84_4.15.txt

网络通信中的MII接口 通常将4位nibble数据送出

--clk是clk2的两倍

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;



entity eighttofour is
port (
clk: IN STD_LOGIC;  --8 bit clock
TXC: IN std_logic;
reset: in std_logic;
data_in: in STD_LOGIC_VECTOR (7 DOWNTO 0) ;
TXEN: in std_logic;      --buffer
TXD: out STD_LOGIC_VECTOR (3 DOWNTO 0) ); 
end eighttofour;

architecture Behavioral of eighttofour is
signal aa : STD_LOGIC_VECTOR(3 Downto 0);
signal bb : STD_LOGIC_VECTOR(3 Downto 0);
signal cc : STD_LOGIC_VECTOR(3 Downto 0);
signal count : STD_LOGIC;
begin



process(TXC, reset)
begin
if (reset = '0') then
  count <= '0';
elsif (TXC'event and TXC ='1') then
 count<= not count;
end if;
end process;

process(clk, reset,TXEN)

begin 
if (reset = '0') then
  aa <= (others => '0');
  bb <= (others => '0');
  
elsif (clk'event and clk ='1') then
if(TXEN='1') then
bb(3)  <= data_in(7);
bb(2)  <= data_in(6);
bb(1)  <= data_in(5);
bb(0)  <= data_in(4); 
aa(3)  <= data_in(3);
aa(2)  <= data_in(2);
aa(1)  <= data_in(1);
aa(0)  <= data_in(0);
end if;
end if;
end process;





process(TXC)
begin
if (reset = '0') then
 cc<= (others => '0');
elsif (TXC'event and TXC ='1') then
if (count  = '0') then
cc<=aa;
else cc <=bb;
end if;
end if;


end process;
TXD<=cc;
end Behavioral;