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Replying to @josecastillo

On the back of the main board: a battery backup for the DS3231 RTC (which I forgot in my last @digikey order, oops), STEMMA port for Neopixeling, STEMMA-QT for adding more I2C gadgets (maybe an OLED?) and some socket header in the shape of an ESP-01 for wifi (as yet untested).

(original)