Function declaration and usage in VHDL - Mini Project: Build & Apply

Function declaration and usage in VHDL

📖 Scenario: You are designing a simple digital circuit in VHDL that needs to calculate the square of an integer input.

🎯 Goal: Build a VHDL design that declares a function to calculate the square of an integer and uses it in a process to assign the squared value to an output signal.

📋 What You'll Learn

Declare a function named square that takes an integer input and returns its square as an integer.

Create a signal named input_val of type integer and set it to 5.

Create a signal named output_val of type integer to hold the squared result.

Use the square function inside a process to assign the squared value of input_val to output_val.

💡 Why This Matters

🌍 Real World

Functions in VHDL help reuse code for calculations like squaring numbers, which is common in digital signal processing and arithmetic circuits.

💼 Career

Understanding function declaration and usage in VHDL is essential for FPGA and ASIC design engineers who write reusable and modular hardware descriptions.

1

Declare the square function

Write a VHDL function named square that takes an integer parameter num and returns the square of num as an integer.

VHDL

-- Declare the function square here
-- Your code here

Hint

Think of the function as a small machine that takes a number and gives back its square.

2

Create input and output signals

Declare two signals: input_val of type integer and set it to 5, and output_val of type integer without initial value.

VHDL

function square(num : integer) return integer is
begin
    return num * num;
end function;

-- Declare input_val and output_val signals here
-- Your code here

Hint

Signals are like wires that hold values in VHDL. Set input_val to 5.

3

Use the square function in a process

Write a process that assigns the result of square(input_val) to output_val.

VHDL

function square(num : integer) return integer is
begin
    return num * num;
end function;

signal input_val : integer := 5;
signal output_val : integer;

-- Write a process to assign square(input_val) to output_val
-- Your code here

Hint

The process runs once and assigns the squared value to output_val. Use wait; to stop the process.

4

Display the output_val

Add a report statement to print the value of output_val inside the process.

VHDL

function square(num : integer) return integer is
begin
    return num * num;
end function;

signal input_val : integer := 5;
signal output_val : integer;

process
begin
    output_val <= square(input_val);
    -- Add a report statement to display output_val
    -- Your code here
    wait;
end process;

Hint

Use report with integer'image to convert the integer to text for printing.

Practice

1. What is the main purpose of a function in VHDL?

easy

A. To return a value based on input parameters

B. To declare signals inside an architecture

C. To define the entity interface

D. To instantiate components

Solution

1. Step 1: Understand function role

   Functions in VHDL are used to perform calculations or operations and return a result.

2. Step 2: Identify correct purpose

   Among the options, only returning a value based on inputs matches the function's role.

3. Final Answer:

   To return a value based on input parameters -> Option A

4. Quick Check:

   Function returns value = B [OK]

Hint: Functions always return a value based on inputs [OK]

Common Mistakes:

• Confusing functions with procedures
• Thinking functions declare signals
• Mixing entity and function roles

2. Which of the following is the correct way to declare a function in VHDL that returns an integer?

easy

A. function add(a : integer; b : integer) return integer is
begin
a + b;
end function;

B. function add(a : integer; b : integer) is
begin
return a + b;
end function;

C. function add(a : integer; b : integer) return integer is
begin
return a + b;
end function;

D. function add(a : integer; b : integer) return integer is
begin
return a - b;
end function;

Solution

1. Step 1: Check function syntax

   Function must specify return type after parameters and use 'return' keyword in body.

2. Step 2: Validate each option

   function add(a : integer; b : integer) return integer is begin return a + b; end function; correctly declares return type and returns sum. function add(a : integer; b : integer) is begin return a + b; end function; misses return type. function add(a : integer; b : integer) return integer is begin a + b; end function; misses 'return' keyword. function add(a : integer; b : integer) return integer is begin return a - b; end function; returns difference, not sum.

3. Final Answer:

   function add(a : integer; b : integer) return integer is begin return a + b; end function; -> Option C

4. Quick Check:

   Correct syntax with return = C [OK]

Hint: Always specify return type and use 'return' keyword [OK]

Common Mistakes:

• Omitting return type
• Forgetting 'return' keyword
• Returning wrong expression

3. Given the function declaration:

function multiply(x : integer; y : integer) return integer is
begin
  return x * y;
end function;

What is the output of the following code snippet?

signal result : integer;
result <= multiply(3, 4);

medium

A. Error

B. 7

C. 34

D. 12

Solution

1. Step 1: Understand function behavior

   The function multiply returns the product of x and y.

2. Step 2: Calculate multiply(3, 4)

   3 multiplied by 4 equals 12.

3. Final Answer:

   12 -> Option D

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply inputs to get output value [OK]

Common Mistakes:

• Adding instead of multiplying
• Concatenating numbers as strings
• Assuming syntax error

4. Identify the error in this VHDL function declaration:

function increment(value : integer) return integer is
begin
  value := value + 1;
  return value;
end function;

medium

A. Missing return type declaration

B. Cannot assign to function parameter 'value'

C. Function must not have parameters

D. Missing 'is' keyword

Solution

1. Step 1: Check parameter usage

   Function parameters in VHDL are constants and cannot be assigned new values.

2. Step 2: Identify error line

   The line 'value := value + 1;' tries to assign a new value to parameter 'value', which is illegal.

3. Final Answer:

   Cannot assign to function parameter 'value' -> Option B

4. Quick Check:

   Parameters are constant = A [OK]

Hint: Function parameters are read-only, no assignment allowed [OK]

Common Mistakes:

• Trying to modify input parameters
• Confusing functions with procedures
• Ignoring VHDL constant parameter rule

5. You want to create a function in VHDL that returns the maximum of two integers. Which of the following function declarations correctly implements this?

hard

A. function max_val(a : integer; b : integer) return integer is
begin
if a > b then
return a;
else
return b;
end if;
end function;

B. function max_val(a : integer; b : integer) return integer is
begin
if a < b then
return a;
else
return b;
end if;
end function;

C. function max_val(a : integer; b : integer) return integer is
begin
return a + b;
end function;

D. function max_val(a : integer; b : integer) return integer is
begin
if a = b then
return 0;
else
return a;
end if;
end function;

Solution

1. Step 1: Understand max function logic

   The function should return the larger of the two inputs.

2. Step 2: Analyze each option

   function max_val(a : integer; b : integer) return integer is begin if a > b then return a; else return b; end if; end function; returns 'a' if a > b, else 'b' -- correct max logic. function max_val(a : integer; b : integer) return integer is begin if a < b then return a; else return b; end if; end function; returns smaller value. function max_val(a : integer; b : integer) return integer is begin return a + b; end function; returns sum, not max. function max_val(a : integer; b : integer) return integer is begin if a = b then return 0; else return a; end if; end function; returns 0 if equal, else 'a', which is incorrect.

3. Final Answer:

   function max_val(a : integer; b : integer) return integer is begin if a > b then return a; else return b; end if; end function; -> Option A

4. Quick Check:

   Return larger value = D [OK]

Hint: Return 'a' if greater, else 'b' for max function [OK]

Common Mistakes:

• Returning smaller value
• Returning sum instead of max
• Incorrect equality handling