Atmega328p at 3.3v

Venturing down the path of low voltage Atmega (Arduino)?  The following references the two steps (set your fuses and add a board entry) to allow you to use an off-the-shelf 328p “naked” and at very low voltage ranges (disabling brown-out).
If your 328 chip is using the default fuses (16 MHz external clock setup), run
the following avrdude commandline to re-write fuses. This disables
brown-out detection completely (allowing for low voltage thresholds), and sets to use the internal, 8MHz clock:

avrdude -c usbtiny -p m328p -B 1.0 -u -U lfuse:w:0xe2:m -U hfuse:w:0xd8:m -U efuse:w:0xff:m

Last but not least, add the following to your “boards” entry in the ArduinoIDE:

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