Do you have a bricked Adafruit SAMD board that won't boot into CircuitPython, or show up as a boot volume? Are you building your own SAMD board and want to flash our UF2-SAMD bootloader onto it?

This guide will cover wiring a J-Link to a SAMD board, flashing the bootloader, and (optionally) installing the latest CircuitPython build.

This process does require extra hardware and some software installation time. It is unfortunate when a microcontroller's firmware is corrupted - it does not happen often. But, rather than buy a new board and have one sitting, this process will get your original board back to 100%.

About the SAMD UF2 Bootloader

You will need to program the Adafruit UF2-SAMD Bootloader onto the affected board. Adafruit SAMD21 (M0) and SAMD51 (M4) boards feature an improved bootloader that makes it easier than ever to flash different code onto the microcontroller. This bootloader makes it easy to switch between Microsoft MakeCode, CircuitPython and Arduino.

Instead of needing drivers or a separate program for flashing (say, bossac, jlink or avrdude), one can simply drag a UF2 file onto a removable drive.

Parts

To flash the bootloader, you'll need a JTAG/SWD debugger. We suggest the J-Link EDU Mini. This version is smaller (the size of a USB drive) and less expensive than the full-sized J-Link EDU, BUT it is for non-commerical use only.

What does that mean?

Basically, if you're making money (or plan to make money) off your project, you'll need to order the full commercial version, or find a different debugger that suits your needs and budget better. But if you're working on personal, non-commercial projects, such as publishing some open source designs you're not selling yourself, you're good. You don't need to be a student, and you can even be a paid engineer during the week, using this on the weekend for personal non-commercial projects. As long are your intentions are non-commercial, the J-Link EDU is an excellent choice!

We also carry the full-sized J-Link EDU and the J-Link base (this model is for commercial use).

If you're a commercial user (not educational/home hobby) - you must use the commercial J-Link Base

The process for using the J-LINK models is identical, only differing in the software you will install for the specific unit on the next page.

You'll also want to get a JTAG to SWD converter board and SWD cable (not needed for the JLink mini), and a SWD breadboard breakout

Installing J-Link

Navigate to the Seger downloads page and install the version of the J-Link Software and Documentation Pack for your operating system:

Grab a Bootloader

You'll also want to download a compiled bootloader binary (.bin file) for the board you're recovering. These can be found on the Adafruit/uf2-samdx1 repository:

(Optional) Grab the Latest CircuitPython UF2

If you want to install CircuitPython onto the UF2 bootloader, you'll need a board-specific .uf2 file. Click here to find the latest build for your hardware:

When the UF2 finishes downloading, move the file to your Desktop. 

We'll leave this file alone and come back to it when we're ready to load CircuitPython onto our board's boot drive.

The Adafruit Metro M0 and M4 have a socket for connecting to a SWD cable built-in - you can connect (and disconnect) a SWD cable quickly. 

The ItsyBitsy has pins SWDIO and SWCLK broken out on the back edge of the board. You'll need a SWD cable breakout to connect the J-Link to an ItsyBitsy:

ItsyBitsy Wiring

Make the following connections between the ItsyBitsy M0/M4 and the SWD Cable Breakout:

• ItsyBitsy GND to Breakout GND
• ItsyBitsy 3V to Breakout VRef
• ItsyBitsy SWCLK to Breakout CLK
• ItsyBitsy SWDIO to Breakout SWIO

The debugging interface on the Feathers are a little bit harder to get to than the ItsyBitsy and the Metro. They're on the bottom of the board - labeled SWDIO and SWCLK. We'll need to solder a wire to each of them.

There are two programming pads located at the bottom of the Trinket M0:

On the bottom of the Gemma, there are three small pads used for programming:

You can use breadboard-friendly Alligator clips to attach the Circuit Playground Express VOut and GND pins to the breakout.

You'll need to install Atmel Studio 7. This software is free and only works on a Windows host computer. 

• If you're running macOS or Linux, you can run Windows in a virtual machine (Parallels, VirtualBox) and install Atmel Studio on the Windows virtual machine.

The download (we recommend the online installer) is available from Microchip's website.

Setting up JLink for Atmel Studio

JLinks ship with the firmware they are programmed with from the factory. We'll want to update ours to the latest version.

From the Windows search bar, search for the Jlink Configurator application and launch it.

Verify Connection in Atmel Studio

Next, open Atmel Studio. From the toolbar, select Tools -> Device Programming.

Flashing a SAMD21 M0 Board with Atmel Studio

Make sure your board is plugged into USB, then click Read in the top nav br. The empty fields for Device Signature and Target Voltage will populate.

Make sure these values appear before proceeding!

• If the board is not detected, or your wiring is not detected, Atmel Studio will throw an error that it could not connect to the board. Check your wiring (especially the SWDIO/SWCLK wires), that your USB cables are connected to the computer, and try to connect again. 

Un-set Bootloader Protection Fuse

The SAMD21 has a BOOTPROT fuse protecting the flash area of the bootloader. You'll want to clear the BOOTPROT fuse before flashing the bootloader.

Go to the Fuses page

Click Read in the bottom right section to read the fuses. Look for the BOOTPROT fuse

For SAMD21, you will need to set it to 0x07 or Zero Bytes

Click Program, wait for a confirmation that the fuses have been set. Then, click Verify.

Program Binary File

On the sidebar, click Memories.

Select the bootloader for the CircuitPython board you're recovering (the .bin file you downloaded earlier).

Select Erase Flash before programming and Verify flash after programming.

Then, click Program.

After flashing, you'll need to set the BOOTPROT fuse back to a 8kB bootloader size.

From Fuses, set BOOTPROT to 0x2 or 8KB and click Program

Check Success!

Open Windows Explorer - there should be a new volume mounted on your machine indicating that the board has been programed with the UF2 bootloader:

If you want to use the SAMD21 board with Arduino or Microsoft MakeCode, you can stop here. 

If you want to use it with CircuitPython, let's continue to installing a CircuitPython build.

Flashing a SAMD51 M4 Board with Atmel Studio

Boards like the Metro M4 and the Grand Central use a different, more powerful chip than the SAMD21, the SAMD51 - the flashing process is a little bit different for these boards.

Make sure your board is plugged into USB, then click Read. The empty fields for Device Signature and Target Voltage will populate.

Make sure these values appear before proceeding!

• If the board is not detected, or your wiring is not detected, Atmel Studio will throw an error that it could not connect to the board. Check your wiring (especially the SWDIO/SWCLK wires), that your USB cables are connected to the computer, and try to connect again. 

On the sidebar, click Memories.

Select the bootloader for the CircuitPython board you're recovering (the .bin file you downloaded earlier).

Select Erase Flash before programming and Verify flash after programming.

Then, click Program.

Open Windows Explorer - there should be a new volume mounted on your machine indicating that the board has been programed with the UF2 bootloader:

If you want to use the SAMD51 board with Arduino or Microsoft MakeCode, you can stop here. 

If you want to use it with CircuitPython, let's continue to installing a CircuitPython build.

A perk of the SAMD UF2 bootloader is the ability to load other firmware (like CircuitPython) onto the board without re-programming the bootloader.

Double tap the Reset button, a new volume should appear on your computer - boardNameBOOT (the name of this volume differs between boards).

Drag and drop the CircuitPython board and language-specific UF2 file we downloaded earlier from the desktop to the boardNameBOOT volume. 

The volume should briefly disappear, and reappear as CIRCUITPY.

Congrats, your board is un-bricked and running CircuitPython again!