TOPIC : Functions in Verilog

Module : TASKS, Functions and UDPs in Verilog

FunctionsFunctions are declared with the keywords

function and endfunction. Functions are used if all of the following conditions are true for the procedure.

There are no delay, timing, or event control constructs in the procedure.

The procedure returns a single value.There is at least one input argument.

Function featuresWhen a function is declared, a register with

name <name_of_function> is declared implicitly inside Verilog. The output of a function is passed back by setting the value of the register <name_of_function> appropriately.

Notice that at least one input argument must be defined for a function.

Functions cannot invoke other tasks. They can only invoke other functions.

Typical Structure of a Function

Example 1The first example models a parity calculator

that returns a 1-bit value.Let us discuss a function that calculates the

parity of a 32-bit address and returns the value.

We assume even parity. Example 1 shows the definition and

invocation of the function calc-parity. The verilog code is shown in the next page.

Example 1 : Parity calculations

module parity;reg [31:0] addr;reg parity;always @(addr)begin

parity = calc_parity(addr);endfunction calc_parity

input [31:0] address;begincalc_parity = ^address; end

endfunctionendmodule

//Return the xor of all address bits.

//invocation of calc_parity function

Example 2 : left-right shifterTo illustrate how a range for the output value

of a function can be specified, let us consider a function that shifts a 32-bit value to the left or right by one bit, based on a control signal.

Example 2 shows the implementation of the left/right shifter. The verilog code has been shown in next page.

Example 2 : Left-Right Shiftermodule shifter:

'define LEFT-SHIFT l'bO'define RIGHT-SHIFT l'bl

reg [31:0] addr, left-addr, right-addr;reg control;always @ (addr )

begin //call the function defined below to do left and right shift.

left-addr = shift(addr, 'LEFT-SHIFT); right-addr = shift(addr, 'RIGHT-SHIFT);end

function [31: 0] shift; //define shift function. The output is a 32-bit value.input [31:0] address;input control;beginshift = (control == 'LEFT-SHIFT) ?(address << 1) : (address >>

1);endendfunction

endmodule

//Compute the right- and left-shifted values whenever//a new address value appears