Thanks for the insight!
So if I understand you correctly and I add an eeprom chip to SDA and S-CL and pull them high with 4k7 resistors (from sensebender schematic) then this should work?
Is there anything I need to add to my sketch to make the eeprom work - or is that handled by MySensors?
I have only tried the most basic of buttonsā¦
Using the example sketch for binary sensor with a push button to ground on D2.
Circuit is on a breadboard with a known good nrf24l01+ radio.
Please do get one - you might be able to help me. 
I have tried a blink sketch - that worked. So I uploaded a mysensors sketch that had worked on an arduino pro mini, unmodified (apart from defining ce and cs) and it compiled and uploaded fine.
I built the circuit with 2 buttons using gpio 2 and 3 and the radio on 6,7,8,9 and 10 on a breadboard and fired it up - and nothing.
No output from serial monitor, nothing appeared in home assistantā¦
Tried a second xiao, same result. Third one failed to uploadā¦ so I went to bed.
The sketch worked using the same setup on the arduino so at this stage Iām not sure what to do.
No i have bought three of them - and they donāt workā¦
Definitely not endorsing these boards - wanting to make them work because I (perhaps naively) believed the marketing of being āarduino compatibleā and have paid money for themā¦
So does anyone have any helpful advice or am I going to be bombarded by this sh#tā¦
The debug output is blank - it freezes the serial monitor in arduino ide.
Donāt really know where to go for help - seeed wiki is way over my headā¦
My training is in electronics - not code, Iāve copy pasted a few of my own sensors using pro minis but I am really lost when it comes to how the back end actually works.
I have a few of these tiny boards (based on samd21)- trying to get them working - anyone got any ideas if itās even possible?
Hoping to make some battery powered devices.
Any advise - or reading I should be doing would be appreciated.
Thanks in advance