70 lines
2.7 KiB
Plaintext
70 lines
2.7 KiB
Plaintext
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 you have to pass the I2C
|
|
bus and address of the installed PCF8575 devices explicitly to the driver at
|
|
load time via the force=... parameter.
|
|
|
|
/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.
|