linux-stable-rt/Documentation/i2c/chips/pcf8574

Kernel driver pcf8574
=====================

Supported chips:
  * Philips PCF8574
    Prefix: 'pcf8574'
    Addresses scanned: none
    Datasheet: Publicly available at the Philips Semiconductors website
               http://www.semiconductors.philips.com/pip/PCF8574P.html

 * Philips PCF8574A
    Prefix: 'pcf8574a'
    Addresses scanned: none
    Datasheet: Publicly available at the Philips Semiconductors website
               http://www.semiconductors.philips.com/pip/PCF8574P.html

Authors:
        Frodo Looijaard <frodol@dds.nl>,
        Philip Edelbrock <phil@netroedge.com>,
        Dan Eaton <dan.eaton@rocketlogix.com>,
        Aurelien Jarno <aurelien@aurel32.net>,
        Jean Delvare <khali@linux-fr.org>,


Description
-----------
The PCF8574(A) is an 8-bit I/O expander for the I2C bus produced by Philips
Semiconductors. It is designed to provide a byte I2C interface to up to 16
separate devices (8 x PCF8574 and 8 x PCF8574A).

This device consists of a quasi-bidirectional port. Each of the eight I/Os
can be independently used as an input or output. To setup an I/O as an
input, you have to write a 1 to the corresponding output.

For more informations see the datasheet.


Accessing PCF8574(A) via /sys interface
-------------------------------------

The PCF8574(A) is plainly impossible to detect ! Stupid chip.
So, you have to pass the I2C bus and address of the installed PCF857A
and PCF8574A devices explicitly to the driver at load time via the
force=... parameter.

On detection (i.e. insmod, modprobe et al.), directories are being
created for each detected PCF8574(A):

/sys/bus/i2c/devices/<0>-<1>/
where <0> is the bus the chip was detected on (e. g. i2c-0)
and <1> the chip address ([20..27] or [38..3f]):

(example: /sys/bus/i2c/devices/1-0020/)

Inside these directories, there are two files each:
read and write (and one file with chip name).

The read file is read-only. Reading gives you the current I/O input
if the corresponding output is set as 1, otherwise the current output
value, that is to say 0.

The write file is read/write. Writing a value outputs it on the I/O
port. Reading returns the last written value. As it is not possible
to read this value from the chip, you need to write at least once to
this file before you can read back from it.