topic Re: Locking an LPC55xx sometimes bricks the device in LPC Microcontrollers

Locking an LPC55xx sometimes bricks the device

janpieterderuit — Thu, 12 Aug 2021 09:16:45 GMT

Hi,

we are using an LPC5526, and we wanted some sort of Code Read Protection like we are used to with for instance LPC546xx devices:

Lock the device in Production, so it can't be read over SWD or SPI/I2C etc (FlashMagic) anymore
Unlock the device, so it can be reprogrammed in Production.

So we implemented the locking mechanism, as described in LPC552x: How to write CMPA to disable debug subdomains from application?

Important note: to be able to unlock, the device should not be sealed (there is no way to unlock a sealed device), so use the 'development' mode.

However, unlocking a device will just do that: it won't erase the device like we are used to with the LPC546xx.
So we added a full Flash erase before unlocking.
To be able to do that we implemented is in a RAM function, see

Now when testing this, we found that sometimes the device gets bricked:

The bootloader/application no longer runs, so we are not able to unlock anymore
The device cannot be reached:
- When trying to connect over SWD connection fails (SW-DAP can't even be found..., which it can when it is correctly locked)
- Also FlashMagic cannot connect

After some testing we suspected that a connected (Segger) debugger causes this.
We can't explain why, but now we always disconnect the debugger (physically!) before locking/unlocking, and we no longer see bricked devices.

Is there some explanation for this, or another way to prevent this situation?
Is there a way to unbrick our products (in case SW-DAP can't be found)?

Thanks in advance,
Jan Pieter de Ruiter
Airios

Re: Locking an LPC55xx sometimes bricks the device

Omar_Anguiano — Tue, 17 Aug 2021 23:43:29 GMT

Hello
Hope you are well.
The reason might be that the image could be corrupted during the loading with the debugger. I suggest you always keep enabled the ISP so it can be unlocked in case of SW-DAP cannot be found. If the SW-DAP and ISP cannot be used the MCU cannot be unlocked.

I apologize for the inconvenience this is might bring you. If you have more questions do not hesitate to ask me.
Best regards,
Omar

Re: Locking an LPC55xx sometimes bricks the device

janpieterderuit — Fri, 20 Aug 2021 07:42:44 GMT

Hi Omar,

Thanks for the confirmation.

The device is completely bricked, so also over ISP not reachable I guess.

Furthermore enabling ISP would make it possible for anyone to unlock it, and that would make it useless in our usecase.

Best regards,

Jan Pieter de Ruiter