A handful MCU pins are enough to implement a temperature logger; Two connect to the sensor via I2C and one more for the interrupt option, four for SPI to connect to the memory chip, two for UART to extract stored data, change settings, user firmware update etc, one for the reset pin for in-system programming (plus SPI), and one for a blinking LED. Add two for the real time crystal and all the power pins and on the 328PB this leaves 14 pins unused.

Before: 14 pins were left unused.

I could go for a lower-count MCU but the only AVR that fits all requirements is the Tiny417/817. This is a new chip and uses some different concepts, which would mean a steep learning curve. While this is an option for future models, I don't have time for it at the moment as I need to produce 50 and program them for deployment by next summer in my spare time.

This leaves me with 14 unused pins, so I opted for making those accessible. I had to redesign the layout to make way for all these lines but, at no cost difference, this temperature logger can now double as a coin-cell powered development board.

After: All MCU pins are used or accessible via 0.05" headers.