Tasks and functions are used to reduce code repetition. If in your project you need to do something many times it is better to use a task or a function that will reduce code writing and it will be more readable.

Task

Tasks are subroutines that can be called anytime in the module they are defined, but it is possible to define them in a different file and include the file in the module. Some charachteristics of tasks are:

• Task can include time delays, functions can’t.
• Tasks can have any number of inputs and outputs, functions can have only one output.
• If a variable is declared within the task it is local to the task and can’t be used outside the task.
• Tasks can drive global variables if no local variables are declared.
• Tasks are called with statements and cannot be used in a expression, functions can be used in a expression.

Task example:

module  task_calling (adc_a, adc_b, adc_a_conv, adc_b_conv);
input [7:0] adc_a, adc_b;
output [7:0] adc_a_conv, adc_b_conv;

reg [7:0] adc_a_conv, adc_b_conv;

task convert;
input [7:0] adc_in;
output [7:0] out;
begin
    out = (9/5) *( adc_in + 32)
end
endtask

always @ (adc_a)
begin
  convert (adc_a, adc_a_conv);
end

always @ (adc_b)
begin
  convert (adc_b, adc_b_conv);
end

endmodule

Functions

Functions are like tasks, with some differences. Functions cannot drive more than one output and cannot have time delays. Some differences are:

• Functions cannot include timing delays, like posedge, negedge, simulation delay, which means that functions implement combitional logic.
• Functions can have any number of inputs but only one output.
• The variables declared within the function are local to that function.
• The order of declaration within the function are considered and have to be the same as the caller.
• Functions can use and modify global variables, when no local variables are used.
• Functions can call other functions, but cannot call tasks. Function example:

module  function_calling(a, b, c, d, e, f);

input a, b, c, d, e ;
output f;
wire f;

function  myfunction;
input a, b, c, d;
begin
    myfunction = ((a+b) + (c-d));
end
endfunction

assign f =  (myfunction (a,b,c,d)) ? e :0;

endmodule

References

Verilog HDL Basics - Altera, ASIC World (http://www.asic-world.com/)