wire can be used on the left-hand side of a given statement.

 

19. How do you write FSM code in Verilog?

In Verilog, there are primarily four approaches to writing FSM code:

  • We are utilizing the first approach, which combines the output decoder, present state, and all input decoders into a single operation.
     
  • Using the second approach, all sequential and combinational circuits are isolated using a different procedure.
     
  • Using the third approach, the output decoder is segregated into different processes while the input decoder and current state are integrated.
     
  • The output decoder, present state, and all three input decoders are divided into three processes using the fourth approach.


20. Which will be updated first between the variable and the signal?

Variables can only be used inside processes, whereas signals are used inside and outside. As a result, signals are updated before variables are updated.

21. Is it required to list every input in the sensitivity disc for a pure combinational circuit? If so, why then?

Yes. If you don't list every input in the sensitivity disc for a pure combinational circuit, the outcome will have a pre- and post-synthesis mismatch.

22. Explain the Verilog concepts of freeze, deposit, force, and drive.

It is important to know the various concepts of Verilog used as they are often asked in Verilog interview questions.

Freeze: For unresolved signals, use freeze. Throughout the simulation, the signal's value is frozen. You cannot modify this value.

Deposit: Deposit ensures that a signal value is kept until it is altered or replaced by the simulation. Throughout the simulation, it serves as value initialization.

Force: It can drive signals at any point during the simulation.

Drive: When the drive command gives a signal a value, it will "resolve" to a new value if the simulation updates the signal.

23. What distinguishes a $monitor from a $display?

The test bench results are displayed using the system commands or functions $monitor and $display. The list of significant changes between $monitor and $display is below.

$monitor

$display

Changes to the signal's value are made via the monitor. An expression, string, or variable can be a signal.

It is used to display a signal's value.

There can be only one execution of the monitor command.

You can use the display command more than once.

24. What does the Verilog code timeframe 1 Ns/ 1 Ps mean?

A compiler directive used to calculate simulation or delay times is called the timeframe directive. The timescale / reference_time_unit defines the unit of measurement for delays and times. The accuracy to which the delays are rounded off is specified by time_precision.

25. In Verilog, what do the casex and casez statements mean?

Verilog has two different forms of case statements: casex and casez. All z values in the case alternatives or the case expression are treated as don't cares in this case by casez. The character? can also represent any bit position with the character z. All z and x values in the case item or the case expression are treated as don't cares by casex. We must use casex, and casez since do not cares are not permitted in a case statement.

26. Write a Verilog program to switch the contents of two registers with and without a temporary register.

An example of a Verilog code that switches the contents of two registers with a temporary register:

always @ (posedge clock)  
begin  
temp=y;  
y=x;  
x=temp;  
end.  

 

Without a temporary register, a Verilog program can swap the contents of two registers:

always @ (posedge clock)  
begin  
x <= y;  
y <= x;  
end

27. How can a sine wave be produced using Verilog coding?

The CORDIC algorithm is the fastest and most effective method for creating sine waves.

28. What is the difference between blocking and non-blocking assignments?

In Verilog, the difference between blocking and non-blocking assignments depends on the way they handle the execution of statements within procedural blocks, such as "always" blocks. Blocking assignments are represented by the "=" operator, and it executes immediately in sequential order. While the "<=" operator represents non-blocking assignments, and it introduces a level of concurrency in Verilog, the RHS statement is computed concurrently with other statements, so it is non-blocking in nature.

29. When should you use tasks instead of functions?

You should use tasks instead of functions in Verilog when you need to perform a sequence of procedural actions or when you require multiple input/output variables. Tasks are more suitable for modeling complex behavior. They can contain both procedural statements and timing controls, allowing you to model more complicated processes.

30. Can race conditions occur in Verilog?

Yes, race conditions can occur in Verilog. A race condition arises due to concurrent events happening in different orders, and the final outcome depends on the order of timing of these events.

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Frequently Asked Questions

What are the interview questions for Verilog?

Some common interview questions for a Verilog can be the differences between a behavioral and structural description in Verilog. Handling delays in a Verilog design. Using a finite state machine in a Verilog design.  Difference between a blocking and non-blocking assignment in Verilog.

What Verilog is used for?

Verilog is primarily used for designing and simulating digital circuits, especially in the field of electronic design automation (EDA). It's instrumental in creating models for FPGA and ASIC designs, allowing hardware engineers to describe, test, and verify complex digital systems.

Which is updated first in Verilog HDL?

In Verilog HDL, the simulation time is updated before any process is executed. Time advances in discrete steps, allowing the simulation to model sequential behavior and synchronize events. This ensures accurate modeling of hardware behavior and timing.

What are the 3 types of coding in Verilog?

Verilog, a hardware description language, has three main types of coding: structural coding, behavioral coding, and register transfer level (RTL) coding. Structural focuses on component connections, behavioral on functionality, and RTL on data flow.

Conclusion

Endless questions may be posted on a topic as broad as Verilog. The Verilog interview questions discussed above should be helpful to you during the recruiting process. Before speaking in front of a panel for an interview, it is advisable to understand the topic beforehand. However, if you thoroughly prepare for this set of Verilog interview questions, you will surely be in a better position to handle any interview.

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