--VHDL code for a 8 bit counter that counts in Gray code
--Date Created: 8/16/2012
--Creator/copyright: Joanne E. DeGroat
--Description:  This counter starts at "00000000" amd counts
--   in Gray Code with the count progressing on the rising edge
--   of input signal clk.  clk is the only input and the 
--   output is a 8-bit bit_vector in signal cnt.
--

ENTITY cnt8 IS                          
 PORT(clk : IN bit;                     
      cnt : OUT bit_vector(7 downto 0)); 
END cnt8;                               
                                        
ARCHITECTURE one OF cnt8 IS             
  SIGNAL state,next_state : bit_vector(7 downto 0) := "00000000";
BEGIN                                   
  --Latching logic specification        
  PROCESS                               
  BEGIN                                 
    WAIT UNTIL clk='1' AND clk'event;   
    state <= next_state;                
  END PROCESS;                          
                                        
  --Assign outputs                      
  cnt <= state;  
     
 --Next State Logic      
 PROCESS (state)                        
 BEGIN                                  
   CASE state IS                        
     WHEN ("00000000") =>
       next_state <= "00000001";
     WHEN ("00000001") =>
       next_state <= "00000011";
     WHEN ("00000011") =>
       next_state <= "00000010";
     WHEN ("00000010") =>
       next_state <= "00000110";
     WHEN ("00000110") =>
       next_state <= "00000111";
     WHEN ("00000111") =>
       next_state <= "00000101";
     WHEN ("00000101") =>
       next_state <= "00000100";
     WHEN ("00000100") =>
       next_state <= "00001100";
     WHEN ("00001100") =>
       next_state <= "00001101";
     WHEN ("00001101") =>
       next_state <= "00001111";
     WHEN ("00001111") =>
       next_state <= "00001110";
     WHEN ("00001110") =>
       next_state <= "00001010";
     WHEN ("00001010") =>
       next_state <= "00001011";
     WHEN ("00001011") =>
       next_state <= "00001001";
     WHEN ("00001001") =>
       next_state <= "00001000";
     WHEN ("00001000") =>
       next_state <= "00011000";
     WHEN ("00011000") =>
       next_state <= "00011001";
     WHEN ("00011001") =>
       next_state <= "00011011";
     WHEN ("00011011") =>
       next_state <= "00011010";
     WHEN ("00011010") =>
       next_state <= "00011110";
     WHEN ("00011110") =>
       next_state <= "00011111";
     WHEN ("00011111") =>
       next_state <= "00011101";
     WHEN ("00011101") =>
       next_state <= "00011100";
     WHEN ("00011100") =>
       next_state <= "00010100";
     WHEN ("00010100") =>
       next_state <= "00010101";
     WHEN ("00010101") =>
       next_state <= "00010111";
     WHEN ("00010111") =>
       next_state <= "00010110";
     WHEN ("00010110") =>
       next_state <= "00010010";
     WHEN ("00010010") =>
       next_state <= "00010011";
     WHEN ("00010011") =>
       next_state <= "00010001";
     WHEN ("00010001") =>
       next_state <= "00010000";
     WHEN ("00010000") =>
       next_state <= "00110000";
     WHEN ("00110000") =>
       next_state <= "00110001";
     WHEN ("00110001") =>
       next_state <= "00110011";
     WHEN ("00110011") =>
       next_state <= "00110010";
     WHEN ("00110010") =>
       next_state <= "00110110";
     WHEN ("00110110") =>
       next_state <= "00110111";
     WHEN ("00110111") =>
       next_state <= "00110101";
     WHEN ("00110101") =>
       next_state <= "00110100";
     WHEN ("00110100") =>
       next_state <= "00111100";
     WHEN ("00111100") =>
       next_state <= "00111101";
     WHEN ("00111101") =>
       next_state <= "00111111";
     WHEN ("00111111") =>
       next_state <= "00111110";
     WHEN ("00111110") =>
       next_state <= "00111010";
     WHEN ("00111010") =>
       next_state <= "00111011";
     WHEN ("00111011") =>
       next_state <= "00111001";
     WHEN ("00111001") =>
       next_state <= "00111000";
     WHEN ("00111000") =>
       next_state <= "00101000";
     WHEN ("00101000") =>
       next_state <= "00101001";
     WHEN ("00101001") =>
       next_state <= "00101011";
     WHEN ("00101011") =>
       next_state <= "00101010";
     WHEN ("00101010") =>
       next_state <= "00101110";
     WHEN ("00101110") =>
       next_state <= "00101111";
     WHEN ("00101111") =>
       next_state <= "00101101";
     WHEN ("00101101") =>
       next_state <= "00101100";
     WHEN ("00101100") =>
       next_state <= "00100100";
     WHEN ("00100100") =>
       next_state <= "00100101";
     WHEN ("00100101") =>
       next_state <= "00100111";
     WHEN ("00100111") =>
       next_state <= "00100110";
     WHEN ("00100110") =>
       next_state <= "00100010";
     WHEN ("00100010") =>
       next_state <= "00100011";
     WHEN ("00100011") =>
       next_state <= "00100001";
     WHEN ("00100001") =>
       next_state <= "00100000";
     WHEN ("00100000") =>
       next_state <= "01100000";
     WHEN ("01100000") =>
       next_state <= "01100001";
     WHEN ("01100001") =>
       next_state <= "01100011";
     WHEN ("01100011") =>
       next_state <= "01100010";
     WHEN ("01100010") =>
       next_state <= "01100110";
     WHEN ("01100110") =>
       next_state <= "01100111";
     WHEN ("01100111") =>
       next_state <= "01100101";
     WHEN ("01100101") =>
       next_state <= "01100100";
     WHEN ("01100100") =>
       next_state <= "01101100";
     WHEN ("01101100") =>
       next_state <= "01101101";
     WHEN ("01101101") =>
       next_state <= "01101111";
     WHEN ("01101111") =>
       next_state <= "01101110";
     WHEN ("01101110") =>
       next_state <= "01101010";
     WHEN ("01101010") =>
       next_state <= "01101011";
     WHEN ("01101011") =>
       next_state <= "01101001";
     WHEN ("01101001") =>
       next_state <= "01101000";
     WHEN ("01101000") =>
       next_state <= "01111000";
     WHEN ("01111000") =>
       next_state <= "01111001";
     WHEN ("01111001") =>
       next_state <= "01111011";
     WHEN ("01111011") =>
       next_state <= "01111010";
     WHEN ("01111010") =>
       next_state <= "01111110";
     WHEN ("01111110") =>
       next_state <= "01111111";
     WHEN ("01111111") =>
       next_state <= "01111101";
     WHEN ("01111101") =>
       next_state <= "01111100";
     WHEN ("01111100") =>
       next_state <= "01110100";
     WHEN ("01110100") =>
       next_state <= "01110101";
     WHEN ("01110101") =>
       next_state <= "01110111";
     WHEN ("01110111") =>
       next_state <= "01110110";
     WHEN ("01110110") =>
       next_state <= "01110010";
     WHEN ("01110010") =>
       next_state <= "01110011";
     WHEN ("01110011") =>
       next_state <= "01110001";
     WHEN ("01110001") =>
       next_state <= "01110000";
     WHEN ("01110000") =>
       next_state <= "01010000";
     WHEN ("01010000") =>
       next_state <= "01010001";
     WHEN ("01010001") =>
       next_state <= "01010011";
     WHEN ("01010011") =>
       next_state <= "01010010";
     WHEN ("01010010") =>
       next_state <= "01010110";
     WHEN ("01010110") =>
       next_state <= "01010111";
     WHEN ("01010111") =>
       next_state <= "01010101";
     WHEN ("01010101") =>
       next_state <= "01010100";
     WHEN ("01010100") =>
       next_state <= "01011100";
     WHEN ("01011100") =>
       next_state <= "01011101";
     WHEN ("01011101") =>
       next_state <= "01011111";
     WHEN ("01011111") =>
       next_state <= "01011110";
     WHEN ("01011110") =>
       next_state <= "01011010";
     WHEN ("01011010") =>
       next_state <= "01011011";
     WHEN ("01011011") =>
       next_state <= "01011001";
     WHEN ("01011001") =>
       next_state <= "01011000";
     WHEN ("01011000") =>
       next_state <= "01001000";
     WHEN ("01001000") =>
       next_state <= "01001001";
     WHEN ("01001001") =>
       next_state <= "01001011";
     WHEN ("01001011") =>
       next_state <= "01001010";
     WHEN ("01001010") =>
       next_state <= "01001110";
     WHEN ("01001110") =>
       next_state <= "01001111";
     WHEN ("01001111") =>
       next_state <= "01001101";
     WHEN ("01001101") =>
       next_state <= "01001100";
     WHEN ("01001100") =>
       next_state <= "01000100";
     WHEN ("01000100") =>
       next_state <= "01000101";
     WHEN ("01000101") =>
       next_state <= "01000111";
     WHEN ("01000111") =>
       next_state <= "01000110";
     WHEN ("01000110") =>
       next_state <= "01000010";
     WHEN ("01000010") =>
       next_state <= "01000011";
     WHEN ("01000011") =>
       next_state <= "01000001";
     WHEN ("01000001") =>
       next_state <= "01000000";
     WHEN ("01000000") =>
       next_state <= "11000000";
     WHEN ("11000000") =>
       next_state <= "11000001";
     WHEN ("11000001") =>
       next_state <= "11000011";
     WHEN ("11000011") =>
       next_state <= "11000010";
     WHEN ("11000010") =>
       next_state <= "11000110";
     WHEN ("11000110") =>
       next_state <= "11000111";
     WHEN ("11000111") =>
       next_state <= "11000101";
     WHEN ("11000101") =>
       next_state <= "11000100";
     WHEN ("11000100") =>
       next_state <= "11001100";
     WHEN ("11001100") =>
       next_state <= "11001101";
     WHEN ("11001101") =>
       next_state <= "11001111";
     WHEN ("11001111") =>
       next_state <= "11001110";
     WHEN ("11001110") =>
       next_state <= "11001010";
     WHEN ("11001010") =>
       next_state <= "11001011";
     WHEN ("11001011") =>
       next_state <= "11001001";
     WHEN ("11001001") =>
       next_state <= "11001000";
     WHEN ("11001000") =>
       next_state <= "11011000";
     WHEN ("11011000") =>
       next_state <= "11011001";
     WHEN ("11011001") =>
       next_state <= "11011011";
     WHEN ("11011011") =>
       next_state <= "11011010";
     WHEN ("11011010") =>
       next_state <= "11011110";
     WHEN ("11011110") =>
       next_state <= "11011111";
     WHEN ("11011111") =>
       next_state <= "11011101";
     WHEN ("11011101") =>
       next_state <= "11011100";
     WHEN ("11011100") =>
       next_state <= "11010100";
     WHEN ("11010100") =>
       next_state <= "11010101";
     WHEN ("11010101") =>
       next_state <= "11010111";
     WHEN ("11010111") =>
       next_state <= "11010110";
     WHEN ("11010110") =>
       next_state <= "11010010";
     WHEN ("11010010") =>
       next_state <= "11010011";
     WHEN ("11010011") =>
       next_state <= "11010001";
     WHEN ("11010001") =>
       next_state <= "11010000";
     WHEN ("11010000") =>
       next_state <= "11110000";
     WHEN ("11110000") =>
       next_state <= "11110001";
     WHEN ("11110001") =>
       next_state <= "11110011";
     WHEN ("11110011") =>
       next_state <= "11110010";
     WHEN ("11110010") =>
       next_state <= "11110110";
     WHEN ("11110110") =>
       next_state <= "11110111";
     WHEN ("11110111") =>
       next_state <= "11110101";
     WHEN ("11110101") =>
       next_state <= "11110100";
     WHEN ("11110100") =>
       next_state <= "11111100";
     WHEN ("11111100") =>
       next_state <= "11111101";
     WHEN ("11111101") =>
       next_state <= "11111111";
     WHEN ("11111111") =>
       next_state <= "11111110";
     WHEN ("11111110") =>
       next_state <= "11111010";
     WHEN ("11111010") =>
       next_state <= "11111011";
     WHEN ("11111011") =>
       next_state <= "11111001";
     WHEN ("11111001") =>
       next_state <= "11111000";
     WHEN ("11111000") =>
       next_state <= "11101000";
     WHEN ("11101000") =>
       next_state <= "11101001";
     WHEN ("11101001") =>
       next_state <= "11101011";
     WHEN ("11101011") =>
       next_state <= "11101010";
     WHEN ("11101010") =>
       next_state <= "11101110";
     WHEN ("11101110") =>
       next_state <= "11101111";
     WHEN ("11101111") =>
       next_state <= "11101101";
     WHEN ("11101101") =>
       next_state <= "11101100";
     WHEN ("11101100") =>
       next_state <= "11100100";
     WHEN ("11100100") =>
       next_state <= "11100101";
     WHEN ("11100101") =>
       next_state <= "11100111";
     WHEN ("11100111") =>
       next_state <= "11100110";
     WHEN ("11100110") =>
       next_state <= "11100010";
     WHEN ("11100010") =>
       next_state <= "11100011";
     WHEN ("11100011") =>
       next_state <= "11100001";
     WHEN ("11100001") =>
       next_state <= "11100000";
     WHEN ("11100000") =>
       next_state <= "10100000";
     WHEN ("10100000") =>
       next_state <= "10100001";
     WHEN ("10100001") =>
       next_state <= "10100011";
     WHEN ("10100011") =>
       next_state <= "10100010";
     WHEN ("10100010") =>
       next_state <= "10100110";
     WHEN ("10100110") =>
       next_state <= "10100111";
     WHEN ("10100111") =>
       next_state <= "10100101";
     WHEN ("10100101") =>
       next_state <= "10100100";
     WHEN ("10100100") =>
       next_state <= "10101100";
     WHEN ("10101100") =>
       next_state <= "10101101";
     WHEN ("10101101") =>
       next_state <= "10101111";
     WHEN ("10101111") =>
       next_state <= "10101110";
     WHEN ("10101110") =>
       next_state <= "10101010";
     WHEN ("10101010") =>
       next_state <= "10101011";
     WHEN ("10101011") =>
       next_state <= "10101001";
     WHEN ("10101001") =>
       next_state <= "10101000";
     WHEN ("10101000") =>
       next_state <= "10111000";
     WHEN ("10111000") =>
       next_state <= "10111001";
     WHEN ("10111001") =>
       next_state <= "10111011";
     WHEN ("10111011") =>
       next_state <= "10111010";
     WHEN ("10111010") =>
       next_state <= "10111110";
     WHEN ("10111110") =>
       next_state <= "10111111";
     WHEN ("10111111") =>
       next_state <= "10111101";
     WHEN ("10111101") =>
       next_state <= "10111100";
     WHEN ("10111100") =>
       next_state <= "10110100";
     WHEN ("10110100") =>
       next_state <= "10110101";
     WHEN ("10110101") =>
       next_state <= "10110111";
     WHEN ("10110111") =>
       next_state <= "10110110";
     WHEN ("10110110") =>
       next_state <= "10110010";
     WHEN ("10110010") =>
       next_state <= "10110011";
     WHEN ("10110011") =>
       next_state <= "10110001";
     WHEN ("10110001") =>
       next_state <= "10110000";
     WHEN ("10110000") =>
       next_state <= "10010000";
     WHEN ("10010000") =>
       next_state <= "10010001";
     WHEN ("10010001") =>
       next_state <= "10010011";
     WHEN ("10010011") =>
       next_state <= "10010010";
     WHEN ("10010010") =>
       next_state <= "10010110";
     WHEN ("10010110") =>
       next_state <= "10010111";
     WHEN ("10010111") =>
       next_state <= "10010101";
     WHEN ("10010101") =>
       next_state <= "10010100";
     WHEN ("10010100") =>
       next_state <= "10011100";
     WHEN ("10011100") =>
       next_state <= "10011101";
     WHEN ("10011101") =>
       next_state <= "10011111";
     WHEN ("10011111") =>
       next_state <= "10011110";
     WHEN ("10011110") =>
       next_state <= "10011010";
     WHEN ("10011010") =>
       next_state <= "10011011";
     WHEN ("10011011") =>
       next_state <= "10011001";
     WHEN ("10011001") =>
       next_state <= "10011000";
     WHEN ("10011000") =>
       next_state <= "10001000";
     WHEN ("10001000") =>
       next_state <= "10001001";
     WHEN ("10001001") =>
       next_state <= "10001011";
     WHEN ("10001011") =>
       next_state <= "10001010";
     WHEN ("10001010") =>
       next_state <= "10001110";
     WHEN ("10001110") =>
       next_state <= "10001111";
     WHEN ("10001111") =>
       next_state <= "10001101";
     WHEN ("10001101") =>
       next_state <= "10001100";
     WHEN ("10001100") =>
       next_state <= "10000100";
     WHEN ("10000100") =>
       next_state <= "10000101";
     WHEN ("10000101") =>
       next_state <= "10000111";
     WHEN ("10000111") =>
       next_state <= "10000110";
     WHEN ("10000110") =>
       next_state <= "10000010";
     WHEN ("10000010") =>
       next_state <= "10000011";
     WHEN ("10000011") =>
       next_state <= "10000001";
     WHEN ("10000001") =>
       next_state <= "10000000";
     WHEN ("10000000") =>
       next_state <= "00000000";
   END CASE;                            
 END PROCESS;                           
     
END one;