How to access servos using pins other than GPIO 2&3

Question

I have an adafruit pca9685 16-servo controller. I was using this guide to control it: https://learn.adafruit.com/adafruit-16-channel-servo-driver-with-raspberry-pi/using-the-adafruit-library

Particularly my code looks something like the following

from adafruit_servokit import ServoKit
import time 
kit = ServoKit(channels=16)

while True:
    kit.servo[0].angle = 180
    time.sleep(5)
    kit.servo[0].angle = 0
    time.sleep(5)

And that all works fine for me if GPIO 2&3 are connected to SDA & SCL respectively.

Now i wanted to get a little creative. I wanted to use GPIO 10 and 9 to control the servo board and have GPIO 2&3 be used somewhere else (it would make the wiring cleaner looking for me and generally I dont want to be stuck using "pin X" only for "purpose Y").

So I moved SDA & SCL to GPIO 9 & 10 respectively, but now my python code freaks out throwing:

"ValueError: No I2C Device at address: 0x40"

From the file "/home/pi/.local/lib/python3.7/site-packages/adafruit_bus_device/i2c_device.py"

I did some research into this and the following website makes it seem as if GPIO 2&3 are the ONLY pins that can be used for I2C: https://pinout.xyz/pinout/i2c, but I wanted to confirm with the community if there is some work around to user other pins?

It is clear to me that I2C and adafruit by default are checking pins 2&3, seeing as (2,3) is never passed as an argument in my code, and I was hopeful that there might be a configuration file or setting that can be changed to support other pins.

Answer 1

You will need to use the device tree to accomplish this.

You may also need to make some changes to the AdaFruit software. It's likely using the default i2c1 channel - you'll have to locate this in the software, and change it to match the i2c channel you choose (as shown below).

RPi 4 gives you a choice of 6 hardware-based i2c channels. These are available through the device tree and its overlays. These are declared & configured in /boot/config.txt. The detailed instructions/documentation for this is mostly in this README file.

Suggest you read this (README) file first - esp the introductory sections, Afterwards, scroll down in the file until you reach a section that begins with the line Name: i2c3. I'll guess the most likely substitutes for i2c1 in your case are the dtoverlays for i2c3 - i2c6. Read through these & select the GPIO pins that suit you. The README file will show you the format & available options for the overlay you choose.

For example, if you choose i2c4, you will edit your /boot/config.txt file to add the following line:

dtoverlay=i2c4,pins_6_7     # *physical* pin #'s 31 & 26

You will also need to add pullups (approx 2Kohm each) to those pins - unless your AdaFruit hardware includes them.

Changes to /boot/config.txt will become effective after a reboot, and will remain in effect until you remove (or comment) the dtoverlay you added, and reboot again.

Answer 2

These are the only accessible pins with hardware support for I²C on most Pi (the Pi4 has additional ports e.g. to enable the i2c3 bus dtoverlay=i2c3)

You can use the dtoverlay=i2c-gpio for software I²C on other pins.

All of these will require pullup and changes to the code.