add8.vhd

出租车计费器,VHDL实现

LIBRARY  IEEE;
USE  IEEE.STD_LOGIC_1164.ALL;
USE  IEEE.STD_LOGIC_UNSIGNED.ALL; 
ENTITY add8 IS
    PORT(
           cin:IN STD_LOGIC;
          a,b :IN STD_LOGIC_VECTOR(7 DOWNTO 0);
            s :OUT STD_LOGIC_VECTOR(7 DOWNTO 0);
          cout:OUT STD_LOGIC);
END  add8;

ARCHITECTURE  struc  OF add8  IS
   COMPONENT  add4
        PORT(
           cin:IN STD_LOGIC;
          a,b :IN STD_LOGIC_VECTOR(3 DOWNTO 0);
            s :OUT STD_LOGIC_VECTOR(3 DOWNTO 0);
          cout:OUT STD_LOGIC);
   END COMPONENT;
   SIGNAL carry_out:STD_LOGIC;
BEGIN
   U1:add4 PORT MAP(cin,a(3 DOWNTO 0),b(3 DOWNTO 0),s(3 DOWNTO 0),carry_out);
   U2:add4 PORT MAP(carry_out,a(7 DOWNTO 4),b(7 DOWNTO 4),s(7 DOWNTO 4),cout);
END struc;