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 has partnered with Renesas and Espressif to introduce the latest addition to its UNO family, the UNO R4, which offers a 3x performance improvement over the UNO R3 and is available in a Wi-Fi ...

“We collaborated with Renesas and Espressif,” said Arduino CEO Fabio Violante. This partly obscured photo is the only one Arduino has allowed out so far “Designed for maximum compatibility, the Uno R4 ...