Using the SERCOM I/O lines requires the I/O pins to be configured using port configuration (PORT).
The PORT Control bit (PINCFGn.DRVSTR) is still effective for the SERCOM output pins. The PORT Control bit (PINCFGn.PULLEN) is still effective on the SERCOM input pins, but is limited to the enabling or disabling of a pull down only (it is not possible to enable or disable a pull up)
The SERCOM has four internal pads, PAD[3:0], and the signals from I2C, SPI, and USART are routed through these SERCOM pads, through a multiplexer. The configuration of the multiplexer is available from the different SERCOM modes. Refer to the mode-specific chapters for additional information.