Welcome back all, I tried changing the extended fuses from 0x05 to 0xFC in boards.txt to get 4.3V level, but it did not work. It showed error : ***failed; avrdude: verification error, first mismatch at byte 0x0000 0xfc != 0x04 avrdude: verification error; content mismatch. Then I tried to change extended fuses to 0x04, then it worked! Thanks to the comment of Jack Christensen : "BTW, the top five bits of the efuse sort of "aren't there", so setting the efuse to 0x04 turns out to be the same as 0xFC."