nRF52840 development kit

[BAEE]

I am trying to connect to another type of board via the UART on the development kit.  However, there seems to be a voltage mismatch.  Are the nRF52840 development kit UART signals TTL level versus CMOS level?

[Sean Taylor]

I am also trying to connect to the nRF52840 development kit UART so as to connect it to another MCU dev. kit.  I can communicate via pins P0.06 and P0.08 using a 3V serial - BUT ONLY if I first communicate via the interface MCU.  If I power cycle the dev. kit and attempt to communicate via P0.06/08 the nRF52840  UART is not responsive.

The  DK User Guide says "The UART signals are routed directly to the interface MCU. The UART pins connected to the interface MCU
are tri-stated when no terminal is connected to the virtual COM port on the computer.
Note: The terminal software used must send a Data Terminal Ready (DTR) signal to configure the UART interface MCU pins."

there is no DTR signal available on the P0 pins - so is there a way I can make the UART always available on P0.06 and P0.08 without having to first connect a terminal via the interface MCU ?  Either in the nRF52840 firmware, or on the dev kit boad (jumper, switch setting etc) ?  Thanks !

[Sean Taylor]

I think I answered my own question - the nRF52840 has hardware flow control enabled by default

pca10056.h:#define HWFC           true

The interface MCU does not drive P0.07 (CTS - input to nRF52840) low until a terminal emulator is connected to the Jlink UART - however I can loop back the CTS/RTS (P0.05, P0.07) and the nRF52840 will communicate over the Tx./Rx (P0.06/08) regardless of the state of the interface MCU.