User Manual

For example,

bossac -p=/dev/cu.usbmodem14301 -e -w -v -R adafruit-circuitpython-feather_m0_express-3.0.0.bin

Using bossac Versions 1.9 or Later

For M0 boards, which have an 8kB bootloader, you must specify -offset=0x2000 , for example:

bossac -p=/dev/cu.usbmodem14301 -e -w -v -R --offset=0x2000 adafruit-circuitpython-feather_m0_express-3.0.0.bin

For M4 boards, which have a 16kB bootloader, you must specify -offset=0x4000 , for example:

bossac -p=/dev/cu.usbmodem14301 -e -w -v -R --offset=0x4000 adafruit-circuitpython-feather_m4_express-3.0.0.bin

This will e rase the chip, w rite the given file, v erify the write and R eset the board. On Linux or MacOS you may

need to run this command with sudo ./bossac ... , or add yourself to the dialout group first.

Updating the bootloader

The UF2 bootloader is relatively new and while we've done a ton of testing, it may contain bugs. Usually these bugs

effect reliability rather than fully preventing the bootloader from working. If the bootloader is flaky then you can try

updating the bootloader itself to potentially improve reliability.

If you're using MakeCode on a Mac, you need to make sure to upload the bootloader to avoid a serious problem with

newer versions of MacOS. See instructions and more details here (https://adafru.it/ECU).

In general, you shouldn't have to update the bootloader! If you do think you're having bootloader related issues,

please post in the forums or discord.

Updating the bootloader is as easy as flashing CircuitPython, Arduino or MakeCode. Simply enter the bootloader as

above and then drag the

update bootloader uf2

file below. This uf2 contains a program which will unlock the

bootloader section, update the bootloader, and re-lock it. It will overwrite your existing code such as CircuitPython or

Arduino so make sure everything is backed up!

After the file is copied over, the bootloader will be updated and appear again. The INFO_UF2.TXT file should show the

newer version number inside.

For example: