Yes, most likely the body diode in the regulator’s MOSFET is conducting the current back through the regulator. So, if one Mini is powered down, the other will be powering both of them. The MOSFET is probably robust enough to handle this current with causing any harm.
It would be best if both units were powered up when connected, though.
Another option, as I’ve said previously, is to use a QWIIC cable without the 3.3V wire and only SDA, SCL and GND connected.
I see a version of road trip where you are racing another player.
Should be able to have several more devices only limited I suppose by the pullup resistance limit from combining them.
@MLXXXp do you know anything about running i2c without the pullups between just two arduinos is it possible?
Stuff like this seems to indicate not needed for direct communication to the two mcu?
Now I need to find out if SBA pins are crossover on a c to c cable,
No, you need pullups somewhere. In I²C mode the data and clock output drivers are “open collector”. They only pull the signal low and are high impedance for a high signal. Therefore, you need a pullup to pull the line high, otherwise it would be floating.
I suggest you include the pullups on the Mini and preferably include bridged solder jumpers that can be cut if you only want pullups elsewhere.
Well I dunno there is input pullup and this guy says it works I’m gonna test it. I think it can’t work in situation where the other chip is not strong enough to pull down the signal but I think one arduino can pull down another.
The only thing left to do now is:
Should have all this by Wednesday. Working on an update (finally) that will get posted to kickstarter tomorrow.
Note: Graffiti edition will not have labeled test pads.
@bateske,
Have you done any testing of the diode (D1) when powered via USB, and MOSFET (Q3) when battery powered, to see if they can handle the specified current (226mA) to the Qwiic interface?
You should also check the 3.3V regulator for the Qwiic interface to see how it holds up.
You could put a 15Ω 1W resistor across the 3.3V and GND pins of the Qwiic connector to draw 220mA.
You might want to label the qwiic connector with the official logo.
https://cdn.sparkfun.com/assets/custom_pages/3/3/4/Qwiic-Registered-Logo.zip
Not yet, I’ve got the battery pigtail coming in tomorrow then I will do charging tests.
Good point to test the load of the 3v3 but do you have any suggestion how to do that? Do I need to invest in a variable load of some kind?
I told you:
If you don’t have a 1W, put 4 15Ω ¼W resistors in series/parallel.
It couldn’t hurt to have one on hand for testing the Mini and other things in the future. I have one of these:
https://www.aliexpress.com/item/32686933876.html
Graffiti done just the testing remaining and submit the new design.
Update: Nope the polarity of the battery connector is backwards, good thing to catch!
Not happy with shrinking pharap name down it needs to be bigger so I think I’m gonna choose a different font and mix it up
It doesn’t matter. It’s not like anyone’s going to be able to read it anyway.
But I will know it’s there! 
Alright, fun times!
| what is happening | what should happen | |
|---|---|---|
| usb only | no status light | the light usually comes on even when there is no battery (even though I would prefer no light) |
| battery only | dim LED | no LED |
| battery charging | bright LED | bright LED |
| 3v3(?) power | dimmer LED | no LED? |
I’ll get the multimeter out and do some tests, but I what I’m suspecting is happening is the mosfet is not blocking the voltage, and so it’s trying to charge itself?
Is it because the other side of the status LED is connected to VCC and not VIN? I suppose?
Would a pull to VCC on status fix this?
There are 3.3V pull ups on the I2C pins and there are protection diodes on the I2C pins inside the atmega. So these may produce a weak Vcc
very plausible.
Try removing the LED from VCC and bodge it to Vin and see how your test above work out then
