I've come to understand fuse bits in the last few weeks. I'm working on a bitbanging project on an Arduino Pro Micro, and the signals I'm working with are around 30uS each. For this reason, I think I'll need to unset the CKDIV8 to unlock the full 16MHz speed of my micro controller.

Regardless, I can't figure out how to even do that. Is it possible to modify fuse bits through the Arduino IDE? If so, where?

The only other info I've found on this subject so far is that I can do it by chaining two micro controllers together, but at that point we're getting into application specific stuff and I'm nervous to play with the fuse bits by myself until after I've followed a step-by-step guide. I'd hate to, say, erase the bootloader.

  • There are 8MHz pro micro boards and there are 16MHz pro micro boards. The difference is a different resonator or crystal (and an other voltage regulator). None of them have the CKDIV8 fuse set. I'm curious, what is your project?
    – Jot
    Dec 9, 2018 at 7:39
  • I'm (for the last 12 months) trying to capture and interpret messages sent by the head unit of my car's stereo. It uses AVCLAN. My intention is to capture specific button presses so that I can interact with my phone over bluetooth through the buttons already in my car. The bluetooth component I'm using is a BK8000L. Dec 10, 2018 at 1:28

1 Answer 1


The Arduinos are shipped without CKDIV8 enabled.

New chips without bootloader and the chips after chip erase are having CKDIV8 active.

And you can change the clock prescaller in software, but it mess up everything that depends on it. Delays, serial baud rate and USB as well.

However fuse setings are stored in boards.txt definition file. None of the bootloaders would work with default settings. They are usually compiled for 16MHz @5V and 8MHz @3.3V with xtal or resonator. But the custom boards may have much more options.

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