Realy nice format and great specs!
I have to do the RF circuit myself.
I’ve never done anything like that but that could be interesting.
Because???
WTF! Well I never…welcome back 
… you are back right? 
Desk looks a lot tidyer judging by the new profile pic. mind you we’re missing the tea-cosy rainbow rasta hat for the forum user image vs blue dot! 
Will DM/email you later…
That one also came up to my mind. 
I only noticed this board yesterday for the first time while searching for SAMD21 specs.
I had seen it before but due to its small size and colorful sticker it appeared more like a gadget to me then.
It’s tiny but has all te common serial interfaces (I2C, SPI, UART, serial over USB).
If needed it can be soldered directly onto a custom PCB (because castellated).
Did you measure any (deep) sleep current usage already?
The Seeeduino XIAO - Arduino Microcontroller - SAMD21 Cortex M0+ is also on back order - no stock! So still doesnt help with the question " Where are all the samd21 chips" 
6 weeks to availability?
If it has to be SAMD21, there exist also (a pin compatible) one from AdaFruit The QT-PY
That one has even a button to get into boot mode, handy.
Even more power has this one , same size but with a 2040
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yes, below 1 mA (leds removed/3v3), the exact figures I have to find… somewhere
left to right : SEEED XIAO - ADAFRUIT QT-PY - PIMORONI TINY 2040
stock NL ! Adafruit QT Py - SAMD21 Dev Board with STEMMA QT
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i think it’s not reasonable to be dependent on anything whose availability is already unclear
On the link provided above i says:
Unfortunately it has a couple of drawbacks: it’s available only in BGA packaging and there’s a significant amount of external RF circuitry required around the antenna.
and on the RAK forum i found this post
Today i ordered some ATmega1284, ATmega4808 and i will give the RAK4260 a try.
Thanks guys for all your great recommendations.
@fishbeetle was referring to the SAMR34 chip which is on BGA which is a total PitA for small production runs and almost impossible to place by hand. The RAK4260 is complete and has castellated edges.
Which confirms you may need to add one capacitor and one ESD suppressor, for which they have given values - so not really any “RF circuit” needed. FishBeetle’s node goes straight from the RFC output to the u.FL connector.
I’m just drawing your attention to the total convenience that the RAK4260 brings.
But it is probably not the module you are looking for if you if you want an AT modem type module to talk to from your AVR. Microchip provide an AT code base that you can download on to the module - so you’ll need to ensure you have access to the programming pins.
On the plus side, if you are comfortable with full-on C type programming, you may find you can put your application on the RAK4260 and skip a second MCU. Or have both and get the benefits of both. If you are going to try out the RAK4260, I’d get a breakout board as it provides Dupont/jumper pins you can just hook up as well as the programming pins and an SMA connector.
If you just want an AT style modem module, then the RAK4270 is the way to go - uses the new low power SX1262 and has the RAK AT firmware pre-installed.
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You would almost certainly be able to put your whole application on the Rak module - unless you hit memory constraints - just a question of dev time!
Ok, thank you for your explanation.
I really like the format and the all in one approach
I think im not able to make it running as stand alone for now but i’m sure there will be further development in future.
I found the arduino supported bastwan board.
But for now i think i will stick with the SX1276 and a microcontroller.
Where are all the samd21 chips
One of them is here running LMIC-node connected to TTN V3.
Thanks for the tip.