VHDL Type Conversions

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This article discusses how to convert between the common VHDL types. The most common VHDL types are std_logic_vector, signed, unsigned, and integer. The std_logic and std_logic_vector types are defined in the standard logic 1164 package of the IEEE library. This package can be imported as follows:

library IEEE;

The signed and unsigned types can be found in the numeric_std package. This package can be imported using the code below. The signed type is represented in two's complement form. The source file for the numeric_std package can be found here. For more information on two's complement, check the Wikipedia page.

use ieee.numeric_std.all

VHDL is a strongly-typed language, and does not have automatic type conversion. As a result, converting between different types is often necessary. The libraries above provide type casts and conversion functions between the common VHDL types. These are depicted by the image below.

How to convert between the most common VHDL types


It is important to note that casting between a std_logic_vector and a signed/unsigned type requires that both signals have the same bit width. Integers do not have a set bit width. Therefore, in the to_signed and to_unsigned conversion functions, note that the first argument is the integer that is being converted and the second argument specifies the bit width of the resulting signed/unsigned type.

It is important to realize that there is no direct conversion between signed and unsigned types. There is also no direct conversion between the std_logic_vector and integer types.

Using these conversions and casts is straightforward. Below is example code illustrating their usage:

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

signal stdlvec : std_logic_vector(31 downto 0);
signal unsgn : unsigned(31 downto 0);
signal sgn : signed(31 downto 0);
signal int : integer;

--Conversion from std_logic_vector to signed and unsigned types
unsgn <= unsigned(stdlvec);
sgn <= signed(stdlvec);

--Conversion from signed/unsigned types back to std_logic_vector
stdlvec <= std_logic_vector(unsgn);
stdlvec <= std_logic_vector(sgn);

--Conversion from signed/unsigned types to integer
int <= to_integer(sgn);
int <= to_integer(unsgn);

--Conversion from integer back to 32-bit signed/unsigned types
unsgn <= to_unsigned(int, 32);
sgn <= to_signed(int, 32);