How to Use $monitor in Verilog for Signal Monitoring

In Verilog, $monitor is used to automatically print signal values whenever any of the specified signals change during simulation. You use it by calling $monitor with a format string and the signals you want to watch, typically inside an initial block.

Syntax

The basic syntax of $monitor is:

$monitor("format string", signal1, signal2, ...);

Here:

• format string: A string with placeholders like %d or %b to format the output.
• signal1, signal2, ...: The signals or variables whose values you want to display.

$monitor prints the message automatically whenever any of the listed signals change.

verilog

$monitor("At time %0t: a=%b, b=%b, sum=%b", $time, a, b, sum);

Example

This example shows how $monitor tracks changes in signals a, b, and sum during simulation.

verilog

module monitor_example;
  reg a, b;
  wire sum;

  assign sum = a ^ b; // XOR operation

  initial begin
    $monitor("At time %0t: a=%b, b=%b, sum=%b", $time, a, b, sum);
    a = 0; b = 0;
    #5 a = 1;
    #5 b = 1;
    #5 a = 0;
    #5 b = 0;
    #5 $finish;
  end
endmodule

Output

At time 0: a=0, b=0, sum=0
At time 5: a=1, b=0, sum=1
At time 10: a=1, b=1, sum=0
At time 15: a=0, b=1, sum=1
At time 20: a=0, b=0, sum=0

Common Pitfalls

• Multiple $monitor calls: Only the last $monitor call is active; previous ones are overridden.
• Using $monitor outside initial or always blocks: It should be inside an initial block to start monitoring at simulation start.
• Forgetting to include $time: Without $time, you won't see when changes happen.
• Confusing $monitor with $display: $display prints once when called; $monitor prints automatically on signal changes.

verilog

/* Wrong: multiple $monitor calls - only last works */
initial begin
  $monitor("a=%b", a);
  $monitor("b=%b", b); // overrides previous monitor
end

/* Right: single $monitor call monitoring multiple signals */
initial begin
  $monitor("a=%b, b=%b", a, b);
end

Quick Reference

Feature	Description
$monitor	Automatically prints when any listed signal changes
Format string	Use %b for binary, %d for decimal, %h for hex, %0t for time
Placement	Inside an initial block to start monitoring at simulation start
Multiple calls	Only the last $monitor call is active
$time	Use to print current simulation time

Key Takeaways

Use $monitor inside an initial block to watch signals automatically on change.

Only one $monitor call is active at a time; multiple calls override each other.

Include $time in the format string to see when changes occur during simulation.

Use format specifiers like %b, %d, and %h to display signals in different formats.

$monitor differs from $display by printing output automatically on signal changes.