thanks for answer. I have check the power of the system, and in my case the problem is not the power supply of raspberry pi but the connection between the rpi and the RAK831, I wired only one wire of 5V supply, probably it didn't bring enough current. I have add another wire and now it seems work properly to long run. We'll see...
I am also curious about the fault on the converter-board. I received it yesterday, integrated it into my working setup and noticed the software-reset was not working anymore, i had to reset by hand on the board to get it to work.
Also something got hot so i already stopped using it.
I am interested to know what has to be changed to make it work..
To update the firmware you'll need to power the module, so VCC and GND. And UART1_TX and UART1_RX to the RX and TX of your USB to serial converter (make sure this is 3.3v, not 5v. I use a 3.3v FTDI cable). Connect BOOT0 to VCC for programming, and GND for normal operation.
That's all it needs.
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