8-bit MCU for low-power control — STM8L101K3T6TR
If your design lives in a conditioned indoor space, the commercial portion of that range is still fine; the full span just gives margin for cold-start or summer enclosure soak.
Memory sizing — 8 KB Flash, 1.5 KB RAM, 2 KB EEPROM
With 8 KB of Flash and 1.5 KB of RAM, this part fits firmware for a single control loop, a UART protocol handler, and a few hundred bytes of data buffer. The 2 KB EEPROM is a separate block — no wear-leveling needed for infrequent parameter writes like calibration offsets or serial numbers. If your application needs over-the-air updates or a larger protocol stack, you will bump into the Flash ceiling; the STM8L151 series with up to 32 KB is the next step up in the same EnergyLite family.