5864ae03ca
Signed-off-by: Bart Van Assche <bart.vanassche@gmail.com> Signed-off-by: Jean Delvare <khali@linux-fr.org>
72 lines
2.9 KiB
Text
72 lines
2.9 KiB
Text
About the PCF8575 chip and the pcf8575 kernel driver
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====================================================
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The PCF8575 chip is produced by the following manufacturers:
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* Philips NXP
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http://www.nxp.com/#/pip/cb=[type=product,path=50807/41735/41850,final=PCF8575_3]|pip=[pip=PCF8575_3][0]
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* Texas Instruments
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http://focus.ti.com/docs/prod/folders/print/pcf8575.html
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Some vendors sell small PCB's with the PCF8575 mounted on it. You can connect
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such a board to a Linux host via e.g. an USB to I2C interface. Examples of
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PCB boards with a PCF8575:
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* SFE Breakout Board for PCF8575 I2C Expander by RobotShop
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http://www.robotshop.ca/home/products/robot-parts/electronics/adapters-converters/sfe-pcf8575-i2c-expander-board.html
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* Breakout Board for PCF8575 I2C Expander by Spark Fun Electronics
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http://www.sparkfun.com/commerce/product_info.php?products_id=8130
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Description
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-----------
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The PCF8575 chip is a 16-bit I/O expander for the I2C bus. Up to eight of
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these chips can be connected to the same I2C bus. You can find this
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chip on some custom designed hardware, but you won't find it on PC
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motherboards.
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The PCF8575 chip consists of a 16-bit quasi-bidirectional port and an I2C-bus
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interface. Each of the sixteen I/O's can be independently used as an input or
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an output. To set up an I/O pin as an input, you have to write a 1 to the
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corresponding output.
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For more information please see the datasheet.
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Detection
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---------
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There is no method known to detect whether a chip on a given I2C address is
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a PCF8575 or whether it is any other I2C device. So there are two alternatives
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to let the driver find the installed PCF8575 devices:
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- Load this driver after any other I2C driver for I2C devices with addresses
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in the range 0x20 .. 0x27.
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- Pass the I2C bus and address of the installed PCF8575 devices explicitly to
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the driver at load time via the probe=... or force=... parameters.
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/sys interface
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--------------
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For each address on which a PCF8575 chip was found or forced the following
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files will be created under /sys:
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* /sys/bus/i2c/devices/<bus>-<address>/read
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* /sys/bus/i2c/devices/<bus>-<address>/write
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where bus is the I2C bus number (0, 1, ...) and address is the four-digit
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hexadecimal representation of the 7-bit I2C address of the PCF8575
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(0020 .. 0027).
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The read file is read-only. Reading it will trigger an I2C read and will hence
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report the current input state for the pins configured as inputs, and the
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current output value for the pins configured as outputs.
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The write file is read-write. Writing a value to it will configure all pins
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as output for which the corresponding bit is zero. Reading the write file will
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return the value last written, or -EAGAIN if no value has yet been written to
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the write file.
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On module initialization the configuration of the chip is not changed -- the
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chip is left in the state it was already configured in through either power-up
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or through previous I2C write actions.
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