Verilog Task

A function is meant to do some processing on the input and return a single value, whereas a task is more general and can calculate multiple result values and return them using output and inout type arguments. Tasks can contain simulation time consuming elements such as @, posedge and others. There are two ways in which tasks can be written, which we'll see next.

Syntax

  
  
	// Style 1
	task [name];
		input  [port_list];
		inout  [port_list];
		output [port_list];
		begin
			[statements]
		end
	endtask

	// Style 2
	task [name] (input [port_list], inout [port_list], output [port_list]);
		begin
			[statements]
		end
	endtask

  

The keyword automatic will make the task reentrant, otherwise it will be static by default. If a task is static, then all its member variables will be shared across different invocations of the same task that has been launched to run concurrently. Note that auomatic task items cannot be accessed by hierarchical references.

Calling a task

If the task does not need any arguments, then a list of arguments can be avoided. If the task needs arguments, they can be provided in the same statement as its invocation.

  
  
	task sum (input [7:0] a, b, output [7:0] c);
		begin
			c = a + b;
		end
	endtask
// or 	
	task sum;
		input  [7:0] a, b;
		output [7:0] c;
		begin
			c = a + b;
		end
	endtask
	
	
	initial begin
		reg [7:0] x, y , z;
		sum (x, y, z);          
	end

  

The task-enabling arguments (x, y, z) correspond to the arguments (a, b, c) defined by the task. Since a and b are inputs, values of x and y will be placed in a and b respectively. Because c is declared as an output and connected with z during invocation, the sum will automatically be passed to the variable z from c.

Global tasks

Tasks that are declared outside all modules are called global tasks as they have a global scope and can be called within any module.

  
  
// This task is outside all modules
task display();
  $display("Hello World !");
endtask

module des;
  initial begin
    display();
  end
endmodule

  

xcelium> run
Hello World !
xmsim: *W,RNQUIE: Simulation is complete.

Difference between function and task