Boron 2G/3G NRF to Ublox UART interface

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I have a Boron 2G/3G that i am programming from scratch (no particle software) but am stuck getting the connection between the nrf52840 and the SARA U201 working.

I have two particular questions that I hope someone can help with.

  1. Should I expect the uart baud rate to be 115200 (factory default) or 921600 (as used in particle firmware)?

  2. Is there a way to enable DTR (drive it low) which is what I think the modem needs to work? I cannot see how to do this from the schematic but is required as described in the Ublox system integration manual.



If you’re going to bypass the Particle system firmware, you’re kind of on your own. However the baud rate on the U201 starts out at 115200, then is changed to a higher baud rate.

The modem uses hardware flow control, so you’ll need to initialize the RTS1 pin as output and initialize the serial port with hardware flow control enabled.


Yes I realise that writing my own firmware puts me out in no mans land, but the Boron hardware is compellingly nice and I just want a simple MQTT client.

I have set RTS to active-low and am pretty sure my uart code is OK as it is also running a serial CLI.

I remain concerned that leaving DTR floating is a possible problem for me.



Hi @abstractsig,

Any news with this? I’m interested on this as well. Have you publish your results or your code in some place?

Thank you,

Manuel Montenegro


Did you manage to do it?