This intermediate bootloader makes sure the upgrade process of the Legacy Bootloader + Softdevice v2.0 to Secure Bootloader + Softdevice v6.1 happens in a fault tolerant way. The following stages are visualized in the process illustration.

This current legacy bootloader is replaced by the next stage bootloader. In the later stages, the bootloader will not start the firmware app in case there exists one.

Present at address 0x79000 (same as the old bootloader), this bootloader expects Bootloader v1.9.0 package via DFU process. Once, the new bootloader (v1.9.0) is received, it copies it to the address 0x71000 and sets the BOOTADDR in UICR region to the same.

Present at address 0x71000, this bootloader expects a DFU Package with both Softdevice v6.1 and bootloader v2. Once this package is received, it copies the softdevice to its address (0x1000), and copies the bootloader to the final address 0x76000. After the copy of the bootloader is done, it verifies if the copy was successful. With a successful copy, the BOOT_ADDR (aka. BOOTLOADER_UICR, address = 0x10001014) in the UICR region is set to the final address 0x76000 and initiates a reset. Upon reset the new bootloader (v2) takes the control along with the new softdevice v6.1.0.

The largest address that the intermediate bootloader (which is 5 pages in size) can be at is 0x76000 - 0x5000 = 0x71000, as to not overlap with the v2 bootloader.

In case app data should be preserved, the intermediate bootloader address should also be adjust as to not overlap the app data of the 1.8.0 bootloader. So in case you have 0x4000 bytes of app data, the intermediate bootloader should be 0x79000 - 0x4000 - 0x5000 = 0x70000.

Copying the app data from the old app data region to the new app data region (in this example from 0x79000 - 0x4000 = 0x75000 to 0x70000 - 0x4000 = 0x6C000, and after that to 0x76000 - 0x4000 = 0x72000) is not implemented yet though.

At first, we thought the bank size was too small, but the update process of SDK 11 is different than SDK 15. See SDK 11 vs SDK 15 documentation.

This means, however that the uploaded softdevice (located at bank0, or 0x1C000) overlaps with where it will be copied to. The bootloader solves this by copying the softdevice in 3 parts. In our case (old softdevice ends at 0x1C000, new softdevice ends at 0x26000 and is of size 0x25000) this becomes:

• Copy 0xD800 bytes from 0x1C000 to 0x1000.
• Copy 0xD800 bytes from 0x29800 to 0xE800.
• Copy 0xA000 bytes from 0x37000 to 0x1C000.

This process is implemented in dfu_sd_img_block_swap(), used by dfu_sd_image_swap(). The values used in the above copies can be calculated via:

• 0xD800 = (0x1C000 - 0x1000) / 2
• 0x29800 = 0x1C000 + 0xD800
• 0xE800 = 0x1000 + 0xD800
• 0xA000 = 0x26000 - 0x1C000
• 0x37000 = 0x1C000 + 0x25000 - 0xA000

For this upgrade, we don't need to preserve the app data. However, changing the reserved app data size, will change the bank1 address, leading to issues when the bootloader image gets validated. See this post and this post.

This can be fixed as done here, but for the intermediate bootloader, we just kept the reserved app data the same, as it doesn't matter anyway.

The program flow can be viewed in the flowchart.

• nRF SDK 11
• nRF SDK 15.3
• nRF Crownstone's New (Current) Builds (Firmware, Bootloader)
• nrfutil version 0.3.0
• Create a file (named paths.config) in the same directory as Makefile. In this file, define the variables, GNU_INSTALL_ROOT to point to the path where gcc-arm is installed and SDK_PATH to top level of nRF SDK 11 directory. This should looks as follows:

GNU_INSTALL_ROOT = /home/workspace/Development/gcc-arm
SDK_PATH = /home/workspace/Development/nRF-sdk-11

To create the intermediate version at once, run the following command $ create.sh debug. This will create two DFU packages namely, debug_v1.8.0.zip and debug_v1.9.0.zip. If this step is used, the first sub-step of both step 1 and 2 can be skipped.

• To create a bootloader of version 1.8.0, run the following command $ create.sh debug 1.8.0. This will create a DFU package of the version v1.8.0 with a package called debug_v1.8.0.zip
• DFU the above created package ONLY when the read firmware version is 1.3.0.

• To create a bootloader of version 1.8.0, run the following command $ create.sh debug 1.9.0. This will create a DFU package of the version v1.8.0 with a package called debug_v1.9.0.zip
• DFU the above created package ONLY when the read firmware version is 1.8.0.

With the penultimate stage completed.

• Create a DFU package for Softdevice 6.1, Crownstone's Bootloader (or Nordic's SDK 15.3 Secure bootloader), with the following command: $ nrfutil dfu genpkg --bootloader <crownstone_bins_dir>/bootloader.hex --softdevice <softdevice_path/>s132_nrf52_6.1.1_softdevice.hex --sd-req 0x81 <package_name>.zip.
• DFU the above created package ONLY when the read firmware version is 1.9.0.

• src/bootloader.c
• src/dfu_dual_bank.c
• src/dfu_transport_ble.c
• src/main.c
• inc/dfu_types.h
• inc/dfu.h