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Completely open-source CPU architecture.
Compatible with Arduino software and accessories, making it an effective drop-in replacement.
Con Designed with Altium
"The design files for the HiFive 1 were made with Altium, a proprietary and non-Free software."
Con Leverages an FTDI chip, so not fully open-source
"This is an Arduino-compatible board with an FTDI chip providing the USB to serial conversion. [...] An FTDI chip is not Open Source, and they have designed drivers to break chips that aren’t theirs."