Verilog Interview Set 2

What are HDL simulators ?

HDL (Hardware Description Language) simulators are software tools used in the design and testing of digital hardware. They simulate the behavior of digital circuits written in hardware description languages such as Verilog and VHDL. HDL simulators allow designers to test the functionality, timing, and performance of their designs before they are implemented in physical hardware. They are essential tools in the design and verification of complex digital systems such as microprocessors, FPGAs, and ASICs. HDL simulators come in different forms, including standalone software tools, integrated development environments (IDEs), and cloud-based platforms.

What do you understand by continuous assignment ?

In Verilog, a continuous assignment statement allows the designer to assign a value to a signal or a wire continuously as long as the input changes. Unlike procedural assignments, which assign values to signals triggered by an event or a condition, continuous assignments are always active and assign values to signals based on their inputs.

A continuous assignment statement in Verilog is represented by the keyword assign followed by the expression that describes the signal. A continuous assignment is typically used with combinational logic circuits where the output depends solely on the input.


// Assigns the logical OR of (a and b) and c to the signal "out"
assign out = (a & b) | c;

// Assigns the logical AND of reset_n and enable_i to "enable"
assign enable = (reset_n & enable_i);

Can `define be used for text substitution through variable instead of literal substitution ?

Unfortunately, no, the `define directive in Verilog does not allow for text substitution through variables.

The `define directive is used in Verilog to define a macro, which is a piece of code that is replaced with a predefined value or string of text during compilation. The `define macro can be used for literal substitutions only, where the pre-defined text is replaced with the actual text value defined.

For example, the following code defines a macro named "DATA_WIDTH" with the value "32":


`define DATA_WIDTH 32

This macro can be used in the Verilog source code to specify a data width of 32 bits, as shown below:


wire [`DATA_WIDTH-1:0] data_bus;

During compilation, the `define macro is replaced with the pre-defined value "32", resulting in the following code:


wire [32-1:0] data_bus;

However, the `define directive does not allow for variable substitution, and it cannot be used to replace text with variables. Therefore, the pre-defined text value cannot be replaced with variables during compilation.

Also read on Verilog `ifdef Conditional Compilation.

What is the difference between $setup and $hold ?

$setup and $hold are timing checks used in Verilog to ensure that the input signals are stable at the input of the flip-flop during the setup and hold time windows of the flip-flop.

$setup is a timing check used to ensure that the input signal to a flip-flop changes sufficiently ahead of the clock rising edge so that the signal can settle stable by the time the clock edge arrives. $setup specifies the minimum time required for the input signal to reach a stable value before the active edge of the clock. Violation of the setup time can cause a race condition, metastability, and unpredictable behavior of the flip-flop.

$hold is a timing check used to ensure that the input signal to a flip-flop does not change while the clock is active (high or low) and the input is being sampled by the flip-flop. $hold specifies the minimum time that the input signal should be held stable after the active edge of the clock. Violation of the hold time can cause a data loss, timing violations, and unpredictable behavior of the flip-flop.

How can you generate a sine wave using the Verilog coding style ?

Generating a sine wave using Verilog requires performing some mathematical operations using the Verilog logic. One way to generate a sine wave is by using a lookup table of precomputed sine values and updating the index into the table using a sine frequency and the sampling frequency.

Here is a Verilog code that generates a sine wave with a 16-bit amplitude at 440Hz using a 8-bit lookup table:


module sine_wave_generator(
    input logic clk,
    input logic reset,
    output logic signed [15:0] sine
);
    // lookup table containing 256 precomputed sine values
    logic signed [15:0] sin_table [0:255];
    // initialize the lookup table with sine values
    initial begin
        integer i;
        for (i = 0; i < 256; i++) begin
            sin_table[i] = $signed(32767 * $sin(2 * $pi * i / 256));
        end
    end
    // sine wave generation
    logic [15:0] index;
    logic [7:0] phase_acc;
    logic [7:0] phase_inc = (440 / 8000) * 256;
    always @(posedge clk) begin
        if (reset) begin
            index <= 0;
            phase_acc <= 0;
        end else if (phase_acc >= 255) begin
            phase_acc <= 0;
            index <= index + 1;
        end else begin
            phase_acc <= phase_acc + phase_inc;
        end
        // output the sine value from the lookup table
        sine <= sin_table[index[7:0]];
    end
endmodule

In the above code, the sine wave is generated by updating the phase accumulator based on the desired frequency and the system clock frequency. The phase accumulator is used as an index into the lookup table, which contains precomputed sine values scaled to the amplitude of 32767. By outputting the sine value from the lookup table, we get a sine wave output of the desired frequency and amplitude.

What do you understand by casex and casez statements in Verilog ?

The casez and casex statements are conditional statements in Verilog that compare a case expression against a set of possible matching patterns. Both statements are used to simplify conditional statements by reducing the number of conditional decisions to be made.

The casex statement matches the case expression with the patterns that have X in the matching bits. The x bits are automatically treated as don't care bits, and can match either 0 or 1. The remaining bits are treated as exact match bits. The casex statement is effective when we have to deal with multi-bit signals that contain unknown or high impedance values where the majority of the bits are either known or known to be zero.

Here's an example of a casez statement in Verilog:


reg [3:0] my_input;
reg [7:0] my_output;
always @ (my_input) begin
    casez(my_input)
        4'b0000? :  my_output = 8'd0;
        4'b0001? :  my_output = 8'd1;
        4'b10??0 :  my_output = 8'd2;
        default  :  my_output = 8'hFF;
    endcase
end

The casez statement above matches the four possible patterns of the input signal my_input . The ? symbol in my_input means the value is don't-care, i.e., the input signal can be either 0 or 1 in that position. If the my_input signal matches any of the patterns specified, then the corresponding output value is assigned to the my_output signal. If my_input does not match any of the defined patterns, the default state is executed.

On the other hand, the case statement uses exact matching logic to compare the case expression to each of the specified patterns. Any bits that are not defined in the specified pattern must match a zero. If the exact matching bits match, the output value assigned to the specified pattern is assigned to the output signal.

Here's an example of a simple case statement in Verilog:


reg [3:0] my_input;
reg [7:0] my_output;
always @ (my_input) begin
    case(my_input)
        4'b0000 : my_output = 8'd0;
        4'b0001 : my_output = 8'd1;
        4'b1010 : my_output = 8'd2;
        default:  my_output = 8'hFF;
    endcase
end

What is duty cycle ?

Duty cycle is a percentage ratio of the time that a signal is ON compared to the total period of the signal. The duty cycle of a periodic waveform is defined as:

Duty cycle = (time the signal is ON / total period of the signal) * 100

A signal with 50% duty cycle means that the signal is ON for half of the total period and OFF for half of the total period.

The concept of duty cycle is used in many electronic devices and circuits, such as pulse width modulation (PWM), signals from oscillators, and digital communication systems. In PWM, the duty cycle of a square wave signal determines the amount of energy that is being delivered to a load, such as a motor or an LED. In digital communication systems, the duty cycle of a signal affects its power consumption and the signal quality.

The duty cycle is an important parameter to consider when designing electronic systems or circuits, as it determines the characteristics and behaviour of the signal. A low duty cycle signal is one that is mostly OFF and has a long period of time between pulses, while a high duty cycle signal is one that is mostly ON and has a shorter period of time between pulses. Understanding the duty cycle of a signal can assist designers to optimize the performance and power consumption of their designs.

What is verilog $random ?

In Verilog, the random system task generates a random value. It is used to simulate unpredictable values in a Verilog testbench. You can use the $random task to generate a random value for a signal every time the module is executed, thereby ensuring that the design is tested with different scenarios.

The $random system task returns a 32-bit signed integer. The range of values that the $random task can generate is dependent on the simulator and the seed value used. The seed value is an initial value that is used by the random number generator to produce the sequence of random numbers.

Here's an example of how to use the $random task in Verilog:


module testbench;
  reg [7:0] data;
              
  initial begin
    $display("Random numbers: ");
              
    // Generate 32b random numbers ten times and display 
    for(int i = 0; i < 10; i++) begin
      $display("%d: %b", i, $random);
    end
  end
endmodule

It is worth noting that the $random task generates random numbers based on the seed that is set. Hence, to ensure that the simulations are reproducible, it is a common practice to initialize or set/note the seed value for $random sequence generator using simulator options.

What are different types of delay control ?

In Verilog, delay control is used to simulate timing delays within a module or at the module interface. There are four types of delay control in Verilog:

The #delay delay control is used to add a specified delay to a procedural block. This delay control specifies a delay in timescale units.
The @posedge delay control is used to trigger a procedural block on the positive edge of a clock signal. The procedural block executes after a delay determined by the simulation scheduler.
The @negedge delay control is used to trigger a procedural block on the negative edge of a clock signal.
The wait delay control is used to pause a module's execution for a specified delay value. This delay is specified using timescale units.

What are parallel threads in Verilog ?

In Verilog, parallel threads refer to code blocks within a module where multiple processes execute concurrently. Each process is a self-contained block of code that runs independently of the other processes. This includes initial and always blocks.

Parallel threads are also created using the fork...join construct in Verilog. The fork statement can be used to start multiple parallel threads of execution, while the join statement can be used to join the threads back together.

Here's an example code snippet that demonstrates the use of fork and join to create parallel threads:


module parallel_threads;
  reg a, b, c, d;

  always @(a or b) begin
    fork
      if (a) begin  // Thread #1
        c = 1;
        #10ns;
        c = 0;
      end

      if (b) begin 	// Thread #2
        d = 1;
        #20ns;
        d = 0;
      end
    join
  end
endmodule

In this example, whenever input signals a or b change, a new parallel thread is initiated. Each thread within the fork...join block runs independently and concurrently. In this case, the execution of the blocks of code inside the fork statement would overlap with each other, allowing for independent processing of the if statements.

Parallel threading is particularly useful in testbenches, where multiple processes can be executed concurrently, simulating different parts of a design module.