Introduction
Difference between a HDL and other programming language is notion of time and concurrancy. In this chapter we will see the time data type in detail.

We will be covering following topics

• sc_time
• sc_start()
• sc_time_stamp()
• wait(sc_time)
• sc_simulation_time()
• sc_event

sc_time

sc_time is special data type which is used to represent simulation time and time intervals, including delays and time-outs. An object of class sc_time is constructed from a double and an sc_time_unit. Time shall be represented internally as an unsigned integer of at least 64 bits. Like any other data type in SystemC, sc_time also allows arthmetic operations.
Enum types that denote various time units are as below

• SC_FS = 0
• SC_PS = 1
• SC_NS = 2
• SC_US = 3
• SC_MS = 4
• SC_SEC= 5

Time Resolution

Time shall be represented internally as an integer multiple of the time resolution. The default time resolution is 1 picosecond. The time resolution can only be changed by calling the function sc_set_time_resolution. This function shall only be called during elaboration, shall not be called more than once, and shall not be called after constructing an object of type sc_time with a non-zero time value.
The function sc_get_time_resolution shall return the time resolution.

SC_ZERO_TIME

SC_ZERO_TIME represents 0 time delay. It is good practice to use this constant whenever writing a time value of zero, for example, when creating a delta notification or a delta time-out.

Example : sc_time

  1 #include

  2

  3 int sc_main (int argc, char* argv[]) {

  4   // sc_set_time_resol... should be called before sc_time

  5   // variable declration

  6   sc_set_time_resolution(1,SC_PS);

  7   // Declare the sc_time variables

  8   sc_time             t1(10,SC_NS);

  9   sc_time             t2(5,SC_PS);

 10   sc_time             t3,t4(1,SC_PS),t5(1,SC_PS);

 11   // Print all the variables

 12   cout <<"Value of t1 "<< t1.to_string() << endl;

 13   cout <<"Value of t2 "<< t2.to_string() << endl;

 14   cout <<"Value of t3 "<< t3.to_string() << endl;

 15   cout <<"Value of t4 "<< t4.to_string() << endl;

 16   cout <<"Value of t5 "<< t5.to_string() << endl;

 17   // Start the sim

 18   sc_start(0);

 19   sc_start(1);

 20   // Get the current time

 21   t3 = sc_time_stamp();

 22   cout <<"Value of t3 "<< t3.to_string() << endl;

 23   // Run for some more time

 24   sc_start(20);

 25   // Get the current time

 26   t4 = sc_time_stamp();

 27   // Print the variables for new time

 28   cout <<"Value of t4 "<< t4.to_string() << endl;

 29   // This is how you do arth operation

 30   t5 = t4 - t3;

 31   cout <<"Value of t5 "<< t5.to_string() << endl;

 32   // This is how we do compare operation

 33   if (t5 > t2) {

 34     cout <<" t5 is greated then t2" << endl;

 35   } else {

 36     cout <<" t2 is greated then t5" << endl;

 37   }

 38   return 1;

 39 }

Simulation Output : sc_time

Value of t1 10 ns

 Value of t2 5 ps

 Value of t3 0 s

 Value of t4 1 ps

 Value of t5 1 ps

 Value of t3 1 ns

 Value of t4 21 ns

 Value of t5 20 ns

  t5 is greated then t2

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