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library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity tmds_encoder is
port (
clk : in std_logic;
sys_rst_n : in std_logic;
blank : in std_logic;
ctrl : in std_logic_vector(1 downto 0);
din : in std_logic_vector(7 downto 0);
dout : out std_logic_vector(9 downto 0)
);
end tmds_encoder;
architecture beh of tmds_encoder is
signal n_ones_din : integer range 0 to 8;
signal xored, xnored : std_logic_vector(8 downto 0);
signal q_m : std_logic_vector(8 downto 0);
-- a positive value represents the excess number of 1's that have been transmitted
-- a negative value represents the excess number of 0's that have been transmitted
signal disparity : signed(3 downto 0) := to_signed(0, 4);
-- difference between 1's and 0's (/2 since the last bit is never used)
signal diff : signed(3 downto 0) := to_signed(0, 4);
begin
-- ones counter for input data
process(din) is
variable c : integer range 0 to 8;
begin
c := 0;
for i in 0 to 7 loop
if din(i) = '1' then
c := c + 1;
end if;
end loop;
n_ones_din <= c;
end process;
-- create xor encodings
xored(0) <= din(0);
encode_xor : for i in 1 to 7 generate
begin
xored(i) <= din(i) xor xored(i - 1);
end generate;
xored(8) <= '1';
-- create xnor encodings
xnored(0) <= din(0);
encode_xnor : for i in 1 to 7 generate
begin
xnored(i) <= din(i) xnor xnored(i - 1);
end generate;
xnored(8) <= '0';
-- use xnored or xored data based on the ones
q_m <= xnored when n_ones_din > 4 or (n_ones_din = 4 and din(0) = '0') else xored;
-- ones counter for internal data
process(q_m) is
variable c : integer range 0 to 8;
begin
c := 0;
for i in 0 to 7 loop
if q_m(i) = '1' then
c := c + 1;
end if;
end loop;
diff <= to_signed(c-4, 4);
end process;
process(clk) is
begin
if rising_edge(clk) then
if blank = '1' then
case ctrl is
when "00" => dout <= "1101010100";
when "01" => dout <= "0010101011";
when "10" => dout <= "0101010100";
when others => dout <= "1010101011";
end case;
disparity <= (others => '0');
else
if disparity = 0 or diff = 0 then
-- xnored data
if q_m(8) = '0' then
dout <= "10" & not q_m(7 downto 0);
disparity <= disparity - diff;
-- xored data
else
dout <= "01" & q_m(7 downto 0);
disparity <= disparity + diff;
end if;
elsif (diff(diff'left) = '0' and disparity(disparity'left) = '0') or
(diff(diff'left) = '1' and disparity(disparity'left) = '1') then
dout <= '1' & q_m(8) & not q_m(7 downto 0);
if q_m(8) = '1' then
disparity <= disparity + 1 - diff;
else
disparity <= disparity - diff;
end if;
else
dout <= '0' & q_m;
if q_m(8) = '1' then
disparity <= disparity + diff;
else
disparity <= disparity - 1 + diff;
end if;
end if;
end if;
end if;
end process;
end beh;
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