numeric_std standard library

Type conversion with the numeric_std library

It is necessary to use an external package (preferably standardized) that defines the operators. This is the role of the numeric_std library. It will then be easy to switch from one format to another.

• to_integer : Transforms a vector of type signed or unsigned to integer
• to_unsigned : Transforms a signal of type integer, into a vector of type unsigned. Also allows to extend the format of an unsigned vector
• to_signed : Same property as the src_vhdl[:exports code]{to_unsigned} function for signed numbers.

integer_Signal <= to_integer(unsigned_Signal);
integer_Signal <= to_integer(signed_Signal);
unsigned_Signal <= to_unsigned(a_natural_signal , length_of_unsigned_Signal);
unsigned_Signal <= to_unsigned(an_integer_signal , length_of_unsigned_Signal);
signed_Signal   <= to_signed(another_natural_signal   , length_of_signed_Signal);
signed_Signal   <= to_signed(another_integer_signal   , length_of_signed_Signal);
--
signal B : signed(3 downto 0) ; -- 4 bits width
A < = to_unsigned(B , 5) ;  -- A is 5 bits wide

Numeral assignement with numeric_std

signal AU : unsigned(3 downto 0); -- 4 bits
signal BU : unsigned(3 downto 0); -- 4 bits
signal SU : unsigned(7 downto 0); -- 8 bits
signal AS : signed(3 downto 0);
signal BS : signed(3 downto 0);
signal SS : signed(7 downto 0);
--
AU <= to_unsigned(15, 4);
BU <= to_unsigned(15, 4);
AS <= to_signed(7, 4);
BS <= to_signed(-8, 4);
AI <= -16;
BI <= -16;

Useful function from numeric_std

resize

The resize function is used to expand the size of an unsigned or a signed signal. This function is signed aware and will expand the MSB to preserve the signe of the signal. This function is particularly useful when doing arithmetic to manage overflow.

signal S : signed(11 downto 0);
signal A : signed 5 downto 0);

...

S <= resize(A, 12);  -- resize A to be on 12 bits