Verilog Interview Questions and Answers
This section covers frequently asked Verilog interview questions.
1. What is Verilog?
Verilog is a Hardware Description Language (HDL) used to model and simulate digital systems (like integrated circuits). It's used for designing and verifying hardware at various levels of abstraction.
2. Who Invented Verilog?
Prabhu Goel, Phil Moorby, Chi-Lai Huang, and Douglas Warmke are credited with Verilog's initial development.
3. What is VHDL?
VHDL (VHSIC Hardware Description Language) is another HDL used for describing digital and mixed-signal systems. It's known for its strong typing and formal approach.
4. VHDL Variants.
- VHDL-1987
- VHDL-1993
5. Usages of VHDL.
- Hardware modeling
- Hardware verification (using testbenches)
- Design entry for EDA (Electronic Design Automation) tools
6. Verilog vs. VHDL.
Both are HDLs, but they differ in syntax, typing, and approach. Verilog is more C-like and concise. VHDL is more formal and verbose.
7. Verilog vs. VHDL (Detailed Comparison).
| Feature | Verilog | VHDL |
|---|---|---|
| Origin | 1984 | 1980 |
| Syntax | C-like | Ada/Pascal-like |
| Typing | Weakly typed | Strongly typed |
| Verbosity | More concise | More verbose |
8. HDL Simulators.
HDL simulators are software tools used to simulate the behavior of hardware designs described using HDLs (like Verilog or VHDL).
9. Blocking vs. Non-blocking Assignments.
Blocking assignments (=) execute sequentially. Non-blocking assignments (<=) schedule assignments for the next time step, allowing for parallel simulation.
10. Verilog case Statements.
- Full case: Every possible case expression is handled (including a default case).
- Parallel case: Only one case item can match per case expression.
11. Tasks vs. Functions in Verilog.
| Feature | Task | Function |
|---|---|---|
| Return Value | No return value | Returns a single value |
| Timing Controls | Supported | Not supported |
| Input/Output | Input/output arguments | Input arguments only |
12. PLI (Programming Language Interface).
PLI lets you integrate Verilog with other programming languages (like C) to extend simulator capabilities.
13. Sensitivity Lists.
In Verilog's always blocks, the sensitivity list specifies which signals trigger the block's execution.
14. == vs. === in Verilog.
| Operator | Comparison |
|---|---|
== |
Logical equality (considers 'x' and 'z') |
=== |
Case equality (does not consider 'x' and 'z') |
15. $monitor, $display, and $strobe.
These are Verilog system tasks for displaying simulation output. $monitor displays continuously; $display and $strobe display once per invocation.
16. $monitor vs. $display.
| Task | Execution |
|---|---|
$monitor |
Continuous monitoring of signals |
$display |
Displays output only when called |
17. wire vs. reg.
| Data Type | Usage |
|---|---|
wire |
Combinational logic; always reflects the current input |
reg |
Sequential or combinational logic; can store a value across clock cycles |
18. Executing Blocking and Non-blocking Assignments.
Blocking assignments (=) execute sequentially. Non-blocking assignments (<=) are scheduled for the next simulation time step, enabling parallel execution.
19. Continuous Assignments.
Continuous assignments (using the assign keyword) model combinational logic in Verilog. The output is always a direct function of the current inputs. They are typically used with `wire` data types.
20. Full Case vs. Parallel Case Statements.
- Full case: All possible input values are handled (including a default case).
- Parallel case: Only one case can match per input value. Overlapping cases are not allowed in a parallel case statement.
21. Transport Delay vs. Inertial Delay.
Transport delay models signal propagation delays in wires. Inertial delay models delays within logic gates, potentially ignoring short pulses.
22. Writing FSM (Finite State Machine) Code in Verilog.
(This section would detail different ways to implement FSMs in Verilog using various coding styles, typically involving `always` blocks with case statements to define state transitions.)
23. Sensitivity Lists in Combinational Circuits.
In a purely combinational circuit (where output depends only on current inputs), the sensitivity list of an `always` block *must* list all the inputs to ensure correct simulation. This prevents the possibility of a timing error between the actual circuit behaviour and the simulated behaviour.
24. Variable vs. Signal Update Order.
Signals are updated first, followed by variables.
25. force, drive, deposit, and freeze Commands.
These commands manipulate signals during simulation:
force: Sets a signal value at a specific time.drive: Sets a signal, but allows it to be overridden by the simulation.deposit: Sets a signal value, which remains until explicitly changed.freeze: Sets a signal to a fixed value for the entire simulation.
26. Swapping Registers in Verilog.
(This section would provide code examples demonstrating register swapping with and without a temporary register.)
27. timescale Directive.
The timescale directive specifies the units and precision for time in a Verilog simulation.
28. $setup and $hold System Tasks.
$setup and $hold check setup and hold time violations in sequential circuits.
29. Generating a Sine Wave in Verilog.
(This section would discuss algorithms or methods for generating sine waves in Verilog, such as the CORDIC algorithm.)
30. casex and casez Statements.
casex and casez are case statements that treat 'x' (unknown) and 'z' (high-impedance) bits as don't cares.
31. repeat Loop.
The repeat loop in Verilog executes a block of code a fixed number of times.