i2c vhdl example code

LIBRARY ieee;
USE ieee.std_logic_1164.all;
USE ieee.std_logic_unsigned.all;

ENTITY i2c_master IS
  GENERIC(
    input_clk : INTEGER := 50_000_000; --input clock speed from user logic in Hz
    bus_clk   : INTEGER := 400_000);   --speed the i2c bus (scl) will run at in Hz
  PORT(	
    clk       : IN     STD_LOGIC;                    --system clock
    reset_n   : IN     STD_LOGIC;                    --active low reset
    ena       : IN     STD_LOGIC;                    --latch in command
    addr      : IN     STD_LOGIC_VECTOR(6 DOWNTO 0); --address of target slave
    rw        : IN     STD_LOGIC;                    --'0' is write, '1' is read
    data_wr   : IN     STD_LOGIC_VECTOR(7 DOWNTO 0); --data to write to slave
    busy      : OUT    STD_LOGIC;                    --indicates transaction in progress
    data_rd   : OUT    STD_LOGIC_VECTOR(7 DOWNTO 0); --data read from slave
    ack_error : BUFFER STD_LOGIC;                    --flag if improper acknowledge from slave
    sda       : INOUT  STD_LOGIC;                    --serial data output of i2c bus
    scl       : INOUT  STD_LOGIC);                   --serial clock output of i2c bus
END i2c_master;

ARCHITECTURE logic OF i2c_master IS
  CONSTANT divider  :  INTEGER := (input_clk/bus_clk)/4; --number of clocks in 1/4 cycle of scl
  TYPE machine IS(ready, start, command, slv_ack1, wr, rd, slv_ack2, mstr_ack, stop); --needed states
  SIGNAL state         : machine;                        --state machine
  SIGNAL data_clk      : STD_LOGIC;                      --data clock for sda
  SIGNAL data_clk_prev : STD_LOGIC;                      --data clock during previous system clock
  SIGNAL scl_clk       : STD_LOGIC;                      --constantly running internal scl
  SIGNAL scl_ena       : STD_LOGIC := '0';               --enables internal scl to output
  SIGNAL sda_int       : STD_LOGIC := '1';               --internal sda
  SIGNAL sda_ena_n     : STD_LOGIC;                      --enables internal sda to output
  SIGNAL addr_rw       : STD_LOGIC_VECTOR(7 DOWNTO 0);   --latched in address and read/write
  SIGNAL data_tx       : STD_LOGIC_VECTOR(7 DOWNTO 0);   --latched in data to write to slave
  SIGNAL data_rx       : STD_LOGIC_VECTOR(7 DOWNTO 0);   --data received from slave
  SIGNAL bit_cnt       : INTEGER RANGE 0 TO 7 := 7;      --tracks bit number in transaction
  SIGNAL stretch       : STD_LOGIC := '0';               --identifies if slave is stretching scl
BEGIN

  --generate the timing for the bus clock (scl_clk) and the data clock (data_clk)
  PROCESS(clk, reset_n)
    VARIABLE count  :  INTEGER RANGE 0 TO divider*4;  --timing for clock generation
  BEGIN
    IF(reset_n = '0') THEN                --reset asserted
      stretch <= '0';
      count := 0;
    ELSIF(clk'EVENT AND clk = '1') THEN
      data_clk_prev <= data_clk;          --store previous value of data clock
      IF(count = divider*4-1) THEN        --end of timing cycle
        count := 0;                       --reset timer
      ELSIF(stretch = '0') THEN           --clock stretching from slave not detected
        count := count + 1;               --continue clock generation timing
      END IF;
      CASE count IS
        WHEN 0 TO divider-1 =>            --first 1/4 cycle of clocking
          scl_clk <= '0';
          data_clk <= '0';
        WHEN divider TO divider*2-1 =>    --second 1/4 cycle of clocking
          scl_clk <= '0';
          data_clk <= '1';
        WHEN divider*2 TO divider*3-1 =>  --third 1/4 cycle of clocking
          scl_clk <= '1';                 --release scl
          IF(scl = '0') THEN              --detect if slave is stretching clock
            stretch <= '1';
          ELSE
            stretch <= '0';
          END IF;
          data_clk <= '1';
        WHEN OTHERS =>                    --last 1/4 cycle of clocking
          scl_clk <= '1';
          data_clk <= '0';
      END CASE;
    END IF;
  END PROCESS;