Hi @johanleroy
I hope that you are doing well!
All of the LPC55 MCUs provide features to disable ISP entry and debug access , even the ones that do not have an S ( LPC55S security modules such as PRINCE , secure boot etc. ). This is done by setting dedicated bits on the CMPA and CFPA areas protected flash region or PFR. Refer to SOCU_PIN and SOCU_DFLT on the user manual
The PFR can be programmed from the application (using API) or using tools such as the MCUXpresso secure provisioning tool , elftosb gui, or by directly writing CMPA and CFPA using bl-host.
Those settings might be confusing at first glance and if there is any improper setting into the PFR you might brick your chip. If you disable ISP and debug access you won't be able to recover the MCU externally.
Let me know if you have any other question
Diego.
