$time $stime $realtime

1,$time

The $time system function returns an integer that is a 64-bit time, scaled to the timescale unit of the module that invoked it.

`timescale 10 ns / 1 ns 
module test; 
    reg set; 
    parameter p = 1.55; 
    initial begin 
        $monitor($time,,"set=",set); 
        #p set = 0; 
        #p set = 1;
    end 
endmodule

结果：

                   0 set=x
                   2 set=0
                   3 set=1
           V C S   S i m u l a t i o n   R e p o r t 
Time: 32 ns

 分析：

a) The simulation times 16 ns and 32 ns are scaled to 1.6 and 3.2 because the time unit for the module is 10 ns; therefore, time values reported by this module are multiples of 10 ns.
b) The value 1.6 is rounded to 2, and 3.2 is rounded to 3 because the $time system function returns an integer. The time precision does not cause rounding of these values

因为精度是1ns，1.55*10=15.5ns，四舍五入，取16ns

假如将精度设置为

 `timescale 10 ns / 1ps  那么，结果将是15.5和31

2，$stime

The $stime system function returns an unsigned integer that is a 32-bit time, scaled to the timescale unit of the module that invoked it. If the actual simulation time does not fit in 32 bits, the low order 32 bits of the current simulation time are returned.

对于$time的example，$stime和$time是一样的。

3，$realtime

The $realtime system function returns a real number time that, like $time, is scaled to the time unit of the module that invoked it.

将上述example改为$realtime之后，结果如下所示：

0 set=x
1.6 set=0
3.2 set=1
           V C S   S i m u l a t i o n   R e p o r t 
Time: 32 ns

其他运行结果相同