ad0809的VHDL驱动程序,START如何启动.最好有注释.

LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;

ENTITY ADC0809 IS
PORT( CLK: IN STD_LOGIC;
DIN: IN STD_LOGIC_VECTOR( 7 DOWNTO 0);
START: OUT STD_LOGIC;
ALE: OUT STD_LOGIC; ---地址信号的锁存信号
OE: OUT STD_LOGIC; --允许三态输出控制线
EOC: IN STD_LOGIC; --转换标志
Q: OUT STD_LOGIC_VECTOR( 7 DOWNTO 0);
ADDA: OUT STD_LOGIC; --最低通道控制，本来是有三个地址，在此只给出一个
LOCK0: OUT STD_LOGIC); --数据锁存时钟
END;

ARCHITECTURE HBV OF ADC0809 IS
TYPE STATE IS ( ST0, ST1, ST2, ST3, ST4);
SIGNAL CURRENT_STATE,NEXT_STATE: STATE := ST0;
SIGNAL REAL: STD_LOGIC_VECTOR( 7 DOWNTO 0);
SIGNAL LOCK: STD_LOGIC;
BEGIN
REG: PROCESS( CLK )
BEGIN
IF CLK'EVENT AND CLK='1' THEN
CURRENT_STATE <= NEXT_STATE;
END IF;
END PROCESS REG;
COM: PROCESS( CURRENT_STATE,EOC)
BEGIN
CASE CURRENT_STATE IS