Instead of entering bootloader mode by pressing a button, you can also integrate BLE DFU Service support into an application and enable the application to automatically enter bootloader mode. Writing to the DFU Service will then trigger the application to restart in bootloader mode.
The general concept of the Device Firmware Update is the same no matter if you use a button to enter bootloader mode or add BLE DFU Service support to the application. In both cases, a DFU bootloader is required, and the DFU is performed when the device is in bootloader mode. The only difference is how the device is put into bootloader mode.
Adding BLE DFU Service support to an application is required whenever you want to be able to start bootloader mode and update a device without physical interaction. Potential reasons for this are, for example, that the device does not have a button or is not in reach.
To get started, program and test the provided Example application. Then check Extending your application for detailed information on how to include BLE DFU Service support in your own application. Switching to bootloader/DFU mode describes how to trigger bootloader/DFU mode.
The example application supports Sharing bonding information, which enables the DFU target to do directed advertising after bootloader mode is started, so that only the DFU controller can reconnect and the link is immediately re-encrypted.