Knowledge of a subject or topic is often challenged before the job interviewers. Therefore, it is of utmost importance to understand interview question patterns before appearing for one. This is to understand the way of Verilog interview questions that are most commonly faced.
Verilog interview questions can sometimes test your knowledge on the subject for which, while answering these Verilog interview questions we made sure to provide in-depth information on the topic.
Before beginning with interview question answers, let us understand a bit more of what we are talking about here.
The basic idea of Verilog:
The word Verilog is a combination of verification and logic and that almost explains what Verilog does at the hardware level.
Verilog is an HDL or Hardware Description Language. As the name suggests, it is a language used in digital systems to design and verify circuits. Verifying analog circuits and mixed-signal circuits are done using Verilog as well as designing Genetic circuits.
Now moving on to the Verilog interview questions.
We have collected 15 most asked Verilog interview questions along with their answers.
These selected Verilog interview questions have answers that can additionally prepare you for topics that are related to the questions.
There will always be Verilog interview questions that are not covered in this set of interview question answers, but these Verilog interview questions are sure to give you some idea on how to answer those.
Verilog Interview Questions for Freshers
1: What is Verilog?
Verilog being a Hardware Description Language, can be used to describe any digital system such as microprocessors, flip-flops, etc..
Verilog works on multiple levels of abstraction, three notable ones of those are:
- Structural or Gate: This being the lowest level is the most detailed.
- Data-flow or Register Transfer: This is less detailed compared to gate and thus is a higher level of abstraction. This level is more abstracted to ensure efficiency. This level of abstraction controls the flow of data.
- Behavioral: As clear from the name, this level describes the functional aspect of the hardware and is at the maximum abstraction. This is often used for simulations and analysis of the model.
2: What is VHDL?
VHDL is short for Very High-speed Integrated Circuit HDL (Hardware Description Language). VHDL allows the user to describe and simulate a system before modeling it into a digital system.
This HDL has features that have made it very popular such as:
- Readable
- Tool availability. It is independent of technology and tool
- Description of concurrent systems. Better efficient and less time consuming
- Design data portability
- Complexity management
3: What is the difference between VHDL and Verilog?
This is one of the most common Verilog interview questions. So, let’s be prepared to answer this in a way that will ensure a positive response.
| VHDL | Verilog |
| Founded in 1980, making it an older language | Comparatively much newer, being founded between late 1983 and early 1984 |
| Uses Ada and Pascal as the base language | C is the base language |
| VHDL is a more compact language | LOC or lines of code are comparatively more |
| VHDL is more robust and more refined as a language that takes up more complex functionality than just modelling systems Considered a much lower level as a language being mainly used for modelling | Considered a much lower level as a language being mainly used for modeling |
| VHDL is not case sensitive | Verilog is case sensitive |
| VHDL not only describes the digital system structure but analyses and simulates its behavior | Used only for describing digital systems |
4: What is the difference between blocking and non-blocking systems in Verilog?
This is another of the very frequently asked Verilog interview questions.
| Blocking | Non-blocking |
| Blocking simply means when one register assignment blocks other assignments to be performed at the same time. In other words, while one register assignment is in progress other assignments will be blocked. This stops concurrent assignments to be performed. | Non-blocking is just the opposite of blocking. Non-blocking allows concurrent execution of assignments irrespective of the sequence of dependence. |
| Blocking will only block assignments in sequence until the preceding assignment is completed. Whereas it does not block in parallel execution. | Non-blocking will schedule the execution of sequential assignments so that assignments can go on executing concurrently. |
5: What do you understand about Verilog full case statements and Verilog parallel case statements?
Before preparing the answer to some of these Verilog interview questions, it’s better to have a clear perspective of the basic idea behind it.
Let us understand what a case statement is in Verilog?
This can be considered another one of the fundamental Verilog interview questions.
Case statements in Verilog act as a decision-making statement such as the function of the if-else statement (commonly used in programming languages).
Case statements are enclosed between “case” and “endcase”. In between are several case expressions, case items, and case item statements.
The case item is used to compare with the case expression and in turn, execute the case item statement if the case item matches the case expression.
Now let us understand and answer the main Verilog Interview question.
An entire case statement refers to statements covering every possible match between case expression and case item or defaults. In other words, all possible binary patterns of case expressions should be matched with some case item or case default.
A parallel case statement refers to case statements where one single match of case items can be found for a case expression. If any case expression finds a match with more than one case item, it will not be a parallel case statement. Instead, it will be overlapping.
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6:What are the main differences between Task and Function in Verilog?
You will often come across Verilog Interview questions that require finding differences, and the key to answering such system Verilog questions is to be thorough and clear with the prominent differences.
Before differentiating task and function let us understand what these are:
Tasks are procedures where data is passed, processed and output is generated. The same thing can be said for functions but with a few differences that we will mention while answering this Verilog Interview question
Task: It can enable a function as well as a task
Function: Can only enable another function
Task: Can be executed in both zero and non-zero simulation time
Function: Execution is done with zero simulation time
Task: Tasks are permitted to have delay statements, events, or statements controlling time
Function: Cannot have any statements for delay or time control or events.
Task: Invocation may or may not have arguments passed (input or output)
Function: A minimum of one argument is required for invocation
Task: No return value. Although can pass values through input/output statement
Function: Returns single value but cannot use input/output statement
7: What is PLI?
PLI stands for Programming Language Interface. As the name suggests it is an interface to invoke C programming into Verilog. This can be used to perform multiple tasks such as:
- Accessing simulator databases
- Passing data
- Returning or altering the value of objects in the design hierarchy
- Monitoring information alterations for objects in the design hierarchy
8: What is the difference between == and ===?
Some Verilog interview questions might seem to be quite simple although they require an in-depth explanation of topics related to them. This is one such Verilog interview question.
While answering this Verilog interview question, you might require knowledge of the 4-state data type. We will discuss that after the answer.
== is known as the logical equality operator.
This equality operator can only compare 0 and 1. In other words, the logical equality operator can be used for 2-state data types or comparing bits. in case of any operand being other than 0 and 1, it computes it as X.
This cannot compare z and x.
== can be implemented in a digital system (X-NOR gate)
=== is known as the logical equality operator and can compare 4-state data types.
=== can compute 0,1, X, and Z but the output can always be 0 and 1.
This cannot be implemented into digital systems and X and Z are not logics in digital systems
Now let us understand what the 4-state data type is.
4-state data type – these data types have more than just known values i.e. 0 and 1. These data types have high-impedance and unknown values logic, reg, int, and time thus making it a 4-state data type.
9: What are $monitor, $display and $strobe?
These are commands that are used to display on the screen. In other words, this acts as print statements in common programming languages.
These commands are syntactically similar, whereas they have certain differences in functionality or output.
$ Monitor:
Syntax:
$Monitor(<format_string>,<parameter>,<parameter>)
This is similar to the printf statement of C.
$ Monitor will execute and display the parameters every time it is changed, i.e. whenever any of the parameters are changed with a new value displayed when $ Monitor is used.
$ Monitor is used to change signal values where signals can be expressions, variables, or strings.
The format string used in $monitor is the same as C programming:
%d for decimal
%h for hexadecimal
%s for string
%b for binary
%c for character
$display:
Syntax: $display(<list of arguments>);
$display is another task for displaying arguments whereas it also helps in debugging.
While displaying the arguments, $display will always append newline by the end of the string
This task can be invoked multiple times unlike $monitor
$Strobe:
Syntax: similar to $display
$strobe is very similar to $display in the format of displaying its arguments whereas it will only display all its arguments at the end of the current time unit.
10: Describe wire and reg in Verilog?
Wire:
To understand the term wire from Verilog, imagine a physical wire.
It is used to connect different ports. It is to be driven in continuous assignments.
Wires cannot store data.
Wires are used in combinational logic alone and thus are used for combinational circuits.
Reg:
These are not always similar to physical registers, although they can store data like registers. The data is assigned and retained unless a new value is provided.
Reg is not only limited to combinational circuits but can also be implemented in flip-flops and latches.
Reg is used for both sequential and combinational circuits.
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11:What do you understand by continuous assignment?
The continuous assignment is when the inputs of the right-hand side always drive the left-hand side. Then, whenever there is a change in the value of the right-hand side, the whole equation will change.
In a continuous assignment, the target is always a wire driven by the inputs of the right-hand side.
This is used to synthesize combinational logic where the output is determined and driven only based on present and past inputs, as seen in sequential logic.
The continuous assignment functions with an assigned statement or wire declaration.
This assignment can have time controls or delays.
12: What are casex and casez statements used for in Verilog?
This question is related to case statements that we have already come across in a previous Verilog interview question.
We had previously mentioned case statements are enclosed in “case” and “endcase”. This Verilog interview question will answer how “case” can also be written as “casex” or “casez” in different circumstances.
Before going on to answering one of the very frequently asked Verilog interview questions, we should understand the building blocks of casex and casez and that is the case statement.
The case statement is basic and simple which is something required and often more efficient to avoid errors. But rigid statements can often not address many scenarios.
The flow of execution is that the expression for the case is executed only one time and will be sequentially compared with the case items. Whenever a match is found, the corresponding case item statement will be executed.
Casez:
Casez allows matching one case item to multiple case expressions by not taking into consideration the Z bit. It is written as Z or ?.
For a case expression 2’1b0, 2’1b1, 2’1bX, or 2’1bZ will be matched to an item2’1bZ
Whenever comparing the case item and case expression, the don’t care bit (represented by Z or ?) will be ignored completely irrespective of its value.
It is recommended to use ? to the don’t care bit.
Casex:
Casex is absolutely similar to casez with the addition of ‘X’ to the don’t care bit. So here in casex ‘Z’, ‘?’ and ‘X’ bits are ignored in case item and/or case expression while comparing.
13: What is a repeat loop in Verilog?
As we know the base of Verilog is C thus in many of the Verilog interview questions you will find answers that are relatable to the format, idea, and syntax of common programming languages.
As the name suggests, repeat loop has a behavior very similar to loops of common programming languages such as for loop.
Repeat loop will repeatedly run a piece of code for the number of times it is mentioned within the code.
The loop cycle will not be mentioned within the repeating block of code.
Syntax: repeat(<no. of times the loop should run>) <statement to be repeated>
Repeat loop is helpful for avoiding redundancy in line of code.
14: What do you understand about the Sensitivity list?
A sensitivity list is used to identify the change in signal or value and in turn specify when the @always statement is executed.
This is used for Flip-flops and combinational circuits, where changes in signals or inputs change the whole expression and determine the execution flow.
Th @always block is used to keep executing over time without any delays or time controls. Thus, the sensitivity list helps to introduce a time-lapse between the executions and avoids the simulation falling into an infinite loop.
Whenever a change in signal of value is encountered, the sensitivity list triggers the execution of the @always statement and that is how the loop is controlled.
15: Elaborate the concept of freeze, deposit, drive, and force command in Verilog?
Freeze: It is used for unresolved signals. It freezes the value of the signal throughout the simulation. This value cannot be changed.
Deposit: Deposit makes sure to retain a signal value till it is changed/overwritten by the simulation. It acts as value initialization throughout the simulation
Force: It can be used at any time of the simulation to drive signals.
Conclusion:
There can be endless possibilities of questions that can be asked from a vast subject like Verilog. Here are some of the most frequently asked Verilog interview questions, which will hopefully help you with your recruitment process.
It is always preferable to gather as much knowledge of a subject as possible before facing an interview panel. With this set of Verilog interview questions, you are sure to be at a more prepared place for the pattern of Verilog interview questions.
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