Protection diode: [AN2519, page 7: Hardware Design Consideration](https://ww1.microchip.com/downloads/en/Appnotes/AN2519-AVR-Microcontroller-Hardware-Design-Considerations-00002519B.pdf) SPI program lines: [AN2519, page 9: Hardware Design Consideration](https://ww1.microchip.com/downloads/en/Appnotes/AN2519-AVR-Microcontroller-Hardware-Design-Considerations-00002519B.pdf) Inductor [ATMEGA32U4 - Datasheet, page 307](https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-7766-8-bit-AVR-ATmega16U4-32U4_Datasheet.pdf)
Protection diode:
AN2519, page 7: Hardware Design Consideration
SPI program lines:
AN2519, page 9: Hardware Design Consideration
Inductor
ATMEGA32U4 - Datasheet, page 307