library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;
use ieee.numeric_std.all;

-- Quadcounter (for IRC)

entity qcounter is
  port
	(
    clk      : in std_logic;
    reset    : in std_logic;
    a0, b0   : in std_logic;

    qcount   : out std_logic_vector (31 downto 0);
    a_rise, a_fall, b_rise, b_fall, ab_event: out std_logic;
    ab_error : out std_logic
  );
end qcounter;

architecture behavioral of qcounter is
  component dff
	port
	(
		clk   : in std_logic;
		reset : in std_logic;
		d     : in std_logic;
		q     : out std_logic
	);
  end component;

subtype std_logic4 is std_logic_vector (3 downto 0);
  signal a, b, a_prev, b_prev: std_logic;
  signal count_prev: std_logic_vector (29 downto 0);
  signal count: std_logic_vector (29 downto 0);
begin
	dff_a: dff
	port map
	(
		clk => clk,
		reset => reset,
		d => a0,
		q => a
	);

  dff_b: dff
	port map
	(
		clk => clk,
		reset => reset,
		d => b0,
		q => b
	);

	qcount(0) <= a xor b;
	qcount(1) <= b;
	qcount(31 downto 2) <= count;

  comb_event: process (reset, a_prev, b_prev, a, b)
  begin
    a_rise <= '0';
    a_fall <= '0';
    b_rise <= '0';
    b_fall <= '0';
    ab_event <= '0';
    ab_error <= '0';

		if reset = '0' then
			if ((a xor a_prev) and (b xor b_prev)) = '1' then
				-- forbidden double transition
				ab_error <= '1';
			else
				a_rise <= (a xor a_prev) and a;
				a_fall <= (a xor a_prev) and not a;
				b_rise <= (b xor b_prev) and b;
				b_fall <= (b xor b_prev) and not b;
				ab_event <= (a xor a_prev) or (b xor b_prev);
			end if;
		end if;
  end process;

  comb_count: process (reset, a_prev, b_prev, a, b, count, count_prev)
  begin
		if reset = '1' then
			count <= count_prev;
		elsif (a_prev = '0') and (b_prev = '1') and (a = '0') and (b = '0') then
      count <= count_prev + 1;
    elsif (a_prev = '0') and (b_prev = '0') and (a = '0') and (b = '1') then
      count <= count_prev - 1;
    else
      count <= count_prev;
    end if;
  end process;

  seq: process (clk, reset, a, b)
  begin
    if reset = '1' then
			count_prev <= (others => '0');
		end if;

		if clk = '1' and clk'event then
			if reset = '0' then
				count_prev <= count;
			end if;
			a_prev <= a;
			b_prev <= b;
		end if;
  end process;

end behavioral;