Enabling access port protection mechanism

Several Nordic Semiconductor SoCs or SiPs supported in the nRF Connect SDK offer an implementation of the access port protection mechanism (AP-Protect). When enabled, this mechanism blocks the debugger from read and write access to all CPU registers and memory-mapped addresses. Accessing these registers and addresses again requires disabling the mechanism and erasing the flash.

Implementation overview

The following table provides a general overview of the available access port protection mechanisms in Nordic Semiconductor devices. For detailed information, refer to the hardware documentation.

AP-Protect implementations

AP-Protect implementation type	Default factory state	How to enable	How to disable
Hardware	Disabled	Writing Enabled to UICR.APPROTECT and performing a reset.	Issuing an ERASEALL command via CTRL-AP. This command erases the flash, UICR, and RAM, including UICR.APPROTECT.
Hardware and software	Enabled	When AP-Protect is disabled, a reset or a wake enables the access port protection again.	Issuing an ERASEALL command via CTRL-AP. This command erases the flash, UICR, and RAM, including UICR.APPROTECT. To keep the AP-Protect disabled, UICR.APPROTECT must be programmed to HwDisabled and the firmware must write SwDisable to APPROTECT.DISABLE.

The following table lists related SoCs or SiPs with information about the AP-Protect mechanism they support. For some SoCs or SiPs, the AP-Protect implementation is different depending on the build code of the device. See the related hardware documentation for more information about which implementation is supported for which build code and about the differences between the supported implementations.

SoC or SiP AP-Protect matrix

SoC or SiP	Hardware AP-Protect support	Hardware and software AP-Protect support	Related hardware documentation	Additional information
nRF9161		✔	AP-Protect for nRF9161	Also supports Secure AP-Protect (see note below)
nRF9160	✔		Debugger access protection for nRF9160	Also supports Secure AP-Protect (see note below)
nRF5340		✔	AP-Protect for nRF5340	Also supports Secure AP-Protect (see note below)
nRF52840	✔	✔	AP-Protect for nRF52840
nRF52833	✔	✔	AP-Protect for nRF52833
nRF52832	✔	✔	AP-Protect for nRF52832
nRF52820	✔	✔	AP-Protect for nRF52820
nRF52811	✔	✔	AP-Protect for nRF52811
nRF52810	✔	✔	AP-Protect for nRF52810
nRF52805	✔	✔	AP-Protect for nRF52805

Note

The SoCs or SiPs that support ARM TrustZone and different Processing environments (nRF5340 and nRF91 Series) implement two AP-Protect systems: AP-Protect and Secure AP-Protect. While AP-Protect blocks access to all CPU registers and memories, Secure AP-Protect limits access to the CPU to only non-secure accesses. This means that the CPU is entirely unavailable while it is running the code in the Secure Processing Environment, and only non-secure registers and address-mapped resources can be accessed.

Configuration overview in the nRF Connect SDK

Based on the available implementation types, you can configure the access port protection mechanism in the nRF Connect SDK to one of the following states:

AP-Protect states

AP-Protect state	Related Kconfig option in the nRF Connect SDK	Description of the AP-Protect state	AP-Protect implementation type
Locked	CONFIG_NRF_APPROTECT_LOCK (CONFIG_NRF_SECURE_APPROTECT_LOCK for Secure AP-Protect)	In this state, CPU uses the MDK system start-up file to enable and lock AP-Protect. UICR is not modified.	Hardware and software
Authenticated	CONFIG_NRF_APPROTECT_USER_HANDLING (CONFIG_NRF_SECURE_APPROTECT_USER_HANDLING for Secure AP-Protect)	In this state, AP-Protect is left enabled and it is up to the user-space code to handle unlocking the device if needed. The MDK will close the debug AHB-AP, but not lock it, so the AHB-AP can be reopened by the firmware. Reopening the AHB-AP should be preceded by a handshake operation over UART, CTRL-AP Mailboxes, or some other communication channel.	Hardware and software
Open (default)	CONFIG_NRF_APPROTECT_USE_UICR (CONFIG_NRF_SECURE_APPROTECT_USE_UICR for Secure AP-Protect) This option is set to y by default in the nRF Connect SDK.	In this state, AP-Protect follows the UICR register. If the UICR is open, meaning UICR.APPROTECT has the value Disabled, the AP-Protect will be disabled. (The exact value, placement, the enumeration name, and format varies between nRF Series families.)	Hardware; hardware and software

Enabling AP-Protect with CONFIG_NRF_APPROTECT_LOCK

Setting the CONFIG_NRF_APPROTECT_LOCK Kconfig option to y and compiling the firmware is enough to enable the access port protection mechanism for SoCs of the nRF53 Series and those SoCs of the nRF52 Series that feature the hardware and software type of AP-Protect. The access port protection configured in this way cannot be disabled without erasing the flash.

Enabling AP-Protect with CONFIG_NRF_APPROTECT_USER_HANDLING

Setting the CONFIG_NRF_APPROTECT_USER_HANDLING Kconfig option to y and compiling the firmware allows you to handle the state of AP-Protect at a later stage. This option in fact does not touch the mechanism and keeps it closed.

You can use this option for example to implement the authenticated debug and lock. See the SoC or SiP hardware documentation for more information.

Enabling AP-Protect with CONFIG_NRF_APPROTECT_USE_UICR

Setting the CONFIG_NRF_APPROTECT_USE_UICR Kconfig option to y and compiling the firmware makes the AP-Protect disabled by default. This is the default setting in the nRF Connect SDK.

You can start debugging the firmware without additional steps needed.

Once you are done debugging, run the following command to enable the access port protection:

nrfjprog --rbp ALL

This command enables the AP-Protect and resets the device.

To enable only the Secure AP-Protect, run the following command:

nrfjprog --rbp SECURE