Gecko bootloader consists of two stages.

• The first stage is dummy. It can only start the second stage and it can overwrite the second stage with an already uploaded upgrade image. The first stage can only be flashed to the device and cannot be upgraded!
• The second stage starts the application and it is able to upgrade the application in several ways, like using UART, SPI, Bluetooth, etc. It has other features as well, like handling multiple storage slots, verifying the images, etc. Unlike the first stage, the second stage is upgradable through the same channels as the application (UART, SPI, Bluetooth, etc.)

Upgrading via UART

This part discusses how you can upgrade the second stage via UART. For this you have to use the BGAPI UART DFU type Gecko Bootloader!

As a first step you should flash both the first and the second stage of the bootloader to your device. To flash the first + second stage bootloader with a Bluetooth application, follow the steps detailed in http://community.silabs.com/t5/Bluetooth-Wi-Fi-Knowledge-Base/Adding-Gecko-Bootloader-to-Bluetooth-projects/ta-p/195432

If you already  have the bootloader flashed, the second stage can be upgraded in 3 steps:

1. Upload new second stage image in the device (it will be put to a predefined address)
2. Reset the device -> the first stage will overwrite the second stage with the new image
3. Upload the application again, since some part of it may have got corrupted when the second stage image was uploaded

In practice, follow these steps to upgrade your second stage:

1. Create a new bootloader project you want to upgrade to
    
2. Bootloader can only be upgraded and not downgraded, hence you need an increased version number with every upgrade. If you want to upload the same bootloader version with another configuration, then you have to increase  BOOTLOADER_VERSION_MAIN_CUSTOMER. Find the "Other" tab in the AppBuilder of the Bootloader project (opening .isc file), and add BOOTLOADER_VERSION_MAIN_CUSTOMER to the defines. Define a value greater than 0, then increase this with every upgrade.
    
3. Generate and build your bootloader project
    
4. Create a bootloader upgrade file (.gbl) from the .s37 output file (*). E.g.

   commander gbl create bootloader.gbl --bootloader bootloader-uart-bgapi.s37

   Use the .s37 file which does not end with „–combined”! Combined image contains first+second stage, but now we need only the second stage.
    
5. Upload the upgrade image. E.g. if you have an BGAPI UART DFU type bootloader, you can use the uart-dfu application (**) to do this:

   uart-dfu COMn 115200 bootloader.gbl

    

6. Now the bootloader image temporarily overwrites the application image, hence you have to upload the application again:
    
7. Build your Bluetooth application
    
8. Create an application upgrade file (.gbl) from the .s37 output file (*). E.g.

   commander gbl create application.gbl --app my_bluetooth_project.s37

    

9. Upload the upgrade image. E.g. if the upgraded bootloader is an BGAPI UART DFU type bootloader, you can use the uart-dfu application (**) to do this:

   uart-dfu COMn 115200 application.gbl

    

10. Now your bootloader is upgraded and your application is restored.

Note:

(*) commander.exe can be found under
C:\SiliconLabs\SimplicityStudio\vX\developer\adapter_packs\commander

(**) uart-dfu.exe can be found under
C:\SiliconLabs\SimplicityStudio\vX\developer\sdks\gecko_sdk_suite\<version>\app\bluetooth_<version>\examples_ncp_host\uart_dfu\exe
after make was run in
C:\SiliconLabs\SimplicityStudio\vX\developer\sdks\gecko_sdk_suite\<version>\app\bluetooth_<version>\examples_ncp_host\uart_dfu\

Upgrading OTA (over-the-air)

This part discusses how you can upgrade the second stage via Bluetooth connection (OTA – over the air). For this you have to use a Bluetooth in-place OTA DFU type Gecko Bootloader, and you have to upgrade to stack version 2.7 or later (If you already have an older stack version running on the device, it is not a problem, but upgrade is necessary.)

As a first step you should flash both the first and the second stage of the bootloader to your device. To flash the first + second stage bootloader with a Bluetooth application, follow the steps detailed in http://community.silabs.com/t5/Bluetooth-Wi-Fi-Knowledge-Base/Adding-Gecko-Bootloader-to-Bluetooth-projects/ta-p/195432

If you already has the bootloader flashed, the second stage can be upgraded in 4 steps:

1. Upload new second stage image along with an apploader image. Note: this will overwrite the current application image.
2. Reset the device -> the first stage will upgrade the second stage with the new bootloader image. Note: this process will overwrite some part of the current apploader image.
3. Reset the device -> the second stage will upgrade the corrupted apploader image with the newly uploaded apploader image.
4. Upload the application image again. This will upgrade the corrupted application image.

In practice, follow these steps to upgrade your second stage:

1. Create a new bootloader project you want to upgrade to. Important: If you upgrade the bootloader OTA (and not via UART), the address of Slot0 should match in the upgraded and in the old bootloader version! If you still want to change your slot configuration, please refer to the following article: https://www.silabs.com/community/wireless/bluetooth/knowledge-base.entry.html/2018/01/23/upgrading_gecko_boot-IUpJ
    
2. Bootloader can only be upgraded and not downgraded, hence you need an increased version number with every upgrade. If you want to upload the same bootloader version with another configuration, then you have to increase BOOTLOADER_VERSION_MAIN_CUSTOMER. Find the "Other" tab in the AppBuilder of the Bootloader project (opening .isc file), and add BOOTLOADER_VERSION_MAIN_CUSTOMER to the definitions. Define a value greater than 0, then increase this with every upgrade.
    
3. Generate and build your bootloader project
    
4. Open your Bluetooth application project or create a new one (SDK v2.7 or later)
    
5. Build your project if you haven’t done so yet
    
6. Copy the second stage bootloader image bootloader-storage-internal-ble.s37 (the .s37 file that does not end with –combined.s37) from your Bootloader project into your Bluetooth application project
    
7. Find the following line in create_bl_files.bat (which can be found in the Bluetooth project):

   %COMMANDER% gbl create "%PATH_GBL%\%OTA_APPLO_NAME%.gbl" --app "%PATH_GBL%\%OTA_APPLO_NAME%.srec"

    

8. And add the following line under it:

   %COMMANDER% gbl create "%PATH_GBL%\%OTA_APPLO_NAME%+bootloader.gbl" --app "%PATH_GBL%\%OTA_APPLO_NAME%.srec" --bootloader bootloader.s37

    

9. Repeat this for signed and encrypted images if you use signed/encrypted images
    
10. Run the create_bl_files.bat file, which creates the GBL files in the output_gbl folder
     
11. Do a full OTA update on your devices the same way as you do a full OTA for upgrading the stack/apploader and application, but now use these two files:

• apploader+bootloader.gbl
• application.gbl

During the OTA process the device resets itself twice, so there is no need for user interaction. After the OTA update, your bootloader is updated. You can check its version in the system_boot event of the Bluetooth stack or in the Gecko Bootloader version characteristic of the Silicon Labs OTA service (see AN1086)

For more information please refer to UG266: Gecko Bootloader User Guide