3. Data Bus Overview
The Universal Asynchronous Receive and Transmit protocol allows full-duplex communication between two nodes. The only requirement is that the two nodes agree upon a common transmission rate (BAUD rate).
Unlike the other communication protocols used in our design, UART requires a cross-over connection. Transmit→Receive and Receive→Transmit.
Uart Connection from CircuitBasics.com shows a cross-over connection between the two ICs.
Since it has a relatively slow transmission rate, UART data lines can run the full length of an entire PCB panel and off-board through several meters of wiring without much an issue.
Asynchronous communication nodes are usually tolerant of a slight timing mismatch. But if the transmission rates are too different than receiving rates, the information ends up garbled. If you ever see random characters or no characters on a serial-debug screen, the usual culprit is a timing mismatch.
In Our Circuit
The only UART line connects the SAMD11 microcontroller to the ATMega328 microcontroller.