# Firmware Startup

Upon boot-up, the bootloader checks whether there is a valid user application or FOTA Bluetooth Low Energy stack programmed. The sequence of operations is as follows:

| 1. | If there is a valid user application, start it.

| 2. | If no valid application is found, start the FOTA Bluetooth Low Energy stack DFU component (so the device can receive FOTA updates).

| 3. | If no valid FOTA Bluetooth Low Energy stack is found, start the bootloader updater (in this case, the device can only receive firmware updates over UART).

The FOTA Bluetooth Low Energy stack DFU component can be activated from the user application at any time, through a call to Sys_Fota_StartDfu(). More details about this are provided in later sections.