Is there any reason not to load the Arduino Pro 8Mhz bootloader onto a 5V arduino (custom design, just a standard ATMega328p)? I'm doing some battery backup stuff with it that doesn't play nice with 16MHz (since the backup is only 3V). But I want to run the rest of the system at 5V under normal power conditions.

Should I be able to just load the Arduino Pro 3.3V 8Mhz bootloader (and select that profile, or many create my own)?

Anything to watch out for?

  • \$\begingroup\$ Please refer to the following question and its answers arduino.stackexchange.com/questions/12/… it discusses several of the issues with overclocking \$\endgroup\$
    – mpflaga
    Mar 9, 2014 at 1:39
  • \$\begingroup\$ @jerry19 if your bootloader uses the internal clock source, adding a crystal won't change that. The bootloader would need to be changed. \$\endgroup\$
    – Passerby
    Mar 27, 2014 at 2:19

1 Answer 1


Only difference between the regular Arduino 16mhz bootloader and the 8mhz bootloader is that the 16mhz requires an external 16mhz crystal. The 8mhz bootloader uses the internal clock source instead. Less accurate.

AND the lower the voltage, the lower the internal crystal can work at. But in your case, there is no problem with running at the internal 8mhz at 5v.

  • \$\begingroup\$ I assume the same would go for using an external 8mhz crystal? Was looking into using the internal oscillator and realized that it would be easier (in terms of arduino compatibility) to use an external crystal. \$\endgroup\$
    – Adam Haile
    Mar 9, 2014 at 1:25
  • \$\begingroup\$ And F_CPU gets defined differently. \$\endgroup\$ Mar 9, 2014 at 2:05

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