When the interface MCU receives a Data Terminal Ready (DTR) signal from a terminal, it performs automatic Hardware Flow Control (HWFC) detection.
Automatic HWFC detection is done by driving Clear to Send (CTS) from the interface MCU and evaluating the state of Request to Send (RTS) when the first data is sent or received. If the state of RTS is high, it is assumed HWFC is not in use. If HWFC is not detected, pins P0.10/P0.21 P0.07 (CTS) and P0.05 (RTS) are free for the nRF application to use.
After a power-on reset of the interface MCU, all UART lines are tri-stated when no terminal is connected to the virtual serial port. If HWFC has been used and detected, P0.07 (CTS) is driven by the interface MCU until a power-on reset has been performed or until a new DTR signal is received and the detection is redone.
P0.05 (RTS) and P0.07 (CTS) can be disconnected from the interface MCU by switching SW7 to OFF. This might be necessary if Universal Asynchronous Receiver/Transmitter (UART) without HWFC is needed while P0.05 (RTS) and P0.07 (CTS) are used for other purposes. To ensure that the P0.06 (TXD) and P0.08 (RXD) lines are not affected by the interface MCU, the solder bridges for these signals can be cut and later resoldered if needed.