SAM sercom UART only works for some pin mappings

[chris-durand]

The SAM sercom peripheral has 4 peripheral signals PAD[0] to PAD[3] whose functions can be assigned in the driver. For example the D21 sercom UART allows to assign either PAD[0] or PAD[2] as UART TX pin:

Currently the configuration is hard-coded in the driver to one specific case:

// Set rx/tx pins
{{ peripheral }}->USART.CTRLA.bit.RXPO = 0x3; // Pad3
{{ peripheral }}->USART.CTRLA.bit.TXPO = 0x1; // Pad2;

These registers have to be set according to the provided pins in the UART connect() method.