Universal Asynchronous Receiver/Transmitter (UART) communicates through the GPIO pins. All GPIO pins can be configured for UART communication in the application firmware. The UART interface can be used to control the Device Under Test (DUT) using AT commands.
The UART interface is useful for logging transactions between the LTE network and the nRF9151 modem. The modem trace log is required for technical support if problems related to the LTE network occur. The modem trace can also be captured over RTT. For more information, see Cellular Monitor app.
If a Universal Serial Bus (USB) interface is required, a USB-UART bridge cable can be used, for example from FTDI. You can also add a USB-UART converter on a test board, as shown in Example of USB-UART converter based on FTDI circuit.
The nRF9151 and USB-UART converter must have the same I/O voltage level. Thus, VCCIO is equal to VDD_GPIO in the following figure.