-- entity declaration for a BCD counter with an 
-- asynchronous reset input.  
-- (reworked from solution to Ashenden problem 5.26)
LIBRARY ieee;  USE ieee.std_logic_1164.all;

architecture spec of bcd_counter is
begin 
   count : process (clock, reset) is
      variable next_q : std_logic_vector(3 downto 0) := "0000";
      variable carry : std_logic;
   begin
      if reset = '1' then
         next_q := "0000";
         q <= "0000";
      elsif en = '1' and rising_edge(clock) then
         carry := '1';  -- to increment
         for n in 0 to 3 loop
            if next_q(n) = '0' then
              next_q(n) := carry;
              carry := '0';
            else 
              next_q(n) := not carry;
            end if;
         end loop;
      -- check for values 10 and over and make them 0
         if next_q(3) = '1' and (next_q(2) = '1' or next_q(1) = '1') then
            next_q := "0000";
            sync <= '1';
         else
            sync <= '0';
         end if;
         q <= next_q;
      end if;
   end process count;
end architecture spec;

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