I would like to use the Shield Debug (P20) on the nRF52840DK as part of a Device Under Test platform for production devices. I also want to be able to transmit/receive data to/from the nRF52840 on the DK as this will be running a test program which is checking the functionality of the Device Under Test.
The P20 header is there so that you can debug an external device (like a shield). The purpose of the GND_DETECT is so that the onboard debugger can know if it should use the pins connected to the onboard nRF5 device, or it it should use the pins on the P20 and debug out port instead. See Debug output.
So if GND_DETECT is pulled low then those SH_SWDIO and SH_SWDCLK are used? Does SH_VTG also need to have voltage on it for those pins to work?
WestCoastDaz said:So if GND_DETECT is pulled low then those SH_SWDIO and SH_SWDCLK are used?
Yes, that is correct.
WestCoastDaz said:Does SH_VTG also need to have voltage on it for those pins to work?
Yes, SH_VTG should be connected to VDD on the external board/shield. I assume the board is powered from elsewhere, if not you also need to connect the same point to VDD on the DK. (That also reminds me to mention that unless there is another connecting ensuring a common ground, then you need to connect SH_GND_DETECT to GND on the DK as well.)