Before updating BNO055 firmware via the 9DOF Debug debugging interface, the Ambiq chip should be flashed with a firmware (such as test_buzzer.c) that does not activate BNO055, otherwise the JLink connection would not be established.
#start JLink
JLinkExe -Device ATSAMD20J18A -if SWD -speed 4000 -AutoConnect 1
#inside JLink program
#this load the new firmware, not including the bootloader
loadbin mybno.bin 0x4000
using a normal tottag, with JLink connection established you can save the original bootloader or the firmware, just incase something goes wrong
savebin bl.bin 0x0000 0x4000 //save the bootloader, from start up to address 0x4000
savebin saved.bin 0x4000 0x3C000 //save the firmware, not including the bootloader
savebin full.bin 0x0000 0x3c00 //save the full firmware, including the bootloader
on a trial device, load the saved firmware
loadfile saved.bin 0x4000
the bno055 bootloader could also be obtained from https://github.com/NightHawk32/BMF055-flight-controller/blob/master/AT04189_bootloader/load%20sam-ba/samd20_sam-ba_image.hex
with a bricked device, the i2c read in imu_init fails
in this case, openocd could be used.
#tab 1: connect to the device
openocd -f interface/jlink.cfg -c "transport select swd" -f target/at91samdXX.cfg -c
#tab 2: tell the device to unlock
telnet localhost 4444
#inside the telnet connection
#this command unlock the protected memory
at91samd bootloader 0 after unlocking the protected memory, use jlink to load the original bootloader
loadbin bl.bin 0x0000
or load the full saved firmware including the bootloader
loadbin full.bin 0x0000