Kernel driver pcf8574 ===================== Supported chips: * Philips PCF8574 Prefix: 'pcf8574' Addresses scanned: I2C 0x20 - 0x27 Datasheet: Publicly available at the Philips Semiconductors website http://www.semiconductors.philips.com/pip/PCF8574P.html * Philips PCF8574A Prefix: 'pcf8574a' Addresses scanned: I2C 0x38 - 0x3f Datasheet: Publicly available at the Philips Semiconductors website http://www.semiconductors.philips.com/pip/PCF8574P.html Authors: Frodo Looijaard , Philip Edelbrock , Dan Eaton , Aurelien Jarno , Jean Delvare , 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 ------------------------------------- ! Be careful ! The PCF8574(A) is plainly impossible to detect ! Stupid chip. So every chip with address in the interval [20..27] and [38..3f] are detected as PCF8574(A). If you have other chips in this address range, the workaround is to load this module after the one for your others chips. 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.