VHDL strukturalny opis funkcji XNOR 3 zmiennych z użyciem komponentu LUT4

Cytat:

// The INIT parameter for the FPGA LUT primitive is what gives the LUT its
// logical value. By default this value is zero thus driving the output to a
// zero regardless of the input values (acting as a ground) however in most
// cases an new INIT value must be determined in order to specify the logic
// function for the LUT primitive. There are a few methods in which the LUT
// value can be determined and two of those methods will be discussed here.
//
// The Truth Table Method
// ----------------------
//
// A common method to determine the desired INIT value for a LUT is using a
// truth table. To do so, simply create a binary truth table of all possible
// inputs, specify the desired logic value of the output and then create the
// INIT string from those output values. An example is shown below:
//
// Example of determining an XOR INIT equation for a LUT4:
//
// _________________
// | I3 I2 I1 I0 | O |
// |-----------------|
// | 0 0 0 0 | 0 |\
// | 0 0 0 1 | 1 | \ = 4'b0110 = 4'h6 ---------------+
// | 0 0 1 0 | 1 | / |
// | 0 0 1 1 | 0 |/ |
// |-------------|---| |
// | 0 1 0 0 | 1 |\ |
// | 0 1 0 1 | 0 | \ = 4'b1001 = 4'h9 |
// | 0 1 1 0 | 0 | / |
// | 0 1 1 1 | 1 |/ |
// |-------------|---| INIT = 16'h6996
// | 1 0 0 0 | 1 |\ |
// | 1 0 0 1 | 0 | \ = 4'b0110 = 4'h9 |
// | 1 0 1 0 | 0 | / |
// | 1 0 1 1 | 1 |/ |
// |-------------|---| |
// | 1 1 0 0 | 0 |\ |
// | 1 1 0 1 | 1 | \ = 4'b1001 = 4'h6 ------------+
// | 1 1 1 0 | 1 | /
// | 1 1 1 1 | 0 |/
// -------------------

Cytat:

VHDL Instantiation Template for LUT4

-- Component Declaration for LUT4 should be placed
-- after architecture statement but before begin keyword
component LUT4
-- synthesis translate_off
generic (
INIT : bit_vector := X"16");
-- synthesis translate_on
port (O : out STD_ULOGIC;
IO : in STD_ULOGIC;
I1 : in STD_ULOGIC;
I2 : in STD_ULOGIC;
I3 : in STD_ULOGIC);
end component;
-- Component Attribute specification for LUT4
-- should be placed after architecture declaration but
-- before the begin keyword
attribute INIT : string;
attribute INIT of LUT4_instance_name : label is "16";
-- values can be 0 through 16
-- Component Instantiation for LUT4 should be placed
-- in architecture after the begin keyword
LUT4_INSTANCE_NAME : LUT4
-- synthesis translate_off
generic map (
INIT => hex_value)
-- synthesis translate_on
port map (O => user_O,
I0 => user_I0,
I1 => user_I1,
I2 => user_I2,
I3 => user_I3);