-- Manchester decoder Chip 
library ieee ;
use ieee.std_logic_1164.all ;
use ieee.std_logic_arith.all ;
entity md is
port (rst: in std_logic ;--復位clk1x,no_bits_rcvd,nrz寄存器
    clk16x: in std_logic ;--用于時鐘，中心采樣
    mdi: in std_logic ;--串行輸入曼碼數據
    rdn : in std_logic ;--控制信號表示可進行一個讀操作
 dout : out std_logic_vector (7 downto 0) ;
 data_ready : out std_logic;--狀態信號，表示數據已準備好，保持在數據總線上等待輸出
    nrz2:    out std_logic;
    sample2: out std_logic;
    nrz3:    out std_logic
) ;
end md ;
architecture v1 of md is
signal clk1x_enable : std_logic ;--使能1x CLOCK接收一個字。高有?
signal mdi1 : std_logic ;
signal mdi2 : std_logic ;--內部寄存器,用于檢測mdi的輸入邊沿，激活clk1x_enable
signal rsr : std_logic_vector (7 downto 0) ;
signal dout_i : std_logic_vector (7 downto 0) ;
signal no_bits_rcvd : unsigned (3 downto 0) ;--控制字大小(位數),及運行譯碼頻率
signal clkdiv : unsigned (3 downto 0) ;
signal nrz : std_logic ;
signal clk1x : std_logic ;
signal sample : std_logic ;--決定接收器對數據進行譯碼的時刻
begin
-- Generate two FF register to accept serial Manchester data in
process (rst,clk16x)
begin
if rst = '1' then
mdi1 <= '0' ;
mdi2 <= '0' ;--
elsif clk16x'event and clk16x = '1' then 
mdi2 <= mdi1 ;
mdi1 <= mdi ;
end if ;
end process ;
-- Enable the clock when an edge on mdi is detected
process (rst,clk16x,mdi1,mdi2,no_bits_rcvd)
begin
if rst = '1' then
clk1x_enable <= '0' ;
elsif clk16x'event and clk16x = '1' then
if mdi1 = '0' and mdi2 = '1' then
clk1x_enable <= '1' ;
else if std_logic_vector(no_bits_rcvd) = "1001" then
clk1x_enable <= '0' ;
end if ;
end if ;
end if ;
end process ;
-- Center sample the data at 1/4 and 3/4 points in data cell
sample <= ((not clkdiv(3)) and (not clkdiv(2)) and clkdiv(1) and clkdiv(0)) or (clkdiv(3) and clkdiv(2) and (not clkdiv(1)) and (not clkdiv(0))) ; 
sample2<=((not clkdiv(3)) and (not clkdiv(2)) and clkdiv(1) and clkdiv(0)) or (clkdiv(3) and clkdiv(2) and (not clkdiv(1)) and (not clkdiv(0)));
--sample;
-- Decode Manchester data into NRZ
process (rst,sample,mdi2,clk16x,no_bits_rcvd)
begin
if rst = '1' then
nrz <= '0' ;
elsif clk16x'event and clk16x = '1' then
if std_logic_vector(no_bits_rcvd) > "000" and sample = '1' then
nrz <= mdi2 xor clk1x ;
nrz2<=mdi2 xor clk1x;
nrz3<=nrz;
end if ;
end if ;
end process ;
-- Increment the clock
process (rst,clk16x,clk1x_enable,clkdiv)
begin
if rst = '1' then
clkdiv <= "0000" ;
elsif clk16x'event and clk16x = '1' then
if clk1x_enable = '1' then
clkdiv <= clkdiv + "0001" ;
end if ;
end if ;
end process ;
clk1x <= clkdiv(3) ;
-- Serial to parallel conversion
process (rst,clk1x,dout_i,nrz)
begin
if rst = '1' then
rsr <= "00000000" ;
elsif clk1x'event and clk1x = '1' then
rsr <= rsr(6 downto 0) & nrz ;
end if ;
end process ;
-- Transfer from shift to data register
process (rst,clk1x,no_bits_rcvd)
begin
if rst = '1' then
dout_i <= "00000000" ;
elsif clk1x'event and clk1x = '1' then
if std_logic_vector(no_bits_rcvd) = "1001" then
dout_i <= rsr ;
end if ;
end if ;
end process ;
-- Track no of bits rcvd for word size 
process (rst,clk1x,clk1x_enable,no_bits_rcvd)
begin
if rst = '1' then 
no_bits_rcvd <= "0000" ;
elsif clk1x'event and clk1x = '1' then
if (clk1x_enable = '0') then
no_bits_rcvd <= "0000" ;
else
no_bits_rcvd <= no_bits_rcvd + "0001" ;
end if ;
end if ;
end process ;
-- Generate data_ready status signal
process (rst,clk1x,clk1x_enable,rdn)
begin
if (rst = '1' or rdn = '0') then
data_ready <= '0' ;
elsif clk1x'event and clk1x = '1' then
if (clk1x_enable = '0') then
data_ready <= '1' ;
else data_ready <= '0' ;
end if ;
end if ;
end process ;
dout <= dout_i ;
end ;