Timing Control

There are three types of timing control, delay based, event based and level sensitive. We will discuss the useful aspects of each in turn.

Delay-Based

Syntax: timing_control_statement::== delay_based statement*
delay_based::== # delay_value

This method introduces a delay between when a statement is encountered and when it is executed.


        initial begin
          a = 0;      // executed at simulation time 0
          #10 b = 2;  // executed at simulation time 10
          #15 c = a;  //    ... at time 25
          #b c = 4;   //    ... at time 27
          b=5;        //    ... at time 27
        end

The delay value can be specified by a constant or variable. Note the time is not in seconds, it is relative to the current unit of time. A common example is the creation of a clock signal:

        initial begin
          clock = 1'b0;
          forever #5 clock = ~clock;
        end

This will make the clock flip every 5 time units giving a wave form as shown.

EXERCISE
In the following module what would be the values of
a) a at time 15 units;
b) b at 17 units
c) c at 18 units
d) a at 30 units

        module testdelay(a, b, c);
           output [3:0]     a, b, c;
           reg [3:0]   a, b, c;

           initial begin
              #10 a = 3;
              #5 b = 4;
              #6 c = 5;
              #8 a = a + b;
           end

        endmodule

Answers

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