MCP23017 Notes

  Supply Voltage      1.8v - 5.5v
  Supply current typ  1mA
  Standby cur.        1-3uA
  Input Lo            0.15 x Vdd
  Input Hi            0.8 x Vdd
  Output Lo           0.6v
  Output Hi           Vdd - 0.7v
  Max src/sink I      25mA

   IODIR     1 = Input Pin  0 = Output Pin
   IPOL      1 = Input is inverted
   GPINTEN   1 = Interrupt enabled on that Pin
   GPIO      read gets vlaue on port / write changes OLAT reg
   GPPU      1 = Pull-Up resistor enabled
   OLAT      read gets value on o/p latches
             write sets latches which sets those pins set to output
   IOCON     BANK 1 adds are porta 0x00 - 0x0A,
                                portb 0x10 - 0x1A
             BANK 0 A/B addresses interleved (default)
             SEQOP = 0 for byte mode