Many Silicon Labs Bluetooth Low Energy devices are shipped with Bluetooth stack v2.0.0. This image includes the NCP application and the UART DFU type legacy bootloader v3.

Since the legacy bootloader is not upgradable, and will not be supported in the future, it is strongly recommended to reprogram all your devices with the latest Gecko Bootloader (https://www.silabs.com/documents/public/user-guides/ug266-gecko-bootloader-user-guide.pdf) as a first step.

However, if there is absolutely no way to connect to your device’s programming pins, and you can only access the device via UART using a host controller, bootloader v3 can still be used as a last solution.

When you use bootloader v3 for upgrade, the images to be uploaded must have a length multiple of 4 bytes! The same applies for legacy bootloader v4 that is part of Bluetooth SDK v2.1.x (as described in https://www.silabs.com/community/wireless/bluetooth/knowledge-base.entry.html/2016/12/06/bootloader_changesi-jsoH). That is, if you use the factory firmware, or if you flashed your device with Bluetooth SDK v2.0.x or v2.1.x, you have to be careful with the uploaded images. Bootloader v5 (introduced in Bluetooth SDK 2.3.0) can handle also images that are not word aligned.

If you use GCC compiler to build the upgrade image, the 4-byte alignment is not always satisfied, hence it can happen, that the bootloader returns with CRC error, even if you uploaded a valid image. To ensure that the image has a correct length you have to modify the linker file in your project before build.

Find the appropriate .ld file in your project and modify it like this:

1. Find the following lines:

     .text_stack_bin :
     {
          KEEP(*(.binstack*))
     }> FLASH

    

2. And modify it to:

     .text_stack_bin :
     {
          KEEP(*(.binstack*))
       . = ALIGN(4);
     }> FLASH

    

Now you can build your project, and generate the .ebl files by running create_bl_files.bat in your project. You will need full.ebl for upgrading your device with a new SW.