The Serial Wire Debug (SWD) interface provides a non-intrusive mechanism for debugging nRF9151. It can also be used for programming firmware and accessing registers.
The SWD interface consists of SWDCLK (pin 3) and SWDIO (pin 4). The SWDIO pin has an internal pull-up resistor whereas the SWDCLK pin has an internal pull-down resistor. The SWD interface's logic level high originates from the VDD_GPIO level. For more information on the SWD interface, see Debug and trace in nRF9151 Product Specification and nRF9151 Hardware Verification Guidelines.