The nRF21540 DK has multiple analog switches that are used to connect and disconnect signals based on different scenarios.
The Universal Serial Bus (USB) and SW6 control the signal switches by using USB_DETECT as an input to SW6. Therefore, the interface MCU can be disconnected either by unplugging the USB cable in J2 or by toggling SW6.
The signal controls a set of switches (U5, U6, U7) that break the connection between the nRF52840 and the interface MCU, and control the power for the interface MCU. For more information, see Interface MCU power.
Switches U5 and U6 break the connection of the Universal Asynchronous Receiver/Transmitter (UART) lines and SWD/RESET lines. In addition, the signal controls the routing of the RESET signal depending on user preference when the interface MCU is connected/disconnected.
- When the interface MCU is disconnected, cutting SB42 disconnects the IF Boot/Reset button from the reset pin (P0.18) of nRF52840.
- When the interface MCU is disconnected, shorting SB45 connects the RESET pin in the Arduino interface to the reset pin (P0.18) of nRF52840.
- When the interface MCU is connected, shorting SB46 connects the RESET pin in the Arduino interface to the BOOT input of the interface MCU.
- Shorting SB43 connects the RESET pin in the Arduino interface to the IF Boot/Reset button.
- Shorting SB44 connects the RESET pin in the Arduino interface to the reset pin (P0.18) of nRF52840.
When a shield is connected, there are two analog switches connecting the pull-up resistors to the I2C bus lines (SDA and SCL). This function is using one ground pin on the Arduino shield to control the switch. This feature can be disabled by cutting SB33. To permanently enable pull-up resistors, short SB32.
The last switch (U8) controls which General-Purpose Input/Output (GPIO)s certain signals are routed to. This is due to some features using the same GPIOs as the Trace output by default. These analog switches are controlled by SW7. See Debug input and trace for more information.