VERILOG TASKS & FUNCTIONS

INTRODUCTION
• The Tasks & Functions in Verilog help to reduce
the complexity by breaking up the large
behavioural designs into smaller pieces and make
the overall code simple and clean.
• Tasks and functions allow the designer to abstract
Verilog code that is used at many places in the
design.
• Tasks have input, output, & inout arguments where
as Functions have input arguments. So, values can
be passed into and out from tasks and functions.

contd
• A task is similar to a function, but unlike a
function it has both input and output ports.
• Therefore tasks do not return values. Tasks are
similar to procedures in most programming
languages.
• The purpose of a function is to respond to an input
value by returning a single value.
• A task can support multiple goals and can
calculate multiple result values.
• It cannot be used in an expression. Parameters may
be passed to it and results are returned.

• A Verilog task is similar to a software procedure. It
is called from a calling statement and after
execution, returns to the next statement.
• A task is defined within a module using the
keywords task and endtask and it is called from
within the always statement.
• Local variables may be declared within it and their
scope will be the task.
• The order of task parameters at the calling site must
correspond to the order of definitions within the
task
TASK

contd
• A task may call itself, or be called from tasks that it
has called.
• However, as in a hardware implementation, there is
only one set of registers to hold the task variables.
• Thus, the registers used after the second call to the task are
the same physical entities as those in the previous call(s).
• The simulator maintains the thread of control so that the
returns from a task called multiple times are handled
correctly.
23 June 2020 yayavaram@yahoo.com 5

Basic structure of a task
• module this_task;
task my_task;
input a, b;
inout c;
output d, e;
reg foo1, foo2, foo3;
begin
<statements> // the set of statements that
performs the work of the task
23 June 2020 yayavaram@yahoo.com 6

contd
c = foo1; // the assignments that initialize
d = foo2; // the results variables
e = foo3;
end
endtask
endmodule
23 June 2020 yayavaram@yahoo.com 7

Ex:Task (Procedure)
task average; // task declaration
input a; // declaration of ports
input b; //
output Av ;
wire s ;
reg Av;
assign s = (a+b+c);
assign Av= s / 3;
endtask
23 June 2020 yayavaram@yahoo.com 8

System Tasks
• Verilog provides standard system tasks for certain
routine operations.
• All system tasks appear in the form $<keyword>.
• Operations such as displaying on the screen,
monitoring values of nets, stopping, and finishing
are done by system tasks.
• For ex: $display ; $stop; $finish ; $ monitor etc.
23 June 2020 yayavaram@yahoo.com 9

$display
• $display is the main system task for displaying
values of variables or strings or expressions.
• Usage: $display(s1, s2, s3,.....,sn); s1, s2, s3,..., sn
can be quoted strings or variables or expressions.
• Note: The format of $display is very similar to printf
in C.
• A $display inserts a newline at the end of the string
by default.
• A $display without any arguments produces a
newline.
23 June 2020 yayavaram@yahoo.com 10

Examples
• $display("Hello Verilog World");
-- Hello Verilog World
//To display value of current simulation time 230
• $display($time);
-- 230
//Display the value of port_id 5 in binary
• reg [4:0] port_id;
• $display("ID of the port is %b", port_id);
-- ID of the port is 00101
23 June 2020 yayavaram@yahoo.com 11

contd
• //Display x characters //Display value of 4-bit bus
10xx (signal contention) in binary
• reg [3:0] bus;
• $display("Bus value is %b", bus);
-- Bus value is 10xx.
23 June 2020 yayavaram@yahoo.com 12

Monitor
• Verilog provides a mechanism to monitor a signal
when its value changes and this is provided by the
task $monitor.
• Usage: $monitor(p1,p2,p3,....,pn);The parameters
p1, p2, ... , pn can be variables, signal names, or
quoted strings.
• Only one monitoring list is active at a time. If there
is more than one $monitor statement in the
simulation, the last $monitor will only be active.
• The earlier $monitor statements will be ignored.
23 June 2020 yayavaram@yahoo.com 13

contd
• Two tasks are used to switch the monitoring on and
off.
For ex: $monitoron ; $monitoroff ;
• The task $monitoron enables monitoring, and the
$monitoroff task disables monitoring during a
simulation.
• Monitoring is turned on by default at the beginning
of the simulation and can be controlled during the
simulation with the $monitoron and $monitoroff
tasks.
23 June 2020 yayavaram@yahoo.com 14

$stop
• The task , $stop is used to stop the simulation.
Usage: $stop ;
• The $stop task puts the simulation in an interactive
mode.
• The $stop task is used whenever the designer wish
to suspend the simulation and examine the values of
signals in the design.
• The $finish task terminates or finishes the
simulation.
Usage: $finish;
23 June 2020 yayavaram@yahoo.com 15

Ex: Stop &Finish
// Stop at time 100 in the simulation and examine the
results.// Finish the simulation at time 1000.
initial begin
clock = 0;
reset = 1;
#100 $stop; // suspend the simulation at time = 100
#900 $finish; // terminate the simulation at time = 1000
end
23 June 2020 yayavaram@yahoo.com 16

FUNCTIONS
• Functions are similar to tasks, except that functions
return only a single value to the expression from
which they are called.
• Like tasks, functions provide the ability to execute
common procedures from within a module.
• A function can be invoked from a continuous
assignment statement or from within a procedural
statement and is represented by an operand in an
expression.
23 June 2020 yayavaram@yahoo.com 17

contd
• Functions cannot contain delays, timing, or event
control statements and execute in zero simulation time.
• Although functions can invoke other functions, they are
not recursive.
• Functions cannot invoke a task.
• Functions must have at least one input argument, but
cannot have output or inout arguments.
• A function is invoked from an expression. The function
is invoked by specifying the function name together
with the input parameters. The syntax is as below.
function name (expr1, expr2, . . . , exprN);
23 June 2020 yayavaram@yahoo.com 18

Function Syntax
function [range or type] function name
input declaration
other declarations
begin
statements
end
endfunction
23 June 2020 yayavaram@yahoo.com 19

Function-Example
function mux;
input a, b, c, d;
input [1:0] select;
case (select)
2'b00: mux = a;
2'b01: mux = b;
2'b10: mux = c;
2'b11: mux = d;
default: mux = 'bx;
endcase
endfunction23 June 2020 yayavaram@yahoo.com 20

Function for a half adder
• //module for a half adder using a function
module funct_half_add;
reg a, b;
reg [1:0] sum;
initial
begin
sum = half_add (1'b0, 1'b0);
$display ("a=0, b=0, cout, sum = %b", sum);
sum = half_add (1'b0, 1'b1);
23 June 2020 yayavaram@yahoo.com 21

contd
$display ("a=0, b=1, cout, sum = %b", sum);
sum = half_add (1'b1, 1'b0);
$display ("a=1, b=0, cout, sum = %b", sum);
sum = half_add (1'b1, 1'b1);
$display ("a=1, b=1, cout, sum = %b", sum);
end
function [1:0] half_add;
input a, b;
reg [1:0] sum;
begin23 June 2020 yayavaram@yahoo.com 22

contd
case ({a,b})
2'b00: sum = 2'b00;
2'b01: sum = 2'b01;
2'b10: sum = 2'b01;
2'b11: sum = 2'b10;
default:sum = 2'bxx;
endcase
half_add = sum;
end
endfunction
endmodule
23 June 2020 yayavaram@yahoo.com 23

Differences

23 June 2020 25yayavaram@yahoo.com
THANQ FOR WATCHINIG
GOOD LUCK !!