References

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