this may be helpful for folks using the latest attiny and atmega chips that are programmed with "synch uart" UPDI rather than "SPI" AVR-ISP: we've updated our self-contained/standalone programming library to add UPDI support!
the library is here, https://github.com/adafruit/Adafruit_AVRProg
example here: https://github.com/adafruit/Adafruit_AVRProg/tree/master/examples/attiny817_updiprog
we can erase-program-verify an 8KB flash chip in 2 seconds, super fast. great for loading firmware in production.
thanks to bradanlane's portaprog for much of the updi support implementation: https://gitlab.com/bradanlane/portaprog/-/blob/master/src/avrupdi.cpp
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