This is a common ‘problem’ that can occur when you indeed enable LPM (or other things like disable debug pins etc). Therefore, most MCUs have certain procedures to keep the MCU halted after a reset, before any code is executed and the MCU gets unreachable. IIRC, ARM Cortex provides a vector-catch operation to halt the MCU after reset. Some IDEs (like Simplicity Studio for EFM32 MCUs) have a special unlock operation.
Your solution isn’t new, but works great to solve/work-around the problem
I had the same issue with openOCD and SAM0 MCU in the past. But I was always to recover my chips using JLinkExe tool from Segger. I just erased the whole memory using JLinkExe to unbrick the device and re-run openocd afterwards.You should probably take a look too.
Low power modes are indeed a difficulty when debugging.
Are you using the Kinetis low power mode PR? https://github.com/RIOT-OS/RIOT/pull/7897
Do you have any feedback that could help getting it ready for merging?
I have had similar issues when experimenting with low power modes. Do you have access to the reset pin on your device?
If you have the reset pin connected on the JTAG header, you can use the OpenOCD option reset_config connect_assert_srst, in combination with the reset halt command to make the CPU halt so that you can reload a new firmware.
I spent a lot of time trying to save these dead MCUs.
Did you manage to recover them?
I was in that situation a couple of times.
It sometimes helps to just retry flashing, hammering the reset button.
The idea is that you want to hit reset so that the debugger tries to
stop the MCU in the (possibly very short) timespan between power-up and
going to sleep.
$ BOARD=xxx make all (to create the binary)
$ while true; do BOARD=xxx make flash-only; done
This will fail on its own. Then just start pushing the reset button/pin
often, like a couple of times per second. At this point you don't care
if flashing succeeds, just the initial flash erase starts so on the next
reboot, the board hard-faults but "grabbable" by the debugger.
Once you see that the flasher didn't fail right away but managed to get
hold of the mcu and start erasing, you can stop, reset once more and
hopefully flash a proper image.
Depending on the user programmer (tool), replacing "make flash-only"
with a faster-executing flash erase command speeds up the process.