The nRF52840 DK supports programming and debugging external boards with nRF51 Series or nRF52 Series System on Chip (SoC)s. To debug an external board with SEGGER J-Link OB IF, connect to the Debug out connector (P19) with a 10-pin cable.
Figure 1. Debug output connector
When the external board is powered, the interface MCU will detect the supply voltage of the board and program/debug the target chip on the external board instead of the onboard nRF52840
SoC.
Note: The voltage supported by external debugging/programming is VDD voltage. Normally, this is 3 V when running from USB, but if the onboard nRF52840
SoC is supplied from either USB or Li-Ion, the nRF power source switch (SW9) is in either Li-Po or USB position, and VDD can be set by the nRF52840 firmware. Make sure the voltage level of the external board matches the VDD of the nRF52840 DK.
You can also use P20 as a debug out connection to program shield-mounted targets. For both P19 and P20, the interface MCU will detect the supply voltage on the mounted shield and program/debug the target.If the interface MCU detects target power on both P19 and P20, it will by default program/debug the target connected to P19.
If it is inconvenient to have a separate power supply on the external board, the nRF52840 DK can supply power through the Debug out connector (P19). To enable this, short solder bridge SB47. While SB47 is shorted, it is not possible to program the onboard nRF52840 SoC even if the external board is unplugged.