Using Normal GPIO Pins as I2C (required)

Question

We are attempting to use the Raspberry Pi as an embedded software platform. We are writing this in python, but currently we have it wired up to use 3 separate I2C busses. We have two of them wired to the normal I2C pins, but we require the use of a normal bog-standard GPIO pin to wire this third bus to.

We have pull-up resisters included in the electrical writing, is it possible to simple have Raspberry Pi treat these pins as I2C pins?

I found other answers on this site, but nearly all tell the user to simply daisy chain their devices onto one bus. I'm not quite sure of the reason why, but that is not currently an option for us. Is Bit-Banging the only option in this situation? Or can Raspberry Pi handle this?

Answer 1

You can use any spare pair of GPIO to create a software I2C bus. As you have noted you will need to use external pull-ups to 3V3 on the chosen GPIO.

You need to add an entry to /boot/config.txt

For details see /boot/overlays/README

Name:   i2c-gpio
Info:   Adds support for software i2c controller on gpio pins
Load:   dtoverlay=i2c-gpio,<param>=<val>
Params: i2c_gpio_sda            GPIO used for I2C data (default "23")

        i2c_gpio_scl            GPIO used for I2C clock (default "24")

        i2c_gpio_delay_us       Clock delay in microseconds
                                (default "2" = ~100kHz)

E.g. to use GPIO 17 for SDA and 23 for SCL add the following entry to /boot/config.txt

dtoverlay=i2c-gpio,i2c_gpio_sda=17,i2c_gpio_scl=23

A new bus will then be available. I'm not sure of the number but I think the first new bus will be /dev/i2c-3, the next (if any) /dev/i2c-4 etc.

This bus may be used in exactly the same way as the hardware buses, e.g. i2cdetect -y 3 will show devices on the new bus.