6 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