Hi everyone,

Today i will show you how to create a state machine and implementation of that. 
Most important thing in designing at state machine is that lines.

type state_type is (zero, one, two, three, four, five, six, seven, eight, nine);
signal state : state_type;

My state are from zero to nine.
After the implementing this code you can get the state diagram.

library ieee;
use ieee.std_logic_1164.all;

entity BCD_counter is
generic(N : integer := 4);
port
(
clk         : in    std_logic;
reset        : in    std_logic;
y     : inout std_logic_vector((N-1) downto 0);
segment1: out std_logic_vector (6 downto 0)    );

end entity;

architecture bevahioral of BCD_counter is

type state_type is (zero, one, two, three, four, five, six, seven, eight, nine);
signal state : state_type;
signal temp : std_logic_vector ((N-1) downto 0);

begin

process (clk, reset)
begin

if reset = '1' then
state <= zero ;
temp <= "0000";

elsif (rising_edge(clk)) then

case state is

when zero =>
temp <= "0000";
state <= one;

when one =>
temp <= "0001";
state <= two;

when two =>
temp <= "0010";
state <= three;

when three =>
temp <= "0011";
state <=four ;

when four =>
temp <= "0100";
state <=five ;

when five =>
temp <= "0101";
state <=six ;

when six =>
temp <= "0110";
state <=seven ;

when seven =>
temp <= "0111";
state <=eight ;

when eight =>
temp <= "1000";
state <=nine ;

when others =>
temp <="1001";
state <=zero ;

end case;

end if;
end process;

y <= temp;

process(y)
begin
case y is
when "0000" => segment1 <= "1000000";
when "0001" => segment1 <= "1001111";
when "0010" => segment1 <= "0100100";
when "0011" => segment1 <= "0110000";
when "0100" => segment1 <= "0011001";
when "0101" => segment1 <= "0100100";
when "0110" => segment1 <= "0000010";
when "0111" => segment1 <= "0111000";
when "1000" => segment1 <= "0000000";
when "1001" => segment1 <= "0011000";
when "1010" => segment1 <= "0100000";
when "1011" => segment1 <= "0000011";
when "1100" => segment1 <= "1000110";
when "1101" => segment1 <= "0100001";
when "1110" => segment1 <= "0000110";
when others => segment1 <= "0001110";
end case;

end process;

end bevahioral;
--ozturkgokhan.com

After implementing the code use this path to get state machine diagram “Tools>Netlist Viewers> State Machine Viewer”

State Machine Diagram

Modelsim Output

best wishes,
gökhan öztürk