Looking at the SAMD21 datasheet, it appears as though we might indeed be able to have 4 UART ports. I see this:
Up to six Serial Communication Interfaces (SERCOM), each configurable to operate as either:
- USART with full-duplex and single-wire half-duplex configuration
- I2C up to 3.4MHz
- SPI
- LIN slave
I see one each SPI and I2C bus on the 28-header, and SerialUSB is one UART. So Serial1 and (now) Serial makes three serial ports (5 total SERCOMs). So it’s conceivable that there might be a Serial2 after all. That might be a reason @ansonhe97 mentioned it by name in his post earlier today.
TB