Verilog's time system tasks - $time, $stime, $realtime

List of articles

         summary

        $time

        $stime

        $realtime

         Summary and reference

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summary

        When doing simulation , It is often necessary to obtain the simulation time in order to understand the test situation of the tested module .Verilog Syntax provides 3 A system task ----$time、$stime、$realtime, this 3 All system tasks can be simulated （ Unable to synthesize ） Get the time value of the current simulation time , But its use also has a little difference .

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$time

        Call the system task $time, Will get one 64 Bit integer Type variable , It represents the simulation time when the system task is called . It should be noted that , Its value will be automatically scaled to `timescale The time unit defined by the task . Here's an example ：

`timescale 10 ns / 1 ns

module tb_time_test;

reg set;
parameter p = 1.55;

initial begin
	$monitor($time,,"set=",set);
	#p set = 0;
	#p set = 1;
end

endmodule

        The simulation results are as follows ： 

• In the example above , The simulation time unit is defined as 10ns, Time accuracy is defined as 1ns.
• The first 1 That's ok ,set Unassigned , So the output set = x, The time is 0
• The first 2 That's ok ,set The assignment is 0, So the output set = 0, The time is 1.55*10=15.5ns, Because the accuracy is 1ns, So round to 16ns, At the same time, because the time unit is 10ns, So it doesn't mean 16ns, It means rounded 2（ Company 10ns）
• The first 3 That's ok ,set The assignment is 1, So the output set = 1, The time is 16+1.55*10=31.5ns, Because the accuracy is 1ns, So round to 32ns, At the same time, because the time unit is 10ns, So it doesn't mean 32ns, It means rounded 3（ Company 10ns）

         Next, we change the simulation time unit of the above code to 1ns, The time precision is reserved as 1ns, It looks easier to understand .

`timescale 1 ns / 1 ns

module tb_time_test;

reg set;
parameter p = 1.55;

initial begin
	$monitor($time,,"set=",set);
	#p set = 0;
	#p set = 1;
end

endmodule

        The simulation results are as follows ：

• In the example above , The simulation time unit is defined as 1ns, Time accuracy is defined as 1ns.
• The first 1 That's ok ,set Unassigned , So the output set = x, The time is 0
• The first 2 That's ok ,set The assignment is 0, So the output set = 0, The time is 1.55*1=1.55ns, Because the accuracy is 1ns, So round to 2ns, The unit of time is 1ns, So directly show 2（ Company 1ns）
• The first 3 That's ok ,set The assignment is 1, So the output set = 1, The time is 2+1.55*1=3.55ns, Because the accuracy is 1ns, So round to 4ns, The unit of time is 1ns, So directly show 4（ Company 1ns）

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$stime

        Call the system task $stime, Will get one 32 Unsigned bit integer Type variable , It represents the simulation time when the system task is called . It should be noted that , Its value will also be automatically scaled to `timescale The time unit defined by the task .

        The usage of this system task is basically the same as  $time It's the same , Just because it only 32 position , Therefore, the time range represented will be relatively small , Be careful not to overflow when using .

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$realtime

         Call the system task $realtime, Will get one real Type variable , It represents the simulation time when the system task is called . It should be noted that , Its value will be automatically scaled to `timescale The time unit defined by the task .

        Because its return value is a real number real type , So it can represent decimal time . Also take the above code as an example ：

`timescale 10 ns / 1 ns

module tb_time_test;

reg set;
parameter p = 1.55;

initial begin
	$monitor($realtime,,"set=",set);
	#p set = 0;
	#p set = 1;
end

endmodule

        The simulation results are as follows ：

• In the example above , The simulation time unit is defined as 10ns, Time accuracy is defined as 1ns.
• The first 1 That's ok ,set Unassigned , So the output set = x, The time is 0
• The first 2 That's ok ,set The assignment is 0, So the output set = 0, The time is 1.55*10=15.5ns, Because the accuracy is 1ns, So round to 16ns, At the same time, because the time unit is 10ns, So it doesn't mean 16ns, It means 1.6（ It can express decimals , Company 10ns）
• The first 3 That's ok ,set The assignment is 1, So the output set = 1, The time is 16+1.55*10=31.5ns, Because the accuracy is 1ns, So round to 32ns, At the same time, because the time unit is 10ns, So it doesn't mean 32ns, It means 3.2（ It can express decimals , Company 10ns）

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Summary and reference

• $time、$stime、$realtime These three system tasks can obtain the current simulation time , The difference lies in the type of time value returned , General applications use $time that will do
• During the simulation, the simulation time can be observed 、 Monitor the test condition of the tested module , It is helpful to improve the test efficiency

         Reference material 1：IEEE Standard for Verilog Hardware Description Language（IEEE Std 1364-2005）

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