VHDL Type Conversion?

Question

How do you convert different types in VHDL? What are some examples?

Answer 1

VHDL type conversion is called casting. It's done by writing the type we want to cast to followed by the object we want to cast in parentheses.

Example :

signal a : std_logic_vector ( 7 downto 0 ) ;
signal b : signed ( 7 downto 0 ) ;
signal c : unsigned ( 7 downto 0 ) ;

signal unsigned_result : unsigned ( 7 downto 0 ) ;
signal signed_result : signed ( 7 downto 0 ) ;

unsigned_result <= unsigned ( a ) + unsigned ( b ) + c ; -- a and b require casting
signed_result <= signed ( a ) + b + signed ( c ) ; -- a and c require casting

Note : Not all types are castable from one to another.

Answer 2

The type of an object defines the kind of values it can have and the operations that can be done on it. Type conversion is an important concept to learn because usually when two objects are used together, they need to be of the exact same type.

The syntax for type conversion is as follows:

Type(expression);

Type is not a keyword, it is a generic word used for the purpose of explaining the syntax. In real code, the word type is replaced by the keyword of the type to which the expression needs to be converted. For example:

y_val := Integer(90.4 * real(x_val));

In the above code, suppose that intially both variables, x_val, and y_val are of integer type. Now look at the code carefully and it can be seen that two type conversions are being done. Firstly, real(x_val) means that x_val is converted to a real number so that it can be multiplied by 90.4.

The result of multiplication, the real number is again converted to integer type so that it can be assigned to integer type variable y_val. This is done by: Integer(90.4 * real(x_val)).