Serial LTE Modem UART2 - 3V or 1.8V ?

[benna]

HI there,

In the Serial LTE Modem documentation for the nRF9160 DK, it is mentioned that the GPIO voltage level should be 3V using SW9 when using an external MCU & UART2.


When I set this to 3V, I get following warning in RTT console (without anything else connected):

 <wrn> spi_nor: Waiting until flash is ready

Sending UART commands when connecting a serial adapter also doesn't work, not responding at anything.
However, when I set the voltage to 1.8V, everything is working just fine and I get proper logs via RTT and AT commands are working.

Is this an error in the documentation?
Or am I interpreting this wrong?

( Used NCS SDK v2.6.1, modem fw on latest v1.3.6)

[benna]

We found the issue.
For the build config we selected the nrf9160 DK and left everything else default. When building, it picked up nrf9160dk_nrf9160_ns.overlay but also nrf9160dk_nrf9160_ns_0_14_0.overlay, which enables SPI3 for external flash. 

This SPI3 is sharing some pins with UART2, causing the problem above. 
So without the nrf9160dk_nrf9160_ns_0_14_0.overlay it works on 3V.