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La Fonera is a small router with wireless functions created by FON.

La Fonera's firmware can easily be replaced by OpenWrt which provides any required elements to enable I2C bus usage. This feature allow us to use La Fonera to drive I2C devices like eeproms, temperature sensors, etc.

If you want to add I2C bus(es) to La Fonera, a this wiki page have been made to help you!

Introduction

This guide will help you to connect La Fonera to real world.

La Fonera is a little router that runs Linux kernel. Like many routers, La Fonera had general purpose inputs/outputs (GPIO) on board. In case of La Fonera, not all these GPIO are used and they can be used to create one or two I2C buses.

Hardware modifications

  • Open the case http://fonera-i2c.googlecode.com/files/FON2100_case_bottom.jpg

Remove the two rubber feet on the opposite site to the antenna jack and unscrew the revealed screws.

  • Locate SW1 http://fonera-i2c.googlecode.com/files/FON2100_pcb_top_small.jpg

On this picture, SW1 is located on top-right corner and already have a tulip connector sold on.

  • Remove caps On these GPIO there is debouncing RC, caps are on top side of PCB, resistors are located on bottom side. To prevent this RC to filter data (SDA) or clock (SCL) I2C signals, its recommended to remove caps.

  • Add pull-ups resistors To works correctly, 2.2KOhms resistor pull-up need to be placed on both signal lines. Note: I made some tests with only one I2C device on bus, 1KOhms and 2.2KOhms but 10KOhms doesn't work, just like if these pull-ups are not present.

  • (Optionnal) Sold internal connector You can add a cheap "easy-to-plug" connector on GPIOs in order to be able to plug/unplug I2C device without dismantle all box each time (i.e. no need to remove PCB from box).

http://fonera-i2c.googlecode.com/files/FON2100_pcb_internal_gpio_connector_small.jpg

As you can see on picture, 5x1 tulip connector (2.54 mm / 0.1 inch) is just perfect if you remove center pin to prevent from short circuits. Even PCB doesn't fit with 2.54 mm / 0.1 inch, this 5x1 connector can be sold on this 6x1 pads on PCB. To ease soldering, you should clean these pads (metalic vias) before attempt to put tulip connector.

  • (Optionnal) Add external connector

Software

  • Install OpenWrt 8.09.x
  • Install kernel modules that allow to use GPIO as I2C bus
  • Configure module loading options
    • Create a new file /etc/modules.d/58-i2c-gpio-custom In this file, you must put options to i2c-gpio-custom module loading. In example, this content allow i2c-gpio-custom module loading in order to create I2C device interface /dev/i2c-0 using GPIO1 as SDA and GPIO7 as SCL.
i2c-gpio-custom bus0=0,1,7
  • Reboot in order to load kmod-i2c-gpio-custom before kmod-i2c-gpio Atfer reboot, dmesg command should say something like that:
i2c /dev entries driver
Custom GPIO-based I2C driver version 0.1.1
i2c-gpio i2c-gpio.0: using pins 1 (SDA) and 7 (SCL)
  • Install i2c-tools
opkg update && opkg install i2c-tools

  • Probe I2C devices (i2cdetect)

  • Test using i2cset/i2cget (e.g. with PCF 8574)

To-do list

HW

  • Attempt to drive I2C devices without removing caps
  • Attempt to drive I2C devices with bottom resistors removed but without pull-ups.
  • Test to driver multiple I2C device

SW

  • Write a little C sample to drive PCF 8574.