I have downloaded the Arduino IDE and that includes a version of BOSSAC that may be useful. That may be a quickest method to find a working replacement version.
My interest in the use of bossac comes from the release of the Arduino Nano 33 BLE (Sense) as they are based on nRF52840 with a special boot-loader designed to work in the arduino environment.
I am looking to see if it is possible to use zephyr on these boards without having to replace the boot-loader which may not be possible. I do know that the boot-loader can easily be replacement via the debug interface pins on the underside of these board but that's not that useful for most people.
