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There are two types of timing controls in Verilog - delay and event expressions. The delay control is just a way of adding a delay between the time the simulator encounters the statement and when it actually executes it. The event expression allows the statement to be delayed until the occurrence of some simulation event which can be a change of value on a net or variable (implicit event) or an explicitly named event that is triggered in another procedure.

Simulation time can be advanced by one of the following methods.

Gates and nets that have been modeled to have internal delays also advance simulation time.

Delay Control

If the delay expression evaluates to an unknown or high-impedance value it will be interpreted as zero delay. If it evaluates to a negative value, it will be interpreted as a 2's complement unsigned integer of hte same size as a time variable.

 
`timescale 1ns/1ps
 
module tb;
  reg [3:0] a, b;
 
  initial begin
    {a, b} <= 0;
    $display ("T=%0t a=%0d b=%0d", $realtime, a, b);
 
    #10;
    a <= $random;
    $display ("T=%0t a=%0d b=%0d", $realtime, a, b);
 
    #10 b <= $random;
    $display ("T=%0t a=%0d b=%0d", $realtime, a, b);
 
    #(a)           $display ("T=%0t After a delay of a=%0d units", $realtime, a);
    #(a+b)         $display ("T=%0t After a delay of a=%0d + b=%0d = %0d units", $realtime, a, b, a+b);
    #((a+b)*10ps) $display ("T=%0t After a delay of %0d * 10ps", $realtime, a+b);
 
    #(b-a)         $display ("T=%0t Expr evaluates to a negative delay", $realtime);
    #('h10)       $display ("T=%0t Delay in hex", $realtime);
 
    a <= 'hX;
    #(a)           $display ("T=%0t Delay is unknown, taken as zero", $realtime);
 
    a <= 'hZ;
    #(a)           $display ("T=%0t Delay is in high impedance, taken as zero", $realtime);
 
    #1ps           $display ("T=%0t Delay of 10ps", $realtime);
  end
 
endmodule
 

Note that the precision of timescale is in 1ps and hence $realtime is required to display the precision value for the statement with a delay expression (a+b)*10ps.

Simulation Log
ncsim> run
T=0 a=x b=x
T=10000 a=0 b=0
T=20000 a=4 b=0
T=24000 After a delay of a=4 units
T=29000 After a delay of a=4 + b=1 = 5 units
T=29050 After a delay of 5 * 10ps
T=42050 Expr evaluates to a negative delay
T=58050 Delay in hex
T=62050 Delay is unknown
T=62050 Delay is in high impedance
T=62051 Delay of 10ps
ncsim: *W,RNQUIE: Simulation is complete.

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Event Control

Value changes on nets and variables can be used as a synchronization event to trigger execution other procedural statements and is an implicit event. The event can also be based on the direction of change like towards 0 which makes it a negedge and a change towards 1 makes it a posedge.

  • A negedge is when there is a transition from 1 to X, Z or 0 and from X or Z to 0
  • A posedge is when there is a transition from 0 to X, Z or 1 and from X or Z to 1

A transition from the same state to the same state is not considered as an edge. An edge event like posedge or negedge can be detected only on the LSB of a vector signal or variable. If an expression evaluates to the same result it cannot be considered as an event.

 
module tb;
  reg a, b;
 
  initial begin
    a <= 0;
 
    #10 a <= 1;
    #10 b <= 1;
 
    #10 a <= 0;
    #15 a <= 1;
 
  end
 
  // Start another procedural block that waits for an update to 
  // signals made in the above procedural block
  initial begin
    @(posedge a);
    $display ("T=%0t Posedge of a detected for 0->1", $time);
 
    @(posedge b);
    $display ("T=%0t Posedge of b detected for X->1", $time);
 
    @(posedge (a + b)) $display ("T=%0t Posedge of a+b", $time);
 
    @(a) $display ("T=%0t Change in a found", $time);    
  end
endmodule
 
Simulation Log
ncsim> run
T=10 Posedge of a detected for 0->1
T=20 Posedge of b detected for X->1
T=30 Posedge of a+b
T=45 Change in a found
ncsim: *W,RNQUIE: Simulation is complete.

Named Events

The keyword event can be used to declare a named event which can be triggered explicitly. An event cannot hold any data, has no time duration and can be made to occur at any particular time. A named event is triggered by the -> operator by prefixing it before the named event handle. A named event can be waited upon by using the @ operator described above.

 
module tb;
  event a_event;
  event b_event[5];
 
  initial begin
    #20 -> a_event;
 
    #30;
    ->a_event;
 
    #50 ->a_event;
    #10 ->b_event[3];
  end
 
  always @ (a_event) $display ("T=%0t [always] a_event is triggered", $time);
 
  initial begin
    #25;
    @(a_event) $display ("T=%0t [initial] a_event is triggered", $time);
 
    #10 @(b_event[3]) $display ("T=%0t [initial] b_event is triggered", $time);
  end
endmodule
 

Named events can be used to synchronize two or more concurrently running processes. For example, the always block and the second initial block are synchronized by a_event. Events can be declared as arrays like in the case of b_event which is an array of size 5 and the index 3 is used for trigger and wait purpose.

Simulation Log
ncsim> run
T=20 [always] a_event is triggered
T=50 [always] a_event is triggered
T=50 [initial] a_event is triggered
T=100 [always] a_event is triggered
T=110 [initial] b_event is triggered
ncsim: *W,RNQUIE: Simulation is complete.

Event or operator

The or operator can be used to wait on until any one of the listed events is triggered in an expression. The comma , can also be used instead of the or operator.

 
module tb;
  reg a, b;
 
  initial begin
    $monitor ("T=%0t a=%0d b=%0d", $time, a, b);
    {a, b} <= 0;
 
    #10 a <= 1;
    #5  b <= 1;
  #5  b <= 0;
  end
 
  // Use "or" between events
  always @ (posedge a or posedge b) 
    $display ("T=%0t posedge of a or b found", $time);
 
  // Use a comma between
  always @ (posedge a, negedge b)
    $display ("T=%0t posedge of a or negedge of b found", $time);
 
  always @ (a, b) 
    $display ("T=%0t Any change on a or b", $time);
endmodule
 
Simulation Log
ncsim> run
T=0 posedge of a or negedge of b found
T=0 Any change on a or b
T=0 a=0 b=0
T=10 posedge of a or b found
T=10 posedge of a or negedge of b found
T=10 Any change on a or b
T=10 a=1 b=0
T=15 posedge of a or b found
T=15 Any change on a or b
T=15 a=1 b=1
T=20 posedge of a or negedge of b found
T=20 Any change on a or b
T=20 a=1 b=0
ncsim: *W,RNQUIE: Simulation is complete.

Implicit Event Expression List

The sensitivity list or the event expression list is often a common cause for a lot of functional errors in the RTL. This is because the user may forget to update the sensitivity list after introducing a new signal in the procedural block.

 
module tb;
  reg a, b, c, d;
  reg x, y;
 
  // Event expr/sensitivity list is formed by all the
  // signals inside () after @ operator and in this case
  // it is a, b, c or d
  always @ (a, b, c, d) begin
    x = a | b;
    y = c ^ d;
  end
 
  initial begin
    $monitor ("T=%0t a=%0b b=%0b c=%0b d=%0b x=%0b y=%0b", $time, a, b, c, d, x, y);
    {a, b, c, d} = 0;
 
    #10 {a, b, c, d} = $random;
    #10 {a, b, c, d} = $random;
    #10 {a, b, c, d} = $random;
  end
endmodule
 
Simulation Log
ncsim> run
T=0 a=0 b=0 c=0 d=0 x=0 y=0
T=10 a=0 b=1 c=0 d=0 x=1 y=0
T=20 a=0 b=0 c=0 d=1 x=0 y=1
T=30 a=1 b=0 c=0 d=1 x=1 y=1
ncsim: *W,RNQUIE: Simulation is complete.

If the user decides to add new signal e and capture the inverse into z, special care must be taken to add e also into the sensitivity list.

 
module tb;
  reg a, b, c, d, e;
  reg x, y, z;
 
  // Add "e" also into sensitivity list
  always @ (a, b, c, d, e) begin
    x = a | b;
    y = c ^ d;
      z = ~e;
  end
 
  initial begin
      $monitor ("T=%0t a=%0b b=%0b c=%0b d=%0b e=%0b x=%0b y=%0b z=%0b", 
                        $time, a, b, c, d, e, x, y, z);
      {a, b, c, d, e} = 0;
 
      #10 {a, b, c, d, e} <= $random;
      #10 {a, b, c, d, e} <= $random;
      #10 {a, b, c, d, e} <= $random;
  end
endmodule
 
Simulation Log
ncsim> run
T=0 a=0 b=0 c=0 d=0 e=0 x=0 y=0 z=1
T=10 a=0 b=0 c=1 d=0 e=0 x=0 y=1 z=1
T=20 a=0 b=0 c=0 d=0 e=1 x=0 y=0 z=0
T=30 a=0 b=1 c=0 d=0 e=1 x=1 y=0 z=0
ncsim: *W,RNQUIE: Simulation is complete.

Verilog now allows the sensitivity list to be replaced by * which is a convenient shorthand that eliminates these problems by adding all nets and variables that are read by the statemnt like shown below.

 
module tb;
  reg a, b, c, d, e;
  reg x, y, z;
 
  // Use @* or @(*)
  always @ * begin
    x = a | b;
    y = c ^ d;
      z = ~e;
  end
 
  initial begin
      $monitor ("T=%0t a=%0b b=%0b c=%0b d=%0b e=%0b x=%0b y=%0b z=%0b", 
                        $time, a, b, c, d, e, x, y, z);
      {a, b, c, d, e} = 0;
 
      #10 {a, b, c, d, e} <= $random;
      #10 {a, b, c, d, e} <= $random;
      #10 {a, b, c, d, e} <= $random;
  end
endmodule
 
Simulation Log
ncsim> run
T=0 a=0 b=0 c=0 d=0 e=0 x=0 y=0 z=1
T=10 a=0 b=0 c=1 d=0 e=0 x=0 y=1 z=1
T=20 a=0 b=0 c=0 d=0 e=1 x=0 y=0 z=0
T=30 a=0 b=1 c=0 d=0 e=1 x=1 y=0 z=0
ncsim: *W,RNQUIE: Simulation is complete.

Level Sensitive Event Control

Execution of a procedural statement can also be delayed until a condition becomes true and can be accomplished with the wait keyword and is a level-sensitive control.

The wait statement shall evaluate a condition and if it is false, the procedural statements following it shall remain blocked until the condition becomes true.

 
module tb;
  reg [3:0] ctr;
  reg clk;
 
  initial begin
    {ctr, clk} <= 0;
 
    wait (ctr);
    $display ("T=%0t Counter reached non-zero value 0x%0h", $time, ctr);
 
    wait (ctr == 4) $display ("T=%0t Counter reached 0x%0h", $time, ctr);
 
    $finish;
  end
 
  always #10 clk = ~clk;
 
  always @ (posedge clk) 
    ctr <= ctr + 1;
 
endmodule
 
Simulation Log
ncsim> run
T=10 Counter reached non-zero value 0x1
T=70 Counter reached 0x4
T=90 Counter reached 0x5
T=170 Counter reached 0x9
Simulation complete via $finish(1) at time 170 NS + 1

Click to try this example in a simulator!   

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