This topic covers how to connect I2C device to Carambola. For this tutorial I will use PCF8575 16 bit I/O expander.

In order to connect I2C device I will need few passive components. See attached schematics.

There is also power selection jumper (5V or 3.3V) and SPI pin header. Discard these for this subject.

• run make menuconfig and select these packages:

Kernel modules
  I2C support
    kmod-i2c-gpio
Utilities
  i2c-tools package

• Type make and rebuild firmware

• Reflash

• If you are unfamiliar with these steps read Where to start

Connect to Carambola with fresh firmware and type i2cdetect -y 0. You should see something like:

root@OpenWrt:/# i2cdetect -y 0
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: 20 -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --   

root@OpenWrt:/# i2cget -y 0 0x20 0x00 w
0xffff  

root@OpenWrt:/# i2cset -y 0 0x20 0x00 0xff

These utilities were written to work with I2C memory devices and lack of full I2C support. Biggest issue - it is possible only communicate with devices that have 2 bytes in I2C protocol. Although I2C on hardware level supports up to 7 (I am not exactly sure right now) bytes for both directions.