nRF51 SDK - S110 SoftDevice
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Architecture of the DFU bootloader

Unless triggered to start in bootloader mode, the DFU bootloader will check if a valid application is present on the device. If there is an application, the bootloader will run it. Otherwise, it will start the DFU procedure, receive DFU packets, and replace the existing firmware.

The following figure displays the blocks in the DFU bootloader and their tasks when performing an application update:

dfu_bootloader_overview.svg
Architectural overview of the DFU bootloader