Found these bootloaders and using the 1MHz for some low power experiments. I did get this error message when burning the bootloader (using USBASP programmer) : "avrdude: WARNING: invalid value for unused bits in fuse "efuse", should be set to 1 according to datasheet This behaviour is deprecated and will result in an error in future version You probably want to use 0xfd instead of 0x05 (double check with your datasheet first)." The bootloader woas succesfully burned so the message was informational. I did look into this and made the following changes in boards.txt : I changed all the lines containing "......bootloader.extended_fuses=" There are multiples sections in the file that have these lines. old value -> new value 0x04 0xFC 0x05 0xFD 0x06 0xFE 0x07 0xFF I used this site link text for calculating the new values for the extended fuse. Now the error message is gone. Maybe this is of any help when using these bootloaders.