Bootloader ATtiny – Micronucleus

Informatie (ENG):

Micronucleus V2.03

Micronucleus is a bootloader designed for AVR ATtiny microcontrollers with a minimal usb interface, cross platform libusb-based program upload tool, and a strong emphasis on bootloader compactness. To the authors knowledge this is, by far, the smallest USB bootloader for AVR ATtiny

The V2.0 release is a complete rewrite of the firmware and offers significant improvements over V1.x:

• Support for the entire ATtiny family instead of only ATtiny85.
• Much smaller size. All configurations are below 2kb.
• Interrupt free V-USB: no patching of the user program INT-vector anymore.
• Faster uploads due to new protocol.
• Far jmp also allows using ATtinies with more than 8kb flash.
• Many robustness improvements, such as compatibility to USB hubs and
less erratic time out behavior.

Due to the many changes, also the upload tool had to be updated. The V2.0 upload tool is backwards compatible to the V1.X tool, though.

The last release of the V1.x can be found here:


The bootloader allows uploading of new firmware via USB. In its usual configuration it is invoked at device reset and will identify to the host computer. If no communication is initiated by the host machine within a given time, the bootloader will time out and enter the user program, if one is present.

For proper timing, the command line tool should to be started on the host computer _before_ the bootloader is invoked.

Windows machines will need to install the libusb drivers found in the /windows_drivers folder. Clean Micronucleus devices without uploaded userprogram will not time out and allow sufficient time for proper driver installation. Linux and OS X do not require custom drivers.

Windows 10: Installing unsigned drivers became more difficult in Windows 10. Please use the Zadig driver installer as provided in the /windows_driver_installer folder.

Please invoke the command line tool with “micronucleus -help” for a list of available options.

The bootloader resides in the same memory as the user program, since the ATtiny series does not support a protected bootloader section. Therefore, special care has to be taken not to overwrite the bootloader if the user program uses the self programming features. The bootloader will patch itself into the reset vector of the user program. No other interrupt vectors are changed.


Micronucleus can be configured to support all devices of the ATtiny series, with the exception of the reduced core ATtiny 4/5/9/10/20/40.

To allow maximum flexibility, micronucleus supports a configuration system. To compile micronucleus with a specific configuration, please invoke the AVR-GCC tool-chain with:

make CONFIG=<config_name>

Currently, the following configurations are included and tested. Please check the subfolders /firmware/configurations/ for details. Hex files can be found in /releases.

You can add your own configuration by adding a new folder to /firmware/configurations/. The folder has to contain a customized “” and “bootloaderconfig.h”. Feel free to supply a pull request if you added and tested a previously unsupported device.

If changes to the configuration lead to an increase in bootloader size, it may be necessary to change the bootloader start address. Please consult “” for details.

Other make options:

Please “make clean” when switching from one configuration to another.

Devices using Micronucleus

Micronucleus is widely installed on thousands of open source hardware devices. Please find an incomplete list here:


This project is released under the GPLv2 license. Code uploaded via the bootloader is not subject to any license.

In addition, we’d like you to consider these points if you intend to sell products using micronucleus:

• Please make your hardware open source. At least the schematic needs to be
published according to the license inherited from V-USB.

• Your documentation should mention Micronucleus and include a link to the
main repository (

• Please do not “rebrand” micronucleus by renaming the USB device.

• Feel welcome to submit a pull request to include your product in the
“Devices using Micronucleus”-list.

Download micronucleus @