Top 25 Verilog Interview Questions You Should Know

Top 25 Verilog Interview Questions You Should Know

Verilog is very important in the VLSI industry, particularly for designing and verifying digital circuits used in modern electronics. Verilog provides a standardized way to describe the functionality and structure of digital circuits. This allows engineers to create a clear and concise representation of the hardware before it’s physically built. Cracking your next Verilog interview can be a breeze with a strong understanding of these essential Verilog interview questions.

 

With Verilog being very important for the VLSI industry, the skills are highly sought after in the job market, particularly for ASIC/FPGA design and verification engineers. Having Verilog expertise can increase your competitiveness and open doors to new opportunities.

 

Enrolling in a comprehensive VLSI course can equip one with the knowledge and expertise needed. mastering these top Verilog interview questions will put you ahead of the competition.

 

The following are a few Verilog questions that can help aspiring engineers to crack their interview questions!

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Verilog Interview Questions

1. What is Verilog and what is it used for?

 

Verilog is a hardware description language (HDL) used to describe the digital logic and behavior of electronic circuits. It operates at a register-transfer level (RTL), meaning it focuses on the functionality of the hardware rather than the exact transistor-level implementation. Verilog is widely used in the design and simulation of digital circuits, including microprocessors, FPGAs (Field-Programmable Gate Arrays), and other integrated circuits (ICs).

 

2. Explain the difference between wire and reg in Verilog.

 

Both wire and reg are used to declare nets and variables in Verilog, but they have distinct purposes:

 

  • wire: Represents a single physical wire in the circuit. Its value is continuously updated based on the logic driving it. Wires are typically used for combinational logic where the output depends on the current inputs.
  • reg: Represents a register that can store a value. Unlike wires, registers hold their value until they are assigned a new one using an always block or an assignment statement. Registers are commonly used for sequential logic where the output depends on both current and past inputs.

3. What are blocking and non-blocking assignments in Verilog?

 

Verilog assignments can be categorized as blocking or non-blocking based on how they affect the simulation flow:

 

  • Blocking assignment (=): This assignment statement halts the simulation at the point of assignment and calculates the new value before proceeding further. Subsequent statements only execute after the blocking assignment is complete.
  • Non-blocking assignment (<=): This assignment schedules the update for the next simulation delta cycle. The simulation continues without interruption, and the new value will be reflected in the next delta cycle.

4. Differentiate between == and === operators in Verilog.

 

Both == and === are used for comparison in Verilog, but they differ in terms of handling unknown values (represented by x):

 

  • == (equality operator): This operator compares only the bit values (0 or 1) and ignores unknown values (x). It returns 1 if both operands are equal (both 0 or 1) and 0 otherwise. Unknown values (x) can lead to unexpected results with ==.
  • === (strict equality operator): This operator performs a bit-wise comparison and considers unknown values (x). It returns 1 only if all bits are identical (including unknown values) and 0 otherwise. === is generally preferred for reliable comparisons where unknown values might be present.

5. How do you write a Verilog code for a D-latch?

 

Here’s an example Verilog code for a D-latch:

 

Verilog

 

module DLatch(

  input D,

  input clk,

  output reg Q

);

 

  always @(posedge clk) begin

    Q <= D;

  end

 

endmodule

 

This code defines a module called DLatch with three ports: D (data input), clk (clock signal), and Q (output latch). The always block is triggered on the positive edge of the clock (posedge clk). Inside the block, the current value of the D input is assigned to the register Q using a non-blocking assignment (<=). This ensures that the latch updates its output value on the next clock cycle.

 

6. Differentiate between Verilog and VHDL?

 

AspectVerilogVHDL
SyntaxSimilar to C programming language.Closer to Ada programming language.
UsageCommon in the United States and Asia.More prevalent in Europe.
ConcisenessTends to be more concise.More verbose.
LibrariesExtensive library of predefined primitives.Rich set of built-in data types and standard libraries.
PortabilityRelatively more portable.Can also be portable but may vary more between tools.
EcosystemLarger ecosystem of tools and community supportStrong presence in academia and defense industries.
Time to learnPerceived as easier to learn.May have a steeper learning curve.

 

7. What is a continuous assignment?

A continuous assignment in VHDL defines how a circuit’s output depends on its current inputs. It uses the ‘assign’ keyword and continuously evaluates an expression whenever an input changes. This is ideal for describing combinational logic (e.g., adders) where the output relies directly on the current input combination.

8. Explain the terms $monitor, $display and $strobe.

$monitor, $display, and $strobe are all VHDL system tasks for printing during simulation. Here’s a quick breakdown:

 

  • $monitor: Continuously prints info and signal values whenever a monitored signal changes. Great for observing ongoing behavior.
  • $display: Prints a message only once when the statement is encountered. Ideal for specific points like initialization.
  • $strobe: Prints a message once at the end of the current simulation time step. Useful for consolidated info after each cycle.

9. What is PLI in Verilog?

 

PLI (Programming Language Interface) in Verilog acts like a bridge. It lets you call C/C++ functions directly from your Verilog code. This expands Verilog’s abilities for complex tasks like:

 

  • Interfacing with external hardware beyond Verilog’s built-in support.
  • Performing advanced calculations more efficiently than native Verilog.
  • Creating custom debugging or analysis tools tailored to your needs.

10. What is a sensitivity list?

A sensitivity list in Verilog is like a watchlist for an always block. It tells the simulator which signal changes trigger a re-evaluation of the block’s statements, ensuring the code runs only when necessary.

11. In Verilog, which will be updated first? Variable and Signal?

In Verilog, a signal will always be updated first compared to a variable within the same simulation delta cycle. This behavior stems from the fundamental differences between signals and variables:

 

Signals: Represent physical wires in the circuit. Assignments using the <= operator schedule the update for the next delta cycle. The actual value change occurs after all concurrent evaluations within that delta cycle are complete.

Variables: Represent temporary storage locations within a process. Assignments using the = operator update the variable’s value immediately.

12. What does timescale 1 Ns/1 Ps mean?

A timescale declaration in Verilog (timescale) defines two key aspects of your simulation:

 

  • Time Unit: The 1ns in this case specifies the basic unit of time used during simulation. This means that any time value you use in your code (delays, event occurrences) will be interpreted relative to nanoseconds (ns).
  • Time Precision: The 1ps in this case indicates the smallest time increment that can be represented during simulation. This means that Verilog can distinguish between delays or events that differ by as little as 1 picosecond (ps).

 

13. Explain the steps involved in writing an FSM code

  • Define states and transitions
  • Choose representation
  • Define Verilog code structure
  • Implement state logic
  • Implement output logic
  • Verification

14. What is transport delay?

In Verilog, transport delay is a modeling concept used to represent the time it takes for a signal to propagate through a wire or gate within a digital circuit. It essentially introduces a latency between the change in an input signal and the corresponding change appearing at the output. Verilog uses the # symbol followed by a time value (e.g., #5ns) within an assignment statement to model transport delay. This delays the assignment of a new value to the target signal by the specified time.

15. What is inertial delay?

Inertial delay is a more advanced concept compared to transport delay for modeling signal propagation. Inertial delay considers the stability of input signals before propagating the change to the output. It ensures the new output value reflects a stable input for a certain duration. Modeling inertial delay can be more complex compared to transport delay.

16. Explain blocking and non-blocking assignments

In Verilog, blocking assignments (=) and non-blocking assignments (<=) are used within always blocks. Blocking assignments evaluate the right-hand side and update the left-hand side immediately, affecting subsequent statements. Non-blocking assignments schedule updates to occur at the end of the current delta cycle, allowing multiple updates to be applied simultaneously. Blocking is used for sequential logic, and non-blocking is used for parallel updates to signals.

17. Explain the concepts of freeze and drive

Freeze and drive are concepts related to forcing signal values during Verilog simulation, but they’re not built-in Verilog commands. Freeze typically refers to forcing a signal to a specific value and keeping it constant throughout the simulation. The value remains frozen until explicitly released. Drive might imply forcing a signal to a specific value but potentially allowing it to change later based on the simulation flow. 

18. Explain the concept of concurrency in Verilog

Verilog is concurrent by nature. This means multiple always blocks, continuous assignments, and procedural blocks can execute seemingly “at the same time” within a simulation delta cycle. The simulator manages the order, ensuring proper evaluation based on dependencies. This allows modeling of parallel hardware behavior efficiently.

19. How do you handle asynchronous resets in Verilog designs?

To handle asynchronous resets in Verilog designs:

 

  • Include the reset signal in the sensitivity list of your always block (@(posedge clk or posedge reset)).
  • Assign a low value (usually 0) to the reset signal’s active state.
  • Within the always block, use an if statement to prioritize the reset signal. When the reset is active, set the desired initial state for your logic regardless of the clock or other inputs.
  • This ensures the reset takes effect immediately, overriding any ongoing operations.

20. Explain the differences between Verilog and SystemVerilog.

 

Verilog is primarily a Hardware Description Langauge (HDL) for describing the structure and behavior of digital circuits. It works excellent for design implementation. SystemVerilog is a superset of Verilog, offering both HDL and Hardware Verification Language (HVL) capabilities. It expands on Verilog with features for advanced verification including object-oriented programming constructs, and advanced constructs for testbench development. 

21. How can you implement a memory module (e.g., RAM) in Verilog?

Here’s how to implement a simple RAM in Verilog (5 lines):

 

Verilog:

 

module RAM #(parameter ADDRESS_WIDTH=4, DATA_WIDTH=8) (

  input clk,

  input we,  // Write enable

  input [ADDRESS_WIDTH-1:0] addr,

  input [DATA_WIDTH-1:0] data_in,

  output reg [DATA_WIDTH-1:0] data_out

);

 

  reg [DATA_WIDTH-1:0] mem [2**ADDRESS_WIDTH-1:0];

 

  always @(posedge clk) begin

    if (we) mem[addr] <= data_in;

    data_out <= mem[addr];

  end

endmodule

22. Describe how to perform verification coverage analysis for a Verilog design.

Verilog verification coverage analysis involves:

 

  • Defining coverage points: Using SystemVerilog covergroups (or similar constructs), you specify signal value combinations or conditions you want to test (e.g., all control signal values, FSM state transitions).
  • Collecting coverage data: During simulation, the testbench tracks how often each coverage point is encountered, indicating which design parts have been exercised.
  • Analyzing coverage: Coverage reports show uncovered sections (i.e., points not reached), helping identify areas needing more test cases in your testbench.

23. What are generate blocks used for in Verilog?

Generate blocks in Verilog offer two main functionalities:

 

  • Conditional Code Inclusion: Based on parameters or logic, you can include or exclude specific code blocks within the generate statement. This allows creating variations of a module based on configuration settings.
  • Loop-based Hardware Replication: Using loops (for or while) within generate blocks, you can efficiently instantiate repetitive hardware structures like memory arrays, adders, or decoders based on a parameter defining the number of elements.

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

Verilog’s casex and casez statements are used for bit-wise comparisons within conditional blocks. They allow matching patterns with “don’t care” conditions for unknown or unspecified bits during signal value comparisons.

  • casex: Treats X (unknown) as a wildcard that can match any value (0 or 1).
  • casez: Treats both X and Z (high-impedance) as wildcards for matching.

25. How does a Verilog loop work?

Verilog loops provide a way to execute a block of code multiple times. This statement is executed only once at the beginning of the loop. It’s typically used to set up a loop counter variable. This statement is executed after each iteration of the loop body. It’s commonly used to increment or decrement the loop counter to control the number of repetitions.

 

The above Verilog interview questions will be very helpful in preparing for technical Verilog interview questions, as they cover a wide range of essential topics and concepts relevant to digital design and hardware description languages. 

To learn more about Verilog interview questions and other important topics in VLSI take a look at VLSI online courses offered by ChipEdge, an established VLSI training institute in Bangalore.

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