Rescue guide for your Adafruit ATmega32u4 breakout board

The ATmega32u4 breakout board is a beta product. The hardware is essentially perfect, but the firmware is wonky. At least once, I’ve convinced myself that mine was broken, but I eventually figured it out. Here’s how to get yours back in shape.

Things required to get all the way through these steps:

• The board
• A USB cable to connect the board to your computer
• A working copy of avrdude. If you have the Arduino IDE on your system, then it’s buried deep inside the IDE.
• Possibly an in-system programmer. These instructions will work with any USBTiny-compatible programmer.

Before we get going, a warning: the fuses and the bootloader must match, because the fuses tell the chip where the bootloader is. If you have the Leonardo bootloader on your board, beware! That one uses a different fuse combination (I think FCD5C3) from the Adafruit one (FCD0C3). Moreover, you need the ISP to change the fuses; an ATmega32u4 unfortunately can’t change its own fuses. It’s completely possible for you to flash the wrong bootloader to the board, leaving it in a zombie state, needing the ISP to fix it. This means that these steps might take you from a sort-of working board (e.g., an early Leonardo bootloader) to one that’s completely broken (the Adafruit bootloader with the wrong fuses), and if you don’t have an ISP to set the fuses, you’ll be stuck. TL;DR: if you have a board that mostly works, and don’t have an ISP, then stop now.

If your board is showing up as a serial device, we can learn a little from it directly, without the ISP. Run avrdude -c avr109 -p m32u4 -v -U lfuse:r:-:i -U hfuse:r:-:i -U efuse:r:-:i -P /dev/tty.usb_path_to_the_device_serial_port. A bunch of information should get spit out, ending with something like this:

avrdude: safemode: lfuse reads as FC
avrdude: safemode: hfuse reads as D0
avrdude: safemode: efuse reads as C3
avrdude: safemode: Fuses OK

If you get an error, it’s possible your board is running the Leonardo bootloader, so you’ll need to ask differently. Instead of avr109 in the previous command, tryarduino.

Now you want to figure out what those fuses mean. Visit Frank Zhao’s AVR Fuse Calculator  (in fact, you might want to set that as your home page) and type the three into the “Current settings” fields at the bottom of the page. Apply values, then go back up to the top of the page and see what the “Boot flash size” dropdown is set to.

If the size is 2048, this is GOOD because it matches the size of the known-good 2K-word (4096-byte) Adafruit build of the LUFA CDC bootloader. If this size is anything else (probably 512, for the 1024-byte Leonardo bootloader), then stop unless you have an ISP because you need an ISP to change the fuses. If the size is not 2048 but you do have an ISP, proceed to change the fuses.

To change the fuses, plug your board into your ISP and run avrdude -v -c usbtiny -p m32u4 -U lfuse:w:0xFC:m -U hfuse:w:0xD0:m -U efuse:w:0xC3:m. Again, skip this step if your boot flash size is already 2048. (Note that these fuse values came from the Adafruit BootloaderCDC makefile, but the F3 verifies as a C3 because not all 8 bits of the extended fuse are writable. Check out the fuse calculator to see for yourself.)

Next, flash the one true bootloader that today is known to work: BootloaderCDC.hex. Run either of these commands, depending on whether your board is connected to the ISP or directly to your machine:

Directly connected: avrdude -c avr109 -p m32u4 -P /dev/tty.usb_path_to_the_board_serial_port -U flash:w:BootloaderCDC.hex