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Somehow during an experiment, I managed to wreck my i2c SDA and SCL pins on my raspberry pi (pins 3 & 5). I was considering converting two of my gpio pins into another i2c bus, but I thought it might simply be easier to somehow re-purpose my EEprom i2c pins (pins 27 & 28) to act in the same fashion as pins I blew, since they're already affiliated with i2c.

I know that generally you shouldn't mess with these pins since they're critical when it comes to communicating with HATs for the raspberry pi. However, in my specific case, I don't plan on having any HATs, just direct communication with one i2c device.

If it's really necessary, the device I'm trying to communicate with is the mcp23008 on the mcp23x08 eval-board.

I'd love to know if this approach is possible and if so, what is the best way to approach?

-Thanks

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  • It's possible but it isn't recommended as GPIO0 & GPIO1 don't have the pullup resistors and they're reserved for use by an official HAT.
    – Dougie
    Apr 8, 2020 at 12:42
  • Even if I were to use GPIO0 & 1, I'm guessing the procedure would be similar to using any other GPIO? ie. utilizing overlays and editing /boot/config.txt?
    – Shaun the Sheep
    Apr 8, 2020 at 12:53

1 Answer 1

Reset to default
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It is simpler just to use another pair of GPIO as a software I2C bus.

There is no real advantage in using GPIO 0/1.

Whatever you do you will need to ensure that the GPIO you use are conneted to pull-ups to 3V3. I suggest you use resistors in the range 2 to 4 k ohm for the pulls.

See /boot/overlays/README for details of creating a software I2C bus.

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)

        bus                     Set to a unique, non-zero value if wanting
                                multiple i2c-gpio busses. If set, will be used
                                as the preferred bus number (/dev/i2c-<n>). If
                                not set, the default value is 0, but the bus
                                number will be dynamically assigned - probably
                                3.

E.g. to add an I2C bus #3 using GPIO 17 for SDA and GPIO 27 for SCL add the following entry to /boot/config.txt

dtoverlay=i2c-gpio,i2c_gpio_sda=17,i2c_gpio_scl=27,bus=3

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  • Alright, thanks for the insight.
    – Shaun the Sheep
    Apr 8, 2020 at 12:07
  • I do have to ask though, what makes the EEPROM pins so different from pins 3 & 5 that would make using them as a regular i2c bus so inconvenient? Also, since the EEPROM are already i2c, doesn't that mean that they already have pull up resistors installed, meaning that I wouldn't need to add any external pull up resistors?
    – Shaun the Sheep
    Apr 8, 2020 at 16:53
  • Use them if you want. I don't know if doing so will screw up the system or not. You will soon find out. The I2C bus requires pull-ups. Breakout modules for I2C devices sometimes have pulls fitted. However they might be to 5V which would damage the Pi GPIO.
    – joan
    Apr 8, 2020 at 17:00
  • What kind of damage do you think it would cause? As in damage with interfacing with devices via i2c or damage to the pi itself and operation?
    – Shaun the Sheep
    Apr 8, 2020 at 17:05
  • Connect 5V to a Pi GPIO? Damage in the sense the GPIO would stop working followed by the Pi not working and needing to buy a new one.
    – joan
    Apr 8, 2020 at 17:07

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