diff --git a/top.vhd b/top.vhd
index 13c9708..1d0ac76 100644
--- a/top.vhd
+++ b/top.vhd
@@ -3,10 +3,10 @@
 -- Project    : tiny-xo2
 -------------------------------------------------------------------------------
 -- File       : top.vhd
--- Author     : マリオ  <mario.huettel@gmx.net>
+-- Author     : Mario Hüttel  <mario.huettel@gmx.net>
 -- Company    : 
 -- Created    : 2017-11-21
--- Last update: 2017-11-22
+-- Last update: 2017-11-23
 -- Platform   : 
 -- Standard   : VHDL'93/02
 -------------------------------------------------------------------------------
@@ -36,30 +36,37 @@ end entity config_top;
 
 
 architecture RTL of config_top is
+  type fsm_state_t is (IDLE, CMD, REPLY, HANG);
+
+
+
   ----------------- RESET LOGIC ------------------------------
-  signal dts_sync      : std_logic_vector(2 downto 0) := (others => '0');
-  signal rst           : std_logic;     -- reset (high active)
+  signal dts_sync       : std_logic_vector(2 downto 0) := (others => '0');
+  signal rst            : std_logic;    -- reset (high active)
   ----------------- UART RX ----------------------------------
-  signal data_rx       : std_logic_vector(7 downto 0);
-  signal byte_ready_rx : std_logic;
-  signal error_rx      : std_logic;
+  signal data_rx        : std_logic_vector(7 downto 0);
+  signal byte_ready_rx  : std_logic;
+  signal error_rx       : std_logic;
   ----------------- UART TX ----------------------------------
-  signal data_tx       : std_logic_vector(7 downto 0);
-  signal byte_ready_tx : std_logic;
-  signal busy_tx       : std_logic;
-
-
-  signal config_busy : std_logic := '1';  -- indicates that the core is in configuration/waiting mode
-                                          -- and the user logic has to stay in reset
-
+  signal data_tx        : std_logic_vector(7 downto 0);
+  signal byte_ready_tx  : std_logic;
+  signal busy_tx        : std_logic;
+  signal config_busy    : std_logic                    := '1';  -- indicates that the core is in configuration/waiting mode
+  -- and the user logic has to stay in reset
+  signal config_active  : std_logic                    := '1';  -- configuration FSM active
   ---------------------------- Wishbone Bus -------------------------------------
-  signal wb_cyc      : std_logic;
-  signal wb_stb      : std_logic;
-  signal wb_we       : std_logic;
-  signal wb_adr      : std_logic_vector(7 downto 0);
-  signal wb_dat_in   : std_logic_vector(7 downto 0);
-  signal wb_dat_out  : std_logic_vector(7 downto 0);
-  signal wb_ack      : std_logic;
+  signal wb_cyc         : std_logic;
+  signal wb_stb         : std_logic;
+  signal wb_we          : std_logic;
+  signal wb_adr         : std_logic_vector(7 downto 0);
+  signal wb_dat_in      : std_logic_vector(7 downto 0);
+  signal wb_dat_out     : std_logic_vector(7 downto 0);
+  signal wb_ack         : std_logic;
+  -------------------------------- State machine -----------------------------------
+  signal state          : fsm_state_t;
+  signal reply_is_error : std_logic;
+  signal bytecounter    : integer range 0 to 3         := 0;
+
 begin  -- architecture RTL
 
 
@@ -80,7 +87,6 @@ begin  -- architecture RTL
       wb_ack_o    => wb_ack,
       wbc_ufm_irq => open);
 
-
   uart_rx_1 : entity work.uart_rx
     port map (
       clk          => clk,
@@ -108,11 +114,25 @@ begin  -- architecture RTL
       tx           => uart_tx);
 
 
-  timeout_counter_gen : process(clk) is
-    variable cnt : integer range 0 to 40000;
+  -- set config_busy on reset and wait 0.25s. If FSM is not active
+  -- deassert config_busy
+  timeout_counter_busy_gen : process(clk, rst) is
+    variable cnt : integer range 0 to 3000000 := 3000000;  -- 0.25 sec counter
   begin
-  -- TODO: implement timeout counter
-  end process timeout_counter_gen;
+    if rst = '1' then
+      cnt         := 3000000;
+      config_busy <= '1';
+    elsif rising_edge(clk) then
+      config_busy <= '1';
+      if cnt /= 0 then
+        cnt := cnt - 1
+      else
+        if config_active = '0' then
+          config_busy <= '0';
+        end if;
+      end if;
+    end if;
+  end process timeout_counter_busy_gen;
 
 
   -- Generate reset pulse on rising edge of dts input
@@ -141,6 +161,43 @@ begin  -- architecture RTL
   end process user_logic_reset_gen;
 
 
+  controlFSM : process (clk, rst) is
+  begin  -- process controlFSM
+    if rst = '1' then
+      state          <= IDLE;
+      config_active  <= '0';
+      bytecounter    <= 0;
+      reply_is_error <= '0';
+      -- Wishbone
+      wb_cyc         <= '0';
+      wb_stb         <= '0';
+      wb_adr         <= (others => '0');
+      wb_dat_in      <= (others => '0');
+      -- UART TX
+      data_tx        <= (others => '0');
+      byte_ready_tx  <= '0';
+    elsif rising_edge(clk) then
+      case state is
+        when IDLE =>
+          if byte_ready_rx = '1' and data_rx = x"D5" then  -- Serial byte received. Is it the preambel?
+            state       <= CMD;
+            bytecounter <= 0;
+          end if;
+        when CMD =>
+          if byte_ready_rx = '1' then
+          -- TODO: Byte received. Do something
+          end if;
+        when REPLY =>
+          -- TODO: if reply_is_error = '1' then: wait for response that
+          -- indicates that the error is handled. Then switch back to command
+          -- mode. Else; Just wait for response to be sent.
+          null;
+        when HANG =>
+          state <= HANG;                -- End of FSM. Only way out is a reset
+      end case;
+    end if;
+  end process controlFSM;
+
 
   ------ Asynchronous output assignments ---------------