Cable to connect neotrellis M4 and plasma 2040

I’d like to power a neotrellis M4 via a plasma2040, and then send messages via SDA (from the neotrellis to the plasma)

But I am not sure what cable i need. Grove? Stemma? Adafruit stemma? Adafruit stemma to pin header?

I also need to know what direction the power goes in, to make sure that I am powering the neotrellis from the Plasma2040 and not vice versa. How would I do that?


The connector on the NeoTrellis is 4 pin JST PH (2mm spacing). The connector on the Plasma is 4 pin JST SH (1mm pitch). There are adapter cables available, but they are hard to get since Adafruit practically abandoned Stemma in favor of Stemma/Qt. 4 pin JST SH with blank wires on the other side are available from various sources and crimping JST PH is doable.

BTW: Stemma and Grove are not mechanically compatible. Even if Adafruit claims the opposite. If you use your knife and cut off enough of the plugs, they will fit and work, but I would not call this to be compatible.

From software-side, it is more complicated. You would have to convince one side to be an I2C master. The NeoTrellis could also do UART, but I’m not sure about the Plasma.

Powering will only be a problem if you supply power to both boards.

I think this is the best idea. The plasma2040 has bare connectors for SDA and SCL.

I guess I will just use the USB power and only connect ground and SDA.

I have a good feeling about this project. I already use I2C on plasma2040, it’s just a case of figuring out.

Thanks for the help with the connector!


I am currently working on this. I have connected both devices and I am running i2c.scan() on the plasma2040.

The bad news is that I am getting no response, no devices are found.
The good news is that when I disconnect the cables, I get an error message: “TimeoutError: Clock stretch too long”.

Any ideas on how to get these two talking?

You have two microcontrollers (M4 in the neotrellis and RP2040 in the plasma). When scanning from the plasma, the plasma is the host (master) and it will only find the M4 if it implements the I2C device.

I was afraid of that. I’m connecting two master devices and neither has a I2C address. I am pretty sure that it’s not possible to achieve what I want.

Well, you can probably turn one of these MCUs to a device. I have seen code, but never tried myself. Something like this pops up every once in a while and it would be an interesting challenge.

Thank you. I will look into that tomorrow.

I’m rather late to this party, but would it be easier to use two USB lines here - from your computer to each of these devices?

It’s a good tip but I’m trying to go computerless for this- I want to control the plasma2040 directly with the neotrellis.

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