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Tuesday 7 June 2016


In this post, let us see the timescale feature and system tasks that are available in Verilog HDL with brief examples.

`timescale directive specifies the time unit and time precision of the modules that follow it. The time unit is the unit of measurement for time values such as the simulation time and delay values.
Syntax:   `timescale <time_unit>/<time_precision>
              The time_unit argument specifies the unit of measurement for times and delays.
The time_precision argument specifies how delay values are rounded before being used in simulation. The smallest time_precision argument of all the 'timescale compiler directives in the design determines the precision of the time unit of the simulation.

The time_precision argument shall be at least as precise as the time_unit argument; it cannot specify a longer unit of time than time_unit. The integers in these arguments specify an order of magnitude for the size of the value; the valid integers are 1, 10, and 100. The character strings "s, ms, us, ns, ps, and fs " represent units of measurement;

Example: `timescale 1ns/100ps
                Here in the above example, time_unit is 1ns & time_precision is 100ps. So the delay of #1 in the code is equivalent to 1ns delay in simulation. The delay of #1.56 will be rounded to 1.6ns. i.e. the decimal fraction will be rounded to in multiples of the time_precision value. Each #delay value is rounded to time delays w.r.t the timescale specified and added to the current simulation time.

Some of the system tasks on timescale are
·         $printtimescale() prints the timescale settings of the current scope of the file.
·         $timeformat(..,..,,,) system task

The $timeformat system task performs the following two operations:
Ø  It sets the time unit for all later-entered delays entered interactively.
Ø  It sets the time unit, precision number, suffix string, and minimum field width for all %t formats specified in all modules that follow in the source description until another $timeformat system task is invoked.

Syntax for $timeformat is
$timeformat ( units_number , precision_number , suffix_string , minimum_field_width ) ;

Example: $timeformat(-9,3,"ns",8);   This will display any %t string in ns with 3 digits precision, and 8 characters string size.

Let us see the code for the example used for different timescale settings & $timeformat system task.
initial begin
    #1     $display("\n1) %t",$realtime); 
    #10.5  $display("\n2) %t",$realtime); 
    #10.56 $display("\n3) %t",$realtime);
    #10.56 $display("\n4) %t",$realtime);
    #11.46 $display("\n5) %t",$realtime);

The output of the above code with different timescale values are illustrated in the below table.
`timescale 1ns/1ps
`timescale 1ns/10ps
`timescale 1ns/100ps
1)  1.000ns
2) 11.500ns
3) 22.060ns
4) 32.620ns
5) 44.080ns
1)  1.000ns
2) 11.500ns
3) 22.060ns
4) 32.620ns
5) 44.080ns
1)  1.000ns
2) 11.500ns
3) 22.100ns
4) 32.700ns
5) 44.200ns

The calculation for timescale 1ns/100ps is as below:
1.       Step 1, time is 1ns
2.       Step 2, time is (1ns+10.5ns)=11.5ns
3.       Step 3, time is (11.5ns + 10.6ns) = 22.1ns
4.       Step 4, time is (22.1ns + 10.6ns) = 32.7ns
5.       Step 5, time is (32.7ns + 11.5ns) = 44.2ns

 Similarly you could calculate for different timescale values. If you've any queries, plz do comment it.

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