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| 1 | About the PCF8575 chip and the pcf8575 kernel driver | ||
| 2 | ==================================================== | ||
| 3 | |||
| 4 | The PCF8575 chip is produced by the following manufacturers: | ||
| 5 | |||
| 6 | * Philips NXP | ||
| 7 | http://www.nxp.com/#/pip/cb=[type=product,path=50807/41735/41850,final=PCF8575_3]|pip=[pip=PCF8575_3][0] | ||
| 8 | |||
| 9 | * Texas Instruments | ||
| 10 | http://focus.ti.com/docs/prod/folders/print/pcf8575.html | ||
| 11 | |||
| 12 | |||
| 13 | Some vendors sell small PCB's with the PCF8575 mounted on it. You can connect | ||
| 14 | such a board to a Linux host via e.g. an USB to I2C interface. Examples of | ||
| 15 | PCB boards with a PCF8575: | ||
| 16 | |||
| 17 | * SFE Breakout Board for PCF8575 I2C Expander by RobotShop | ||
| 18 | http://www.robotshop.ca/home/products/robot-parts/electronics/adapters-converters/sfe-pcf8575-i2c-expander-board.html | ||
| 19 | |||
| 20 | * Breakout Board for PCF8575 I2C Expander by Spark Fun Electronics | ||
| 21 | http://www.sparkfun.com/commerce/product_info.php?products_id=8130 | ||
| 22 | |||
| 23 | |||
| 24 | Description | ||
| 25 | ----------- | ||
| 26 | The PCF8575 chip is a 16-bit I/O expander for the I2C bus. Up to eight of | ||
| 27 | these chips can be connected to the same I2C bus. You can find this | ||
| 28 | chip on some custom designed hardware, but you won't find it on PC | ||
| 29 | motherboards. | ||
| 30 | |||
| 31 | The PCF8575 chip consists of a 16-bit quasi-bidirectional port and an I2C-bus | ||
| 32 | interface. Each of the sixteen I/O's can be independently used as an input or | ||
| 33 | an output. To set up an I/O pin as an input, you have to write a 1 to the | ||
| 34 | corresponding output. | ||
| 35 | |||
| 36 | For more information please see the datasheet. | ||
| 37 | |||
| 38 | |||
| 39 | Detection | ||
| 40 | --------- | ||
| 41 | |||
| 42 | There is no method known to detect whether a chip on a given I2C address is | ||
| 43 | a PCF8575 or whether it is any other I2C device, so you have to pass the I2C | ||
| 44 | bus and address of the installed PCF8575 devices explicitly to the driver at | ||
| 45 | load time via the force=... parameter. | ||
| 46 | |||
| 47 | /sys interface | ||
| 48 | -------------- | ||
| 49 | |||
| 50 | For each address on which a PCF8575 chip was found or forced the following | ||
| 51 | files will be created under /sys: | ||
| 52 | * /sys/bus/i2c/devices/<bus>-<address>/read | ||
| 53 | * /sys/bus/i2c/devices/<bus>-<address>/write | ||
| 54 | where bus is the I2C bus number (0, 1, ...) and address is the four-digit | ||
| 55 | hexadecimal representation of the 7-bit I2C address of the PCF8575 | ||
| 56 | (0020 .. 0027). | ||
| 57 | |||
| 58 | The read file is read-only. Reading it will trigger an I2C read and will hence | ||
| 59 | report the current input state for the pins configured as inputs, and the | ||
| 60 | current output value for the pins configured as outputs. | ||
| 61 | |||
| 62 | The write file is read-write. Writing a value to it will configure all pins | ||
| 63 | as output for which the corresponding bit is zero. Reading the write file will | ||
| 64 | return the value last written, or -EAGAIN if no value has yet been written to | ||
| 65 | the write file. | ||
| 66 | |||
| 67 | On module initialization the configuration of the chip is not changed -- the | ||
| 68 | chip is left in the state it was already configured in through either power-up | ||
| 69 | or through previous I2C write actions. | ||