Disengaging APPROTECT

[Richard Yoo]

Hi,

On nRF52840, we are able to engage APPROTECT by setting NRF_UICR->APPROTECT to 0.
However, writing 0xFF to NRF_UICR->APPROTECT does not seem to disengage APPROTECT.

For example, after executing below code, APPROTECT still remains engaged:

NRF_UICR->APPROTECT = 0x00;
NRF_UICR->APPROTECT = 0xFF;

This behavior is different from nRF52832, where writing 0xFF to NRF_UICR->APPROTECT disengaged APPROTECT.

What is the proper way to disengage APPROTECT on nRF52840?
Thank you in advance.

[Vidar Berg]

Hi,

Unfortunately, on the 52840, the only way to disable APPROTECT is through a full chip erase. UICR is placed in FLASH memory, and on the nRF52832 you could always erase it with NVMC->ERASEUICR and thus allow you to disable readback protection from the program code. However, on the nrf52840, this operation becomes unavailable once read-back protection is enabled.

Excerpt from the NVMC chapter of the 52840 PS:

Access port protection behavior

When access port protection is enabled, parts of the NVMC functionality will be blocked in order to prevent intentional or unintentional erase of UICR.

Table 1. NVMC Protection

	CTRL-AP ERASEALL	NVMC ERASEPAGE	NVMC ERASEPAGE PARTIAL	NVMC ERASEALL	NVMC ERASEUICR
APPROTECT
Disabled	Allowed	Allowed	Allowed	Allowed	Allowed
Enabled	Allowed	Allowed	Allowed	Allowed	Blocked

Best regards,

Vidar

[Richard Yoo]

That explains what we're observing. Thank you!