Adding I2C Connection

[ecburns33]

I'm trying to add a device using an I2C connection. Following along with the examples on the Notion page, it seems I should just be able to use the connection initialized by pycubed.py ("i2c = cubesat.i2c1"), but this requires me to connect to the SDA/SCL pins. How would I do this? I also see in other examples that PA08 and PA09 are the defaults for these, but I can't find them on the board. I also understand I can maybe use GPIO pins for this and do an I2C scan to find the address of the external device, but I can't get that to work either. If I could get just a brief explanation of how to wire an I2C connection and how to initialize the interface, I'd really appreciate it.

[maholli]

Hey @ecburns33

I didn't make external wiring to I2C bus #1 very easy on mainboardv05 and this is something I have in my notes for potential future revisions. That being said, you have many options since you can configure many different pins to be additional I2C buses. I'll answer your specific questions below:

1. It's doable to access I2C bus #1 (aka i2c1) but it would require soldering wires to some resistors and whatnot on the board. A better solution is to use one of the options below, but if you reeeallllyyy wanted to do i2c1 you would start by looking at the kicad schematic+layout to determine which pads to access.
2. The I2C scan is used to look for devices already attached to an I2C bus. The I2C scan example illustrates this by showing you the devices on i2c1. This might help you troubleshoot later, but not the answer you're looking for.
3. The simplest answer for your situation is to use the SDA2 and SCL2 pins (on the payload connector) as described on the payloads & interfacing page and define another I2C bus in the code.
   • There's example code for this (also on that page) under the "Using I2C (SD2/SCL2)" section. I'll repeat it below:

     from pycubed import cubesat
     import board
     import busio
     
     i2c2 = busio.I2C(board.SDA2,board.SCL2)      

   • For your wiring, you would connect your external board to the payload header using 3V3 (optional), GND, PA17, and PA16. Like this:
     
   • Notice how you can call the PA17 & PA16 pins by multiple names (like SDA2 and SCL2). Visit the Pin Muxing page to learn more about this.
     • For example, this would achieve the exact same thing as the code above:

       from pycubed import cubesat
       import board
       import busio 
       i2c2 = busio.I2C(board.PA17,board.PA16)

   • After wiring, try adapting the I2C scan code to scan this new i2c2 bus you defined and see if it finds your device (some I2C chips don't respond to the "scan" protocol but most do).

Hope this helps! Let me know if you need more details.

[ecburns33]

Okay, I tried that and is still says the SDA connection is in use. The board we're trying to interface with is an Adafruit Motor Shield v2.3 . We're able to successfully use the Motor Shield board with Arduino, but are trying to convert it to PyCubed. I switched over the power supplies and connected the PA16/PA17 pins to the SDA and SCL pins on the Motor Shield board.

[maholli]

Did you also connect GND b/t the motor shield and PyCubed?

[ecburns33]

Yes, I have pins running from 3.3V and GND on the PyCubed to Vin and GND on the Motor Shield. I also have an external battery pack connected to the Motor Shield board via the 5V and GND on the Motor Shield.

[maholli]

Taking a quick look at the adafruit tutorial on the motor shield you should NOT have PyCubed 3.3V connected to VIN.

Having not used this motor shield before, I interpret the instructions like this:

1. Cut the 5V jumper and make a solder bridge to 3.3V.
   
2. Connect PyCubed 3.3V to 3.3V on the motor shield (and connect PyCubed GND to motor shield GND)
3. Connect your external 5V battery back to the motor power here:
   

I'm referencing the motor shield schematics and adafruit's faq on the product

[maholli]

OH! and I just realized why your code wasn't working. The library was written to explicitly set an i2c argument otherwise it defaults to the board's default i2c.

your code needs to look like this:

from pycubed import cubesat
import time
import busio
import board
from adafruit_motorkit import MotorKit

i2c2 = busio.I2C(board.SCL2, board.SDA2)

kit = MotorKit(i2c=i2c2)
kit.motor2.throttle = 1.0
time.sleep(0.5)
kit.motor2.throttle = 0

But DON'T try this until you've fixed the power issue above