Type casting and conversion in VHDL

Introduction

Type casting and conversion let you change data from one type to another so you can use it where needed.

When you want to add a number stored as an integer to one stored as a real number.

When you need to display a number as text on a screen.

When you want to convert a bit vector to an integer to do math.

When you receive data in one format but your code needs it in another.

When you want to compare values of different types.

Syntax

VHDL

target_type(expression)

Use the target type as a function with the expression inside parentheses.

Make sure the conversion makes sense, or you may get errors.

Examples

Convert a bit vector to an unsigned type, then to an integer.

VHDL

integer_value := to_integer(unsigned(bit_vector_value));

Convert an integer to a real number.

VHDL

real_value := real(integer_value);

Convert an integer to an unsigned, then to a bit vector of fixed length.

VHDL

bit_vector_value := std_logic_vector(to_unsigned(integer_value, bit_vector_length));

Sample Program

This program converts a 4-bit vector "1010" to an integer, then to a real number, and prints both values.

VHDL

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity TypeConversionDemo is
end entity;

architecture Behavioral of TypeConversionDemo is
  signal bit_vec : std_logic_vector(3 downto 0) := "1010";
  signal int_val : integer;
  signal real_val : real;
begin
  process
  begin
    -- Convert bit vector to integer
    int_val <= to_integer(unsigned(bit_vec));
    wait for 10 ns;
    -- Convert integer to real
    real_val <= real(int_val);
    wait for 10 ns;
    -- Print results (simulation output)
    report "Integer value: " & integer'image(int_val);
    report "Real value: " & real'image(real_val);
    wait;
  end process;
end Behavioral;

Important Notes

Use ieee.numeric_std library for safe conversions between std_logic_vector and numeric types.

Direct casting between incompatible types without proper conversion functions will cause errors.

Remember that converting from real to integer truncates the decimal part.

Summary

Type casting changes data from one type to another to use it properly.

Use conversion functions like to_integer, to_unsigned, and real() in VHDL.

Always include the right libraries for numeric conversions.