Before you layout a circuit, it's generally a good idea to understand what is happening in your schematic. And before you lay out a schematic, it's generally a good idea to understand what is happening in your block diagram.
"This book is a bit technical. If you are a beginner, it's okay that you don't understand everything that follows -- your first project will still be successful. Try to pick up as much as you can, and ask questions in the forums!"
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.