The Arduino Uno-compatible board has an MCS-51 (often called 8051 instead) instead of the usual ATmega328P/ATmega168. Specifically, [ElectroBoy] uses the AT89S52.

LUGANO, Switzerland--(BUSINESS WIRE)--Arduino, the world's leading open-source hardware and software platform, today announced the launch of its next-generation UNO board, a significant revision ...

The long-awaited Arduino Due just hit the market, replacing the 8-bit, 16 MHz brain of the popular Uno microcontroller prototyping platform with a 32-bit, 84 MHz processor, while augmenting inputs ...