| Bart Van Assche | 5864ae0 | 2008-01-27 18:14:45 +0100 | [diff] [blame] | 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 there are two alternatives |
| 44 | to let the driver find the installed PCF8575 devices: |
| 45 | - Load this driver after any other I2C driver for I2C devices with addresses |
| 46 | in the range 0x20 .. 0x27. |
| 47 | - Pass the I2C bus and address of the installed PCF8575 devices explicitly to |
| 48 | the driver at load time via the probe=... or force=... parameters. |
| 49 | |
| 50 | /sys interface |
| 51 | -------------- |
| 52 | |
| 53 | For each address on which a PCF8575 chip was found or forced the following |
| 54 | files will be created under /sys: |
| 55 | * /sys/bus/i2c/devices/<bus>-<address>/read |
| 56 | * /sys/bus/i2c/devices/<bus>-<address>/write |
| 57 | where bus is the I2C bus number (0, 1, ...) and address is the four-digit |
| 58 | hexadecimal representation of the 7-bit I2C address of the PCF8575 |
| 59 | (0020 .. 0027). |
| 60 | |
| 61 | The read file is read-only. Reading it will trigger an I2C read and will hence |
| 62 | report the current input state for the pins configured as inputs, and the |
| 63 | current output value for the pins configured as outputs. |
| 64 | |
| 65 | The write file is read-write. Writing a value to it will configure all pins |
| 66 | as output for which the corresponding bit is zero. Reading the write file will |
| 67 | return the value last written, or -EAGAIN if no value has yet been written to |
| 68 | the write file. |
| 69 | |
| 70 | On module initialization the configuration of the chip is not changed -- the |
| 71 | chip is left in the state it was already configured in through either power-up |
| 72 | or through previous I2C write actions. |