About the PCF8575 chip and the pcf8575 kernel driver
====================================================

The PCF8575 chip is produced by the following manufacturers:

  * Philips NXP
    http://www.nxp.com/#/pip/cb=[type=product,path=50807/41735/41850,final=PCF8575_3]|pip=[pip=PCF8575_3][0]

  * Texas Instruments
    http://focus.ti.com/docs/prod/folders/print/pcf8575.html


Some vendors sell small PCB's with the PCF8575 mounted on it. You can connect
such a board to a Linux host via e.g. an USB to I2C interface. Examples of
PCB boards with a PCF8575:

  * SFE Breakout Board for PCF8575 I2C Expander by RobotShop
    http://www.robotshop.ca/home/products/robot-parts/electronics/adapters-converters/sfe-pcf8575-i2c-expander-board.html

  * Breakout Board for PCF8575 I2C Expander by Spark Fun Electronics
    http://www.sparkfun.com/commerce/product_info.php?products_id=8130


Description
-----------
The PCF8575 chip is a 16-bit I/O expander for the I2C bus. Up to eight of
these chips can be connected to the same I2C bus. You can find this
chip on some custom designed hardware, but you won't find it on PC
motherboards.

The PCF8575 chip consists of a 16-bit quasi-bidirectional port and an I2C-bus
interface. Each of the sixteen I/O's can be independently used as an input or
an output. To set up an I/O pin as an input, you have to write a 1 to the
corresponding output.

For more information please see the datasheet.


Detection
---------

There is no method known to detect whether a chip on a given I2C address is
a PCF8575 or whether it is any other I2C device. So there are two alternatives
to let the driver find the installed PCF8575 devices:
- Load this driver after any other I2C driver for I2C devices with addresses
  in the range 0x20 .. 0x27.
- Pass the I2C bus and address of the installed PCF8575 devices explicitly to
  the driver at load time via the probe=... or force=... parameters.

/sys interface
--------------

For each address on which a PCF8575 chip was found or forced the following
files will be created under /sys:
* /sys/bus/i2c/devices/<bus>-<address>/read
* /sys/bus/i2c/devices/<bus>-<address>/write
where bus is the I2C bus number (0, 1, ...) and address is the four-digit
hexadecimal representation of the 7-bit I2C address of the PCF8575
(0020 .. 0027).

The read file is read-only. Reading it will trigger an I2C read and will hence
report the current input state for the pins configured as inputs, and the
current output value for the pins configured as outputs.

The write file is read-write. Writing a value to it will configure all pins
as output for which the corresponding bit is zero. Reading the write file will
return the value last written, or -EAGAIN if no value has yet been written to
the write file.

On module initialization the configuration of the chip is not changed -- the
chip is left in the state it was already configured in through either power-up
or through previous I2C write actions.