This installment of Elektor Books presents roughly a quarter chapter excerpted from Warwick A. Smith’s Ultimate Arduino Uno Hardware Manual published by Elektor in June 2021. The Arduino Uno is almost a commodity these days. Being cheap too, it’s ever so easy to overlook important hardware specifications and then run into trouble in your target application. Issues with the Uno Bootloader and the Fuse Settings still confound many readers and rank high in Elektor Lab's Tech Queries Top-10. So, it’s time for some “ultimate” answers given by this new Elektor book.