Verilog elementary tutorial (18) functions and tasks in Verilog

2020-11-13 10:25:08

verilog elementary tutorial functions tasks

List of articles

Preface

Functions and tasks are common syntax in simulation , But fair use can also be used in design , It can be integrated .

Text

function

A lot of times we find that some code is repetitive , stay RTL Is called multiple times in . Most of them don't consume simulation time , Complex calculations may involve , It needs to be done with different data values . under these circumstances , We can declare a function , Put the duplicate code in the function , Let it return the result . This will greatly reduce RTL The number of lines in , Because now all you need to do is make a function call , And pass the data that needs to be calculated on it . in fact , This is related to C The functions in the language are very similar .

The purpose of the function is to return a value to be used in an expression . A function definition is always defined by keywords function Start , And then Return type 、 name And in brackets Port list . When Verilog find endfunction When a keyword , I know that a function definition is over . Be careful , A function must declare at least one input , If the function doesn't return anything , The return type is void.

grammar

function [automatic] [return_type] name ([port_list]);
[statements]
endfunction

keyword automatic Will cause the function to be reentrant , Items declared in tasks are dynamically assigned , Instead of sharing between different calls to a task . This is for recursive functions , And when the same function is forked by N Concurrent processes , Will be very useful .

Function declaration

There are two ways to declare the input to a function .

function [7:0] sum;
input [7:0] a, b;
begin
sum = a + b;
end
endfunction
function [7:0] sum (input [7:0] a, b);
begin
sum = a + b;
end
endfunction

Returns a value from a function

The function definition implicitly creates an internal variable with the same name as the function . therefore , It is illegal to declare another variable with the same name in the scope of a function . The return value is initialized by assigning the result of the function to an internal variable .

sum = a + b;

Function call

A function call is an operand with an expression , The syntax is shown in the figure below .

reg [7:0] result;
reg [7:0] a, b;
initial begin
a = 4;
b = 5;
#10 result = sum (a, b);
end

Function rules

Function cannot contain any time controlled statements , Such as #, @, wait, posedge, negedge
Function cannot call task , Because it may consume simulation time , But you can call other functions .
Function should have at least one input
Function cannot have a nonblocking assignment or force…release or assign…deassign.
Function cannot have any triggers
Functions cannot have output perhaps inout

Mission

The function is used to process input , And return a single value , And tasks are more general , It can calculate multiple result values , And use output and inout Parameters of type return . Tasks can include @、posedge Simulation time-consuming elements . There are two ways to write a task , We're going to see .

grammar


// 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

keyword automatic Will re-enter the task , Otherwise it will default to static . If a task is static , Then all its member variables will be shared in different calls of the same task , The task has been started to run concurrently . Be careful ,auomatic Task items cannot be accessed through hierarchical references .

Task call

If the task does not require any parameters , So you can avoid using parameter lists . If the task requires parameters , You can provide parameters in the same statement when calling a task .


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

Task enabling parameters （x,y,z） Parameters corresponding to the task definition （a,b,c）. because a and b It's input ,x and y The values of will be placed in a and b in . because c Is declared as output , And in the call process with z Connect , So the sum will automatically be from c Pass to variable z in .

Overall task

Tasks declared outside of all modules are called global tasks , Because they have global scope , It can be invoked in any module .


// This task is outside all modules
task display();
$display("Hello World !");
endtask
module des;
initial begin
display();
end
endmodule

Simulation results ：

Hello World !

The difference between functions and tasks

function	Mission
There can't be time control statements / Delay , So it is executed in the same simulation time unit .	Can contain time control statements / Delay , And only at other times .
Unable to enable task	Other tasks and functions can be implemented
There must be at least one input , Functions cannot have output perhaps inout.	You can have zero or more parameters of any type .
Only one value can be returned	Cannot return a value , But you can use output parameters to achieve the same effect .