FPGA设计：如何用半加器和全加器构成四位全加器

今天来分享一下关于FPGA设计的文章，如何用半加器和全加器构成四位全加器~

首先，我们看一位半加器的代码:
1.一位半加器的程序代码及ＲＴＬ图

library ieee;
use ieee.std_logic_1164.all;

entity halfadd is 
	port(a,b:in std_logic;
		s,c:out std_logic);
		end;
		
architecture rtl of halfadd is
begin 
	s<=a xor b; 
	c<=a and b;
end;

再看一位全加器的程序代码：
2.一位全加器的程序代码及ＲＴＬ图

library ieee;
use ieee.std_logic_1164.all;

entity fulladd is
	port(a,b,cin:in std_logic;
		ss,cc:out std_logic);
		end;
	
architecture rtl of fulladd is
component halfadd is 
	port(a,b:in std_logic;
		s,c:out std_logic);
		end component;
signal s1,c1,c2:std_logic;
begin 
	u1:halfadd port map(a,b,s1,c1);
	u2:halfadd port map(s1,cin,ss,c2);
	cc<=c1 or c2;
	end;

最后结合上面，我们来看四位全加器的程序：
3.四位全加器的程序

library ieee;
use ieee.std_logic_1164.all;

entity full4add is
	port(a,b:in std_logic_vector(3 downto 0);
		sum:out std_logic_vector(3 downto 0);
		carry:out std_logic);
		end;
		
architecture rtl of full4add is
component halfadd is 
	port(a,b:in std_logic;
		s,c:out std_logic);
		end component;
component fulladd is
	port(a,b,cin:in std_logic;
		ss,cc:out std_logic);
		end component;
signal cc0,cc1,cc2:std_logic;
begin
	u1:halfadd port map(a(0),b(0),sum(0),cc0);
	u2:fulladd port map(cc0,a(1),b(1),sum(1),cc1);
	u3:fulladd port map(cc1,a(2),b(2),sum(2),cc2);
	u4:fulladd port map(cc2,a(3),b(3),sum(3),carry);
	end;

这里最后我们只用全加器来看看如何实现四位全加器：
4.只用全加器实现四位全加器

library ieee;
use ieee.std_logic_1164.all;

entity full4add is
	port(a,b:in std_logic_vector(3 downto 0);
		sum:out std_logic_vector(3 downto 0);
		carry:out std_logic);
		end;
		
architecture rtl of full4add is
component halfadd is 
	port(a,b:in std_logic;
		s,c:out std_logic);
		end component;
component fulladd is
	port(a,b,cin:in std_logic;
		ss,cc:out std_logic);
		end component;
signal cc0,cc1,cc2:std_logic;
begin
	u1:fulladd port map('0',a(0),b(0),sum(0),cc0);
	u2:fulladd port map(cc0,a(1),b(1),sum(1),cc1);
	u3:fulladd port map(cc1,a(2),b(2),sum(2),cc2);
	u4:fulladd port map(cc2,a(3),b(3),sum(3),carry);
	end;

好了，分享就到这里了~

本人水平有限，上述信息仅供学习参考，如有错误和不妥之处，请多多指教。
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