USB detect
To detect when USB for the interface MCU is connected, there is a circuit sensing the VBUS of USB connector J2.
nRF only mode
The nRF only mode disconnects the power supply of the interface MCU, the external memory, and the LEDs. It also disconnects the signal lines between the nRF52840 SoC and the interface MCU using analog switches.
Signal switches
The nRF21540 DK has multiple analog switches that are used to connect and disconnect signals based on different scenarios.