-三人表決器（三種不同的描述方式） vhdl

-- Three-input Majority Voter
-- The entity declaration is followed by three alternative architectures whichachieve the same functionality in different ways.

ENTITY maj IS
PORT(a,b,c : IN BIT; m : OUT BIT);
END maj;
--Dataflow style architecture
ARCHITECTURE concurrent OF maj IS
BEGIN
--selected signal assignment statement (concurrent)
WITH a&b&c SELECT
m <= '1' WHEN"110"|"101"|"011"|"111",'0' WHENOTHERS;
END concurrent;

--Structural style architecture
ARCHITECTURE structure OF maj IS

--declare components used in architecture
COMPONENT and2 PORT(in1, in2 : IN BIT; out1 : OUT BIT);
END COMPONENT;
COMPONENT or3 PORT(in1, in2, in3 : IN BIT; out1 : OUT BIT);
END COMPONENT;
--declare local signals
SIGNAL w1, w2, w3 : BIT;

BEGIN
--component instantiation statements.
--ports of component are mapped to signals
--within architecture by position.
gate1 : and2 PORT MAP (a, b, w1);
gate2 : and2 PORT MAP (b, c, w2);
gate3 : and2 PORT MAP (a, c, w3);
gate4 : or3 PORT MAP (w1, w2, w3, m);
END structure;

--Behavioural style architecture using a look-up table
ARCHITECTURE using_table OF maj IS
BEGIN
PROCESS(a,b,c)
CONSTANT lookuptable : BIT_VECTOR(0 TO 7) := "00010111";
VARIABLE index : NATURAL;
BEGIN
index := 0; --index must be cleared each time process executes
IF a = '1' THEN index := index + 1; END IF;
IF b = '1' THEN index := index + 2; END IF;
IF c = '1' THEN index := index + 4; END IF;
m <= lookuptable(index);
END PROCESS;
END using_table;