Display Tasks in Verilog Programming Language

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"noreferrer noopener">Verilog Programming Language. Display tasks are built-in procedures that allow you to output information during simulation, making it easier to debug and monitor the behavior of your digital designs. These tasks play a crucial role in testing and validating your code by printing values, expressions, and system variables. In this article, we’ll explore some examples of display tasks and how they can help you improve the accuracy and reliability of your Verilog simulations.

What are Display Tasks in Verilog Programming Language?

In Verilog, display tasks are built-in system functions that provide a way to output information during simulation. These tasks are primarily used to print variables, signals, and messages to the console or log files, which makes them essential tools for debugging and verifying the behavior of your digital circuits.

Display tasks are especially helpful in testbenches, where they allow you to monitor how your design is operating during simulation. They can output information at specific simulation times, and they come in several forms depending on how the output needs to be formatted.

Argument	Description
%h, %H	Display in hexadecimal format
%d, %D	Display in decimal format
%b, %B	Display in binary format
%o or %O	Display octal format
%m, %M	Display hierarchical name
%s, %S	Display as a string
%t, %T	Display in time format
%f, %F	Display ‘real’ in a decimal format
%e, %E	Display ‘real’ in an exponential format

Types of Display Tasks in Verilog

1. $display

• This is the most commonly used display task. It prints a message or the value of a variable to the console.
• Every time $display is called, it prints the output and moves the cursor to the next line (like a newline in typical programming languages).

$display("The value of A is %d", A);

This would print the current value of variable A and then move to the next line.

2. $monitor

• Unlike $display, $monitor continuously monitors changes to specified signals. Whenever any of the signals in the list change, it automatically prints the new values.
• It is useful for tracking variables throughout the simulation without explicitly calling the display task at every step.

$monitor("At time %0t, A = %d, B = %d", $time, A, B);

This will print the values of A and B every time they change, along with the simulation time.

3. $strobe

The $strobe task is similar to $display, but it prints values only after all other statements in the current time step have been evaluated. This ensures that it outputs the final stable values of signals.

$strobe("Final value of A at this time step: %d", A);

This will print the value of A after all updates have taken place for that time step.

4. $write

This task behaves like $display, but without the automatic newline. It can be useful when you need to output multiple values on the same line.

$write("A = %d, ", A);
$write("B = %d", B);

This will print the values of A and B on the same line.

5. $fdisplay

This is similar to $display, but it allows you to direct the output to a file instead of the console.

integer file;
file = $fopen("output.txt");
$fdisplay(file, "The value of A is %d", A);
$fclose(file);

This will write the value of A to a file named output.txt.

Formatting in Display Tasks

Display tasks in Verilog use format specifiers to output values in a structured way. Some common format specifiers include:

• %d for decimal numbers.
• %b for binary numbers.
• %h for hexadecimal numbers.
• %o for octal numbers.
• %s for strings.

Example:

$display("Decimal: %d, Binary: %b, Hex: %h", 10, 10, 10);

This will print:

Decimal: 10, Binary: 1010, Hex: a

Why do we need Display Tasks in Verilog Programming Language?

We need Display Tasks in Verilog because they provide an essential way to observe and monitor the internal behavior of a digital circuit during simulation. Since hardware description languages (HDLs) like Verilog are primarily used to model and simulate digital systems before they are physically built, display tasks allow engineers to debug, validate, and analyze their designs effectively. Here are the key reasons why display tasks are necessary:

1. Debugging and Troubleshooting

Observing Internal Signals: When simulating complex digital circuits, internal signals and variables might not be directly observable. Display tasks, like $display, help output these signals’ values, allowing designers to understand how the circuit is functioning at specific simulation times.

Error Identification: Display tasks help pinpoint errors in logic, timing, or signal flow. By displaying real-time values of signals during simulation, engineers can trace where unexpected or incorrect behavior occurs.

2. Simulation Monitoring

Track Design Changes: With tasks like $monitor, you can automatically track and report changes in signals or variables. This is extremely useful for observing the dynamic behavior of a design without needing to manually insert display statements at every stage.

Timing Analysis: Verilog display tasks allow you to print messages or values alongside the simulation time ($time). This helps in understanding how a design reacts over time, especially in clock-driven circuits where timing is critical.

3. Verifying Expected Behavior

Testbench Validation: In testbenches, display tasks are essential for verifying that the circuit under test (CUT) behaves as expected. For example, $strobe can be used to capture final stable values after a clock cycle, ensuring that signals settle correctly.

Comparing Results: You can use display tasks to compare actual simulation results with expected values, helping you validate the functional correctness of your design.

4. Logging and Reporting

Generating Logs: With tasks like $fdisplay and $fwrite, you can generate detailed log files containing signal values at various stages of simulation. These logs can be used for further analysis or shared with other team members to track the simulation progress and identify problems.

Continuous Feedback: Display tasks provide continuous feedback during simulation, showing the state of your design at different times and under different conditions. This makes it easier to understand the overall behavior and performance of the circuit.

5. Ease of Use in Simulation

Quick Prototyping: Display tasks make it easy to quickly check the behavior of small design changes or modifications without needing complex waveform viewers. They offer a quick and direct way to print values in a human-readable format.

Simplifying Testing: By displaying the output of variables and signals, you simplify the process of writing and running testbenches. Rather than manually checking the results through waveforms, you can rely on display tasks to show critical values.

6. Visibility of Hidden Signals

Internal Signals: Many signals or variables within a Verilog design are not directly visible on simulation waveform viewers. Display tasks allow you to monitor these hidden signals that may not be routed to outputs, providing a deeper understanding of the circuit’s internal workings.

Example of Display Tasks in Verilog Programming Language

Display tasks in Verilog are crucial for printing information about variables, signals, and the overall behavior of your digital designs during simulation. The most common display tasks include $display, $monitor, $strobe, $write, and $fdisplay. Below, we’ll walk through an example that uses multiple display tasks and explain each in detail.

Example: A 4-bit Counter with Display Tasks

Let’s create a simple 4-bit counter in Verilog, and we’ll use various display tasks to observe its behavior at different points during the simulation.

Verilog Code

module counter_test;

  // Declare variables
  reg clk;           // Clock signal
  reg reset;         // Reset signal
  reg [3:0] counter; // 4-bit counter
  integer file;      // File handle for logging

  // Initialize signals
  initial begin
    clk = 0;         // Start clock at 0
    reset = 1;       // Activate reset initially
    counter = 0;     // Initialize counter to 0
    #10 reset = 0;   // Deactivate reset after 10 time units
    #100 $finish;    // End simulation after 100 time units
  end

  // Toggle clock every 5 time units
  always #5 clk = ~clk;

  // Counter logic: increment on positive clock edge, reset on reset signal
  always @(posedge clk or posedge reset) begin
    if (reset) begin
      counter = 0;  // Reset counter to 0
      $display("Counter reset at time %0t", $time); // Display message when reset
    end else begin
      counter = counter + 1;  // Increment counter
      $display("Counter incremented to %d at time %0t", counter, $time); // Display updated counter value
    end
  end

  // Monitor changes in clock and counter
  initial begin
    $monitor("At time %0t, clk = %b, counter = %d", $time, clk, counter);
  end

  // Use strobe to display counter value at the end of each time step
  always @(posedge clk) begin
    $strobe("At the end of time step, counter = %d", counter);
  end

  // Write the counter value to a file
  initial begin
    file = $fopen("counter_log.txt", "w"); // Open file for writing
    $fdisplay(file, "Starting counter simulation..."); // Write header to file
  end

  always @(posedge clk) begin
    $fdisplay(file, "At time %0t, counter = %d", $time, counter); // Log to file
  end

  // Close file at the end of simulation
  initial begin
    #100 $fclose(file);
  end

endmodule

Explanation of Display Tasks:

1. $display

$display prints messages to the simulation console, showing the current state of variables or signals at specific points.

Example in Code:

$display("Counter incremented to %d at time %0t", counter, $time);

This line will print the value of counter and the current simulation time ($time) whenever the counter is incremented.

Output Example:

Counter incremented to 1 at time 10
Counter incremented to 2 at time 20

$display moves the cursor to a new line after each call, similar to a newline in other programming languages.

2. $monitor

$monitor continuously monitors the specified signals and automatically prints their values whenever a change occurs.

Example in Code:

$monitor("At time %0t, clk = %b, counter = %d", $time, clk, counter);

This will print the values of clk and counter whenever they change.

Output Example:

At time 0, clk = 0, counter = 0
At time 5, clk = 1, counter = 0
At time 10, clk = 0, counter = 1
At time 15, clk = 1, counter = 1

$monitor is very useful when you want to track signal changes over time without explicitly adding display statements at every step.

3. $strobe

$strobe works similarly to $display, but it prints values only after all events in the current time step have been evaluated. This ensures that the final, stable values of signals are printed.

Example in Code:

$strobe("At the end of time step, counter = %d", counter);

This will print the value of counter at the end of each clock cycle after the value has settled.

Output Example:

At the end of time step, counter = 1
At the end of time step, counter = 2

$strobe ensures that you’re printing the final, stable values of signals after all updates have been applied in that time step.

4. $write

$write is similar to $display, but it does not automatically add a newline at the end of the output. This is useful when you want to print multiple values on the same line without breaks.

Example:

$write("Counter = %d, ", counter);
$write("Clock = %b\n", clk);

In this case, the output will appear on the same line, giving you control over where newlines are placed.

5. $fdisplay

$fdisplay is used to write output to a file instead of the console. This is helpful when creating logs for later analysis.

Example in Code:

file = $fopen("counter_log.txt", "w");
$fdisplay(file, "At time %0t, counter = %d", $time, counter);

This will write the current value of the counter and the simulation time to the file counter_log.txt.

File Output Example:

Starting counter simulation...
At time 10, counter = 1
At time 20, counter = 2

6. $fclose

This system task closes the file after writing is complete. It’s important to ensure that files are properly closed at the end of the simulation.

Example in Code:

$fclose(file);

Simulation Time: Display tasks often use the built-in $time function to print the simulation time, making it easier to track when certain events or signal changes occur.

Format Specifiers: In all display tasks, format specifiers like %d (decimal), %b (binary), %h (hexadecimal), and %0t (time) are used to format the output correctly.

Advantages of Display Tasks in Verilog Programming Language

Display tasks in Verilog, such as $display, $monitor, $strobe, and $fdisplay, offer a range of advantages for debugging, monitoring, and analyzing digital designs during simulation. Here are some of the key benefits:

1. Simplifies Debugging

Visibility into Internal Signals: Display tasks allow you to observe and print internal signals and variables during simulation. This is critical for debugging as you can see the real-time behavior of your design without the need for specialized hardware or waveform viewers.

Error Detection: By printing signal values at specific times or when certain events occur, you can easily spot where logic errors, timing issues, or unexpected behaviors arise in your design.

2. Enhances Simulation Monitoring

Continuous Signal Tracking: With tasks like $monitor, you can continuously track changes in signals and variables. This eliminates the need to manually insert multiple display statements, making it easier to catch changes automatically during the simulation.

Time-based Observation: Display tasks can include simulation time ($time), which helps designers understand how signals evolve over time, especially in time-sensitive designs like clock-driven circuits.

3. Flexible Output Formatting

Customizable Output: Display tasks support various format specifiers (e.g., %d for decimal, %b for binary, %h for hexadecimal), allowing you to format the output according to your needs. This flexibility is important when dealing with different types of data in your design.

Structured Information: By using format specifiers and custom messages, you can print specific details about the design, helping you structure the output in a meaningful and readable way.

4. Reduces Dependency on Waveform Viewers

Quick and Direct Feedback: Instead of relying solely on waveform viewers to monitor signals, display tasks give you quick and direct feedback in the console. This can save time, especially for small simulations or during the early stages of development when rapid feedback is needed.

Immediate Insight: Display tasks provide immediate insight into signal values without needing to open external tools or files. This is particularly useful in testbenches where you want to quickly check results.

5. Supports File Logging

Logging to Files: With $fdisplay and $fwrite, you can log signal values to external files during simulation. This is helpful for creating persistent logs for later analysis, reporting, or sharing with others in your team.

Detailed Analysis: These logs can be used to trace the behavior of the design over time, especially for complex designs where tracking multiple signals over a long period is necessary.

6. Facilitates Functional Verification

Testbench Integration: Display tasks are widely used in testbenches to validate the behavior of the design under test (DUT). They help in checking whether the DUT behaves as expected by comparing actual outputs with expected results.

Stability Checking: With $strobe, you can check stable values at the end of a time step, ensuring that signal values have settled correctly after updates. This is useful for detecting race conditions and other timing issues.

7. Low Overhead During Simulation

Minimal Performance Impact: Display tasks are lightweight and typically do not introduce significant overhead into the simulation. They can be added and removed easily without affecting the actual logic of the design.

Non-intrusive Monitoring: Since display tasks do not interfere with the operation of the design itself, they allow for non-intrusive monitoring and debugging.

8. Assists in Real-Time Verification

Immediate Results: Display tasks provide real-time updates on signal values during simulation, which helps in quickly verifying the functional correctness of your design.

Event-Based Reporting: Tasks like $monitor and $display allow for event-driven reporting, where you can see values as soon as they change or when specific events occur (e.g., clock edges, resets).

9. Useful for Prototyping and Learning

Fast Prototyping: For small designs or when learning Verilog, display tasks provide an easy way to see how the design is functioning without needing complex simulation setups.

Learning Tool: Beginners in Verilog can use display tasks to gain a deeper understanding of how the language works and how signals propagate through a digital design.

10. Provides Granular Control Over Output

Selective Output: You can place display tasks exactly where needed, allowing for fine-grained control over what information is output and when. This makes it easy to focus on the most important signals or events in your design.

Customizable Message Content: You can create custom messages that provide context or explanations in the output, making the results more meaningful and easier to interpret.

Disadvantages of Display Tasks in Verilog Programming Language

While display tasks in Verilog are highly useful for debugging and monitoring simulations, they also come with certain limitations. Understanding these disadvantages helps designers make informed decisions when using them in large or complex projects.

1. Limited Use in Synthesis

Simulation-Only Tools: Display tasks like $display, $monitor, and $strobe are non-synthesizable. This means they are purely for simulation purposes and cannot be used in hardware synthesis. When a design is being synthesized for actual hardware (FPGA or ASIC), these tasks are ignored. Thus, display tasks have no role in the final hardware implementation and are limited to the debugging and verification stages.

Not Portable to Hardware: If you are developing a design that needs to transition to physical hardware, relying heavily on display tasks during the design phase may limit your ability to debug on the hardware itself.

2. Increased Simulation Overhead

Performance Impact in Large Designs: In large-scale or complex designs, excessive use of display tasks can increase simulation time and impact performance. This is because printing to the console or writing to files takes time, especially when large amounts of data or frequent signal changes are involved.

Slower Simulations: Continuous monitoring with $monitor or frequent printing with $display can slow down the simulation, especially if these tasks are placed inside frequently triggered blocks (e.g., clock edges or large loops).

3. Console Clutter

Overwhelming Output: When too many display tasks are used, especially in complex designs with many signals, the output can become overwhelming and difficult to interpret. A cluttered console with excessive messages can make it harder to focus on the important data and might lead to missed issues.

Difficult Debugging: Instead of helping debugging, too much printed information can result in information overload, making it harder to identify critical problems or trends in the output.

4. Limited Flexibility in Complex Scenarios

Basic Functionality: Display tasks provide basic printing and logging functionality but lack advanced features like graphical visualization or conditional breakpoints found in dedicated debugging tools. For more complex or intricate debugging needs, such as deep timing analysis, signal tracing over time, or waveform generation, display tasks may not be sufficient.

Limited Control Over Output: You can format the output to some extent, but the functionality of display tasks is limited to text-based output. More sophisticated tools are needed for deep analysis, especially when large amounts of data are involved.

5. No Control Over Output Timing in Multi-Threaded Simulations

Potential Timing Issues in Output: In multi-threaded or complex simulations, output from multiple $display or $monitor statements can get jumbled, making the output harder to interpret. Since multiple events may happen simultaneously, ensuring that the output order matches the actual sequence of operations can be challenging.

Inconsistent Output Ordering: If several events happen at the same simulation time, the order of the messages printed by different display tasks might not always reflect the exact order in which events occurred, leading to confusion.

6. Resource-Heavy Logging to Files

File I/O Overhead: Writing extensive logs to files using $fdisplay or $fwrite can slow down simulations, especially if large amounts of data are being recorded continuously. Frequent file I/O operations increase simulation overhead and can result in longer simulation times.

Log File Size Management: Continuous logging can result in large file sizes, which can become cumbersome to manage, analyze, or transfer. Sorting through large log files to find relevant data can also be time-consuming.

7. Manual Analysis Required

No Automated Analysis: Display tasks only print the data; interpreting and analyzing that data is left to the designer. There is no built-in mechanism for automatically highlighting problematic signal changes, errors, or deviations from expected behavior. This manual analysis can be time-consuming, especially for larger designs.

Tedious Debugging Process: While display tasks are useful for simple debugging, they are not ideal for more advanced debugging processes where automated tools (like waveform viewers or formal verification tools) can quickly pinpoint issues.

8. Non-Dynamic Nature

Static in Nature: Display tasks typically print static information at fixed points in time, often at clock edges or specific signal changes. For real-time, dynamic debugging, where immediate interactive feedback is required, display tasks may not be the most efficient tool.

No Interactive Control: Unlike debugging environments that allow you to pause, step through code, or examine variables interactively, display tasks are passive. They simply print predefined information during simulation, offering no control over how or when the information is displayed interactively.

9. Verbose Output for Minor Changes

Over-Detailed for Small Changes: When monitoring a signal that changes frequently, display tasks like $monitor can produce a large amount of output even for small changes. This level of verbosity can make it harder to focus on significant events or transitions in your design.

Signal Bursts: When multiple signals change at once (such as during a reset or initialization sequence), a flood of information can be printed, making it difficult to track which changes are most important.

10. Lack of Conditional Displaying

No Native Conditional Filtering: Display tasks do not inherently offer conditional printing or filtering of output. For instance, if you only want to print a signal value when it crosses a threshold or during an error condition, you would need to manually implement conditional checks in your code. This can make the debugging process slightly more complex than necessary.

Increased Code Complexity: Adding too many display tasks with conditional logic for specific signals or events can clutter your code, making it harder to maintain and understand, especially for more complex designs.