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thanks in advance for any help, this is quite the sticking point for me unfortunately,

Basically I'm coding a program in python for my raspberry pi 3 to read values from two voltmeters, through an ADC, and display them on a TFT screen. The program works, but in order to use the program at the same time as the TFT screen, I need to move the i2c bus to GPIO pins 5 and 6.

(Side note: yes, I'm using external resistors to pull up to 3V3)

I have done this on the end of the raspberry pi by altering config.txt, and I can detect the ADC using i2cdetect. So all good there.

In my python program, it's really only one line of code for the I2C initialization:

i2c = busio.I2C(board.D6, board.D5)

This returns ValueError: No Hardware I2C on (slc,sda)=(6, 5) when I try to run the program.

I won't claim to be a great or even experienced programmer, and am honestly open to any ideas at this point. I've been all over using google and it seems like this is just a limitation of busio.py and there's no real way around it.

I guess at this point I'm wondering if there's another way to code an i2c bus into python, possibly using an alternative library other than busio? Or should I instead rethink my whole project? It might be annoying to change to a different board that either has two i2c hardware busses or analog pins, but at this point maybe it'd be worth it - or I could abandon an LCD display and do something else instead. I tried to use an HDMI display, losing touchscreen functionality but there are very few out there that size. I'd hate to rewrite my program in another language, but... maybe I could just put in a section of code in C, if C doesn't have the limitations of busio?

Thanks again for any thoughts or ideas!

2 Answers 2

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RPi 3 has only 2 "hardware" i2c buses: i2c0 & i2c1. If you'll read through the "documentation" in /boot/overlays/README, you can confirm that for yourself. I think you'll find that the RPi 4 is the only model with more than 2 hardware i2c channels.

I can detect the ADC using i2cdetect. From that & other comments in your question, I'll assume you've configured a software i2c channel in /boot/config.txt? Software i2c can be a poor substitute for the hardware i2c. Also, the error message "ValueError: No Hardware I2C on (slc,sda)=(6, 5)" suggests that opinion is shared by your Python library :)

For RPi 3, there are two hardware i2c channels; i2c0 & i2c1. I assume from your question that your TFT screen is using i2c1. There have been some vague warnings re use of i2c0 in the past; these warnings may still be valid for some models, but I've encountered no problems using i2c0 on my RPi 3B+ model.

Here's how to use i2c0:

  1. Check your /boot/config.txt file for the following line: dtparam=i2c_vc=on. If it's not in the file, then add it now. This will enable i2c0 for your use with the ADC.

  2. If you're using a dtoverlay for your ADC, make sure it calls out i2c0 for the GPIO pin.

  3. With the RPi powered OFF, move your ADC i2c to GPIO 0 (SDA) & GPIO 1 (SCL). Connect your other hardware (pullups incl), boot up & test the connection using sudo i2cdetect -y 0 .

  4. Assuming that checks out, re-code your Python script to reflect use of i2c0 & try again.

Let us know how it works, and the specific model # of your RPi:

$ cat /proc/cpuinfo | awk '/Model/' 
# post the output from this command
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  • Unfortunately, that would not work either. The screen uses the first 26 pins. Unless it doesn't actually need all of them...? And the output is, Raspberry Pi 3 Model B Rev 1.2 Mar 21 at 20:54
  • @Chaos_Spear: ?!? 26 pins... does it actually use the pins, or just cover them up? What's the model # & source for this screen?
    – Seamus
    Mar 22 at 9:33
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First off, sorry if this looks weird, apparently according to SE my registered account is different than my guest account so I'm just going to put an answer down because I found a way to sidestep the issue.

Basically the previous screen I tried to use had the option of working just as an HDMI display, no touchscreen capability, not using any GPIO pins. I had trouble getting this to work before but decided to revisit the idea, and this time the screen worked, so that means I can use the hardware i2c like busio wants me to! Much easier this way.

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