Cheap AVR UPDI Programmer from ATtiny817 Xplained Mini – Wired && Coded;
Programming the new ATtiny from Arduino using UPDI [Beginner Tutorial] - YouTube
A simple and inexpensive way to program new AVR ATtiny214, ATtiny414, ATtiny814 and Atmega microcontrollers using by the UPDI interface - ООО "НПК "Спецтехнаука"
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UPDI pin noise robustness in ATtiny | AVR Freaks
BASCOM-AVR
Create Your Own UPDI Programmer - Arduino Project Hub