References
1.
AVR317: Using the USART on the ATmega48/88/168 as a SPI master
2.
AVR054: Run-time calibration of the internal RC oscillator via the UART
3.
STK600 Routing Cards and Socket Cards
4.
ATmega324PB Xplained Pro User Guide
5.
Atmel Studio - Programming Dialogue
6.
ATTINY104-XNANO User Guide