topic Re: LPC55SXX secure boot fall through mode in LPC Microcontrollers

LPC55SXX secure boot fall through mode

johannes_ferch — Tue, 21 Apr 2026 11:38:24 GMT

Hi everyone,

we have a question regarding the secure boot process on LPC55SXX microcontrollers. We've observed that when experimenting with invalid images during the secure boot process the part is not responsive anymore. This is somewhat expected as the part prohibits the boot in that scenario. We are able to recover from this condition by asserting the ISP pin as described.

The user manual (UM11126 rev. 2.8, p. 144) shows the following flow diagram and also mentions a fall through mode highlighted in yellow below:

We however couldn't find any more information about the fall through mode throughout the technical documentation or the forums.

Is there any way to configure this mode somewhere? For our application it might be interesting to assess if jumping into the bootloader after a failed image validation might be possible or not. This could simplify the programming of valid images in case something might have compromised the boot image in the field.

Thanks in advance,

Johannes

Re: LPC55SXX secure boot fall through mode

Alice_Yang — Fri, 24 Apr 2026 09:27:11 GMT

Hello @johannes_ferch

The fall-through mode configuration is controlled by DEFAULT_ISP_MODE in CMPA.
When it is set to 111, the ISP fall-through function is disabled.

Thank you.

Alice

Re: LPC55SXX secure boot fall through mode

johannes_ferch — Tue, 28 Apr 2026 08:11:15 GMT

@Alice_Yang Thanks for your reply, this seems like a good pointer but I believe that the fall-through is enabled by default anyway. Will check and get back to you though.

Best regards

Re: LPC55SXX secure boot fall through mode

becelin223 — Tue, 05 May 2026 03:30:05 GMT

sdf

Re: LPC55SXX secure boot fall through mode

johannes_ferch — Thu, 07 May 2026 08:52:18 GMT

This was helpful information, after further investigation we found out that the underlying issue had a different and unrelated cause altogether. Topic can be closed, thanks.