Verilog Design Interview Questions and Answers

Verilog is a hardware description language (HDL) used to model, design, and simulate digital systems such as integrated circuits (ICs), microprocessors, and Field Programmable Gate Arrays (FPGAs). It allows engineers to describe the structure, behavior, and functionality of digital hardware at various levels of abstraction, including:

• Behavioral: High-level, algorithmic descriptions.
• Register Transfer Level (RTL): Focused on data flow between registers and the logic operations performed.
• Gate-Level: Detailed descriptions of how gates are connected.
• Switch-Level: Lowest level, describing transistors and their interconnections.

Verilog is widely used in electronic design automation (EDA) and is an IEEE standard (IEEE 1364).

Why is Verilog Used?

1. Hardware Design:

   • Verilog is used to design digital systems such as processors, memory, and custom hardware blocks by describing their functionality and structure.

2. Simulation and Debugging:

   • It allows engineers to simulate and verify digital designs before physical hardware is manufactured. This helps detect and fix errors early in the design cycle.

3. Synthesis to Hardware:

   • Verilog code can be synthesized into a netlist (a representation of logic gates and their connections) that can be implemented on FPGAs or used to fabricate ICs.

4. Support for Different Abstractions:

   • It supports different abstraction levels, making it easier to transition from high-level functional design to low-level implementation details.

5. Standardized Language:

   • As an industry-standard language, Verilog ensures compatibility and interoperability between tools and vendors.

6. Portability and Reusability:

   • Designs written in Verilog are portable across different tools and platforms. Components can also be reused in multiple projects, saving time and effort.

7. Extensive Tool Support:

   • Verilog is supported by most EDA tools for simulation, synthesis, and layout, such as ModelSim, Synopsys, and Cadence.

8. Ease of Use:

   • Compared to other HDLs like VHDL, Verilog's syntax is simpler and more C-like, making it easier for engineers familiar with software programming to learn and use.

9. Testbench Creation:

   • Verilog enables the creation of testbenches, which simulate and verify the behavior of a hardware design by applying stimulus and observing the output.

10. Timing and Delay Modeling:

    • Verilog allows precise timing and delay modeling, essential for designing high-speed circuits.

Key Features of Verilog :

• Concurrent Execution: Models the inherent parallelism of hardware.
• Modules: Supports modular design for large systems.
• Support for Bit-Level Operations: Ideal for digital logic design.
• Event-Driven Simulation: Allows simulation based on events and time changes.
• Built-in Primitives: Includes predefined gate-level primitives like AND, OR, and XOR.

Applications of Verilog :

• ASIC (Application-Specific Integrated Circuit) Design
• FPGA Design
• Processor Design
• Digital Signal Processing (DSP) Applications
• Memory Design
• Testbench and Verification (e.g., functional testing)

Verilog supports several levels of abstraction that allow engineers to design and describe hardware systems at varying degrees of detail. These levels range from high-level behavioral descriptions to low-level physical implementation. Each level serves a specific purpose in the hardware design and verification process.

1. Behavioral Level

• Description: At the behavioral level, the focus is on what the system does, not how it is implemented. This is the highest level of abstraction in Verilog.
• Purpose: Used for describing the algorithm or functionality of the design.
• Key Constructs:
  • Procedural blocks: initial and always
  • Control statements: if, case, for, while
  • Task and function declarations

always @(posedge clk) begin
    if (reset)
        count <= 0;
    else
        count <= count + 1;
end
?

• Applications:
  • Used for initial design exploration and functional verification.
  • Writing testbenches to simulate and verify hardware behavior.

2. Register Transfer Level (RTL)

• Description: RTL describes how data flows between registers and the operations performed on the data at each clock cycle. This is the most commonly used level for hardware design.
• Purpose: Focuses on the functional design of digital circuits while being specific enough for synthesis tools to generate hardware.
• Key Constructs:
  • Always blocks
  • Continuous assignments (assign)
  • Flip-flops, registers, and combinational logic

always @(posedge clk or posedge reset) begin
    if (reset)
        q <= 0;
    else
        q <= d;
end
?

• Applications:
  • Describing hardware for synthesis.
  • Designing finite state machines, arithmetic units, and control logic.

3. Gate Level :

• Description: At the gate level, the design is described in terms of logic gates (AND, OR, NOT, etc.) and their interconnections. This is closer to the physical hardware implementation.
• Purpose: Provides a lower-level representation of the circuit for synthesis and verification.
• Key Constructs:
  • Predefined gate primitives: and, or, nand, nor, xor, xnor, buf, not

and G1 (out, a, b);
or G2 (out, c, d);
not G3 (out_inv, out);?

• Applications:
  • Low-level circuit design.
  • Post-synthesis netlist generation for simulation and debugging.

4. Switch Level :

• Description: The switch level is the lowest abstraction level in Verilog. It describes circuits in terms of transistors (e.g., MOSFETs) and their connections.
• Purpose: Used to model the physical layout of the design and perform detailed analysis of circuit behavior.
• Key Constructs:
  • Primitive switches: nmos, pmos, cmos

nmos (out, in, control);
pmos (out, in, control);?

• Applications:
  • Transistor-level design and simulation.
  • Modeling custom logic cells in ASICs or full-custom ICs.

In Verilog, wire and reg are fundamental data types with distinct characteristics:

Wire

• Represents a physical connection: Think of it as a simple wire connecting two points in a circuit.
• No storage capability: A wire cannot store a value on its own. It simply transmits the value it receives from its source.
• Driven by continuous assignments: wires are typically driven by continuous assignments (assign) or as outputs of modules.

Reg

• Represents a storage element: A reg can hold a value and retain it until a new value is assigned.
• Used within procedural blocks: regs are primarily used within procedural blocks (like always blocks) to model registers, flip-flops, and other memory elements.
• Can be used for both combinational and sequential logic: While often associated with sequential logic, reg can also be used for combinational logic within procedural blocks.

// Wire declaration
wire a, b, c; 
assign c = a & b; 

// Reg declaration
reg d; 
always @ (posedge clk) begin
    d <= a | b; 
end

In this example :

• a, b, and c are declared as wires. The assign statement defines a continuous assignment, meaning the value of c is always updated based on the values of a and b.
• d is declared as a reg. The always block defines a sequential assignment, meaning the value of d is updated only on the rising edge of the clock signal (posedge clk).

By understanding the distinctions between wire and reg, you can effectively model various hardware components and behaviors in Verilog.

In Verilog, initial and always blocks are both procedural constructs used to describe the behavior of digital circuits. However, they have key differences in their execution:

initial Block

• Executes only once: The statements within an initial block are executed only at the beginning of the simulation, and then the block terminates.
• Typically used for:
  • Initialization: Setting initial values for registers and variables.
  • Testbench stimulus generation: Generating test vectors and applying them to the design under test.
• Not synthesizable (generally): initial blocks are primarily used in testbenches and are not typically synthesizable into hardware.

always Block

• Executes repeatedly: The statements within an always block are executed repeatedly, based on the specified sensitivity list or timing control.
• Used for:
  • Modeling sequential logic: Implementing registers, flip-flops, and other sequential circuits.
  • Modeling combinational logic: Describing combinational circuits with appropriate sensitivity lists.
• Synthesizable: always blocks are essential for synthesizing hardware designs.

// Initial block to set initial value of register
initial begin
  reg_a = 1'b0; 
end

// Always block for a D flip-flop
always @ (posedge clk) begin
  if (reset) 
    reg_b <= 1'b0; 
  else
    reg_b <= data; 
end

In this example :

• The initial block sets the initial value of reg_a to 0 at the start of the simulation.
• The always block describes the behavior of a D flip-flop, where the value of reg_b is updated on the rising edge of the clock signal (posedge clk).

By understanding the differences between initial and always blocks, you can effectively model and simulate digital circuits in Verilog.

In Verilog, blocking and non-blocking assignments are two distinct ways to assign values to variables within procedural blocks (like always blocks). They have significant implications for how your Verilog code simulates and synthesizes.

Blocking Assignment (=)

• Sequential Execution: Blocking assignments are executed sequentially, one after another.
• Immediate Update: The value on the right-hand side of the assignment is immediately assigned to the variable on the left-hand side.
• Example :

• reg a, b, c; 
  
  always @ (posedge clk) begin
    a = 1; // Assign 1 to a
    b = a; // Assign the current value of a (which is now 1) to b
    c = b; // Assign the current value of b (which is 1) to c
  end

  In this example, a is assigned 1 first. Then, b is assigned the current value of a, which is 1. Finally, c is assigned the current value of b, which is also 1.

Non-Blocking Assignment (<=)

• Concurrent Execution: Non-blocking assignments are scheduled to happen concurrently at the end of the current time step.
• Delayed Update: The values on the right-hand side of all non-blocking assignments are evaluated first. Then, at the end of the time step, the values are assigned to the corresponding variables simultaneously.
• Example :

• reg a, b, c; 
  
  always @ (posedge clk) begin
    a <= 1; 
    b <= a; 
    c <= b; 
  end

  In this example :

• At the beginning of the time step, the right-hand sides of all assignments are evaluated.
  • a is scheduled to be assigned 1.
  • b is scheduled to be assigned the current value of a (which is its previous value).
  • c is scheduled to be assigned the current value of b (which is its previous value).
• At the end of the time step, all scheduled assignments are performed simultaneously.

In Verilog, posedge and negedge are keywords used within always blocks to specify the edge-triggered events that should trigger the execution of the block's statements.

• posedge:

  • Indicates the rising edge of a signal.
  • The always block will execute only when the specified signal transitions from 0 to 1.

• negedge:

  • Indicates the falling edge of a signal.
  • The always block will execute only when the specified signal transitions from 1 to 0.

Example :

always @ (posedge clk) begin
  // Code to be executed on the rising edge of the clock signal
end

always @ (negedge rst_n) begin
  // Code to be executed on the falling edge of the reset signal
end

Key Points :

• posedge and negedge are crucial for designing synchronous circuits where events are triggered by specific transitions of a clock signal.
• They are used in conjunction with always blocks to create flip-flops, registers, and other sequential logic elements.
• The choice between posedge and negedge depends on the specific requirements of your design and the desired behavior of the circuit.

By understanding the use of posedge and negedge, you can effectively model and design digital circuits with precise timing and synchronization in Verilog.

module DFF (
    input clk,
    input d,
    output reg q
);
always @(posedge clk) begin
    q <= d;
end
endmodule

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

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

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

module my_module #(parameter WIDTH = 8)(input [WIDTH-1:0] data_in, output [WIDTH-1:0] data_out);
  assign data_out = data_in;
endmodule?

module synchronizer (
   input clk_dest, reset,
   input signal_in,
   output reg signal_out
);
reg meta;
always @(posedge clk_dest or posedge reset) begin
   if (reset) begin
       meta <= 1'b0;
       signal_out <= 1'b0;
   end else begin
       meta <= signal_in;
       signal_out <= meta;
   end
end
endmodule?

module handshake_cdc (
   input clk_src, clk_dest, reset,
   input [7:0] data_in,
   input send,
   output reg [7:0] data_out,
   output reg received
);
reg [7:0] data_reg;
reg send_toggle, recv_toggle;
wire send_sync, recv_sync;
// Source domain
always @(posedge clk_src or posedge reset) begin
   if (reset) begin
       data_reg <= 8'b0;
       send_toggle <= 1'b0;
   end else if (send) begin
       data_reg <= data_in;
       send_toggle <= ~send_toggle;
   end
end
// Destination domain
always @(posedge clk_dest or posedge reset) begin
   if (reset) begin
       data_out <= 8'b0;
       received <= 1'b0;
       recv_toggle <= 1'b0;
   end else if (send_sync != recv_toggle) begin
       data_out <= data_reg;
       received <= 1'b1;
       recv_toggle <= send_sync;
   end else begin
       received <= 1'b0;
   end
end
// Synchronizers
synchronizer sync_send (.clk_dest(clk_dest), .reset(reset), .signal_in(send_toggle), .signal_out(send_sync));
synchronizer sync_recv (.clk_dest(clk_src), .reset(reset), .signal_in(recv_toggle), .signal_out(recv_sync));
endmodule?

module mux (
    input a, b, sel,
    output y
);
assign y = sel ? b : a;
endmodule

module bidirectional (
    inout wire bus,
    input enable,
    input data_in,
    output data_out
);
assign bus = enable ? data_in : 1'bz;
assign data_out = bus;
endmodule?

module testbench;
reg clk, d;
wire q;

// Instantiate the DUT
DFF dut (.clk(clk), .d(d), .q(q));

initial begin
    clk = 0; d = 0;
    #5 d = 1;
    #10 d = 0;
    #20 $finish;
end

always #5 clk = ~clk; // Clock generation
endmodule

initial begin
    $dumpfile("waveform.vcd");
    $dumpvars(0, testbench);
end?