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

Arduino-UNO-PCB-Design This repository contains full PCB design of Arduino UNO. This design was done completely from scrath, starting with selecting components from digikey, creating schematic symbols ...

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