...
    -- SMBUS bus signals
    sda : inout std_logic;
    scl : inout std_logic;
...
  -- set SDA and SCL
    sda <= '0' when sda_oe = '1' else 'Z';
    scl <= '0' when scl_out_reg = '0' else 'Z';
...
  -- sda_oe is set when master and arbitration is not lost and data to be output = 0 or
  -- when slave and data to be output is 0
    sda_oe <= '1' when ((master_slave = '1' and arb_lost = '0' and sda_out_reg = '0') or
                 (master_slave = '0' and slave_sda = '0')
                  or stop_scl_reg = '1') else '0';