Configuring partitions on an nRF91 Series device

The default SRAM and Flash partition layouts in Zephyr allocate space for all Trusted Firmware-M (TF-M) features and require you to enable MCUboot. As MCUboot is disabled by default in nRF Connect SDK, and the modem handles the security requirements, all applications are expected to set the partition tables using a devicetree overlay. This ensures that the partitions match the actual requirements and maximizes the available memory for your application.

Including default partition layouts

The easiest way is to include two of the .dtsi files from the nrf/dts/samples/cellular folder to your board’s .overlay file.

#include <samples/cellular/nrf91_no_bootloader_partitions.dtsi>

#include <samples/cellular/nrf91_sram_partitions.dtsi>

• There are four sample flash layouts:

  • nrf91_no_bootloader_partitions.dtsi - This layout works with minimal TF-M configurations without a bootloader. This layout is used by most samples.

  • nrf91_bootloader_partitions.dtsi - This layout works with minimal TF-M setups that include MCUboot.

  • nrf91_immutable_bootloader_partitions.dtsi - This layout works with minimal TF-M and both an nRF Secure Immutable Bootloader and an updatable MCUboot. Both the b0 and MCUboot are built separately and must also know about these partitions. You need to add a b0 and mcuboot folder inside a sysbuild folder with a prj.conf and the required overlay in a boards folder.

  • nrf91_crypto_partitions.dtsi - This layout works for the full TF-M features without a bootloader. You can use it to test TF-M functionalities.

• There are two sample SRAM layouts:

  • nrf91_sram_partitions.dtsi - This layout works with minimal TF-M configurations. This layout is used by most samples.

  • nrf91_sram_crypto_partitions.dtsi- This layout works for the full TF-M features. You can use it to test TF-M functionalities.

Creating a custom partition layout

Instead of including these .dtsi files, you can copy their contents and modify them as required.

The flash memory uses 4 KB pages, so each partition must start on a 4 KB boundary. The flash memory has 32 KB blocks that can be marked as secure or non-secure. Any 32 KB block that contains a slot0_ns_partition, slot1_ns_partition, or storage_partition will be marked as non-secure. Secure partitions must not share a 32 KB block with any non-secure partition. In addition, the storage_partition must not share a 32 KB block with the slot0_ns_partition. Any partitions that need to be accessed by the application must be part of (or included within) the storage partition. See TF-M memory partitioning for more information.

The SRAM has 8 KB blocks that can be marked as secure or non-secure. You need to add a specific block of non-secure memory that is shared with the modem as described in Devicetree integration.

References

See Migrating from Partition Manager to devicetree (DTS) for instructions on how to migrate your project to devicetree in general.