ButtLoad (short for Butterfly Loader) is created by Dean Camera.
Dean's project can be found on AVRFreaks.net, direct link here
and on his personal website (link updated March 27, 2007)
On this page you can find the information to turn ButtLoadButterfly (BLBF) into a USB-device.
Download here the package with all necessary info: BLBF-USB Package
With the release of version 2.1 (currently 2.0), this package will be included in Dean's release.
To make BLBF (RS232 or USB-version) work with 3.3V and 5V
target-devices, some circuitry is necessary to protect BLBF from too
Some protection against damage due to wiring-errors on the target-board is a good idea as well.
I did quite some tests with a simple, but effective resistor-zener-network.
(click to enlarge)
It works well on 3.3V targets, 5V targets, and even on STK200 ! For
that to work, it was necessary to lower the value of the resistors in
The circuit protects BLBF against voltages higher than 3.4 V, and
that's safe for the flash-memory on the Butterfly. The flash-memory is
the only Vcc-critical component on the board.