The good news is that the ATmega168 chips being used have a built-in interrupt based SPI protocol.

The chip has a Multi-Protocol Synchronous Serial Engine (MPSSE) which can speak the I2C and SPI protocols, you just need to know how to active them in your code.

Woodcliff Lake, New Jersey — November 19, 2024 — Semiconductor intellectual property provider CAST, in collaboration with Silesia Devices, today announced a new IP core that implements the Serial ...