use other gpio pins as i2c

As of June 2016 Raspbian already has the necessary modifications. You only need to enable the overlay in config.txt (step 6 onwards).

I finally got this working as follows:

1. Rebuild the kernel with i2c_gpio module. You must edit the bcm2708_defconfig or bcmrpi_defconfig and add the line CONFIG_I2C_GPIO=m before running the make defconfig.
2. Install kernel and modules using mkknlimg for device tree support as described in the kernel building docs.

3. Convert the example i2c_gpio config to a device tree fragment as follows:

   // Overlay for i2c_gpio bitbanging host bus.
   /dts-v1/;
   /plugin/;
   
   / {
           compatible = "brcm,bcm2708";
   
           fragment@0 {
                   target-path = "/";
                   __overlay__ {
                           i2c_gpio: i2c@0 {
                                   compatible = "i2c-gpio";
                                   gpios = <&gpio 23 0 /* sda */
                                            &gpio 24 0 /* scl */
                                           >;
                                   i2c-gpio,delay-us = <2>; /* ~100 kHz */
                                   #address-cells = <1>;
                                   #size-cells = <0>;
                           };
                   };
           };
           __overrides__ {
                   i2c_gpio_sda = <&i2c_gpio>,"gpios:4";
                   i2c_gpio_scl = <&i2c_gpio>,"gpios:16";
                   i2c_gpio_delay_us = <&i2c_gpio>,"i2c-gpio,delay-us:0";
           };
   };
   

   • This will default to using gpio 23 and 24 for software i2c.
   • delay-us should be 2 for 100kHz operation.
   • The following lines should be removed because they will prevent the driver from working on Raspberry Pi: i2c-gpio,sda-open-drain; i2c-gpio,scl-open-drain;

4. Build the device tree blob with dtc -@ -I dts -O dtb -o i2c-gpio.dtb i2c-gpio.dts

5. Copy the blob to /boot/overlays/

6. Add a line in /boot/config.txt dtoverlay=i2c-gpio

   If you want to use different pins, put dtoverlay=i2c-gpio,i2c_gpio_sda=<pin>,i2c_gpio_scl=<pin> instead. You can also change the rate with i2c_gpio_delay_us=<usecs>.

7. Reboot.
8. modprobe i2c-dev and you now should have /dev/i2c-3 in addition to any others you previously configured.