Verilog Timeformat

Verilog timescale directive specifies time unit and precision for simulations.

Verilog $timeformat system function specifies %t format specifier reporting style in display statements such as $display and $strobe.

Syntax

$timeformat(<unit_number>, <precision>, <suffix_string>, <minimum field width>);

unit_number is the smallest time precision out of all `timescale directives used in the source code.
precision represents the number of fractional digits for the current timescale.
suffix_string is an option to display the scale alongside real time values.

Unit Number	Time Unit
-3	1ms
-6	1us
-9	1ns
-12	1ps
-15	1fs

Example 1: 1ns/1ps

Below is an example of how $timeformat affects the format of the time unit display.

`timescale 1ns/1ps

module tb;
  bit a;

  initial begin

    // Wait for some time
    // precision is 1/1000 of
 the main scale (1ns), the 3rd position will truncate
 this delay 
    #10.512351;

    // Display current time with default timeformat parameters
    $display("[T=%0t] a=%0b", $realtime, a);

    // Change timeformat parameters and display again
    $timeformat(-9, 2, " ns");
    $display("[T=%0t] a=%0b", $realtime, a);

    // Remove the space in suffix, and extend fractional digits to 5
    $timeformat(-9, 5, "ns");
    $display("[T=%0t] a=%0b", $realtime, a);

    // Here suffix is wrong, it should not be "ns" because we are
    // setting display in "ps" (-12)
    $timeformat(-12, 3, " ns");
    $display("[T=%0t] a=%0b", $realtime, a);

    // Correct the suffix to ps
    $timeformat(-12, 2, " ps");
    $display("[T=%0t] a=%0b", $realtime, a);
  end
endmodule

Now executes the above code, and it will give the following output, such as:

xcelium> run
[T=10512] a=0
[T=10.51 ns] a=0
[T=10.51200ns] a=0
[T=10512.000 ns] a=0
[T=10512.00 ps] a=0
xmsim: *W,RNQUIE: Simulation is complete.

Example 2: 1ns/100ps

We consider the same example from above with a different timescale.

`timescale 1ns/100ps

module tb;
  bit a;

  initial begin

    // Wait for some time 
    // precision is 1/1000 of
 the main scale (1ns), the 3rd position will truncate
 this delay 
    #10.512351;

    // Display current time with default timeformat parameters
    $display("[T=%0t] a=%0b", $realtime, a);

    // Change timeformat parameters and display again
    $timeformat(-9, 2, " ns");
    $display("[T=%0t] a=%0b", $realtime, a);

    // Remove the space in suffix, and extend fractional digits to 5
    $timeformat(-9, 5, "ns");
    $display("[T=%0t] a=%0b", $realtime, a);

    // Here suffix is wrong, it should not be "ns" because we are
    // setting display in "ps" (-12)
    $timeformat(-12, 3, " ns");
    $display("[T=%0t] a=%0b", $realtime, a);

    // Correct the suffix to ps
    $timeformat(-12, 2, " ps");
    $display("[T=%0t] a=%0b", $realtime, a);
  end
endmodule

Above code produce the following output:

xcelium> run
[T=105] a=0
[T=10.50 ns] a=0
[T=10.50000ns] a=0
[T=10500.000 ns] a=0
[T=10500.00 ps] a=0
xmsim: *W,RNQUIE: Simulation is complete.

Example 3: 100ns/1ns

`timescale 100ns/1ns

module tb;
  bit a;

  initial begin

    // Wait for some time - note that because precision is 1/1000 of
    // the main scale (1ns), this delay will be truncated by the 3rd
    // position
    #10.512351;

    // Display current time with default timeformat parameters
    $display("[T=%0t] a=%0b", $realtime, a);

    // Change timeformat parameters and display again
    $timeformat(-9, 2, " ns");
    $display("[T=%0t] a=%0b", $realtime, a);

    // Remove the space in suffix, and extend fractional digits to 5
    $timeformat(-9, 5, "ns");
    $display("[T=%0t] a=%0b", $realtime, a);

    // Here suffix is wrong, it should not be "ns" because we are
    // setting display in "ps" (-12)
    $timeformat(-12, 3, " ns");
    $display("[T=%0t] a=%0b", $realtime, a);

    // Correct the suffix to ps
    $timeformat(-12, 2, " ps");
    $display("[T=%0t] a=%0b", $realtime, a);
  end
endmodule

NOTE: #1 represents 100ns and hence #10 yields 1000ns.

And the output looks like:

xcelium> run
[T=1051] a=0
[T=1051.00 ns] a=0
[T=1051.00000ns] a=0
[T=1051000.000 ns] a=0
[T=1051000.00 ps] a=0
xmsim: *W,RNQUIE: Simulation is complete.

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