Bootloaders and Device Firmware Updates

There are two types of bootloaders used by the nRF Connect SDK:

• Immutable first-stage bootloaders.

• Upgradable second-stage bootloaders that can perform device firmware updates to both themselves and the application.

The bootloaders support two types of updates:

• Direct updates, where an in-place substitution of the image takes place.

• Background updates, where the updated image is obtained and stored, but the update is completed later on.

You can deliver the updated images to the device in two ways:

• Wired, where updates are sent through a wired connection, like UART, or delivered by connecting a flash device.

• Over-the-air (OTA), where updates are sent through a wireless connection, like Bluetooth® Low Energy.

You can use a second-stage bootloader only in combination with a first-stage one. Also, not all bootloaders supported by the nRF Connect SDK can be used as either first-stage or second-stage ones. You can find an overview of currently supported bootloaders in the table below:

Bootloaders supported by nRF Connect SDK

Bootloader	Can be first-stage	Can be second-stage	Key type support	Public key revocation	SMP updates	Downgrade protection	Versioning	Update methods (supported by nRF Connect SDK)
nRF Secure Immutable Bootloader	Yes	No	See list	Yes	No	Yes	Monotonic (HW)	Dual-slot direct-xip
MCUboot	Yes	Yes	See imgtool	No	Yes	Yes	Semantic (SW)	Image swap - single primary Dual-slot direct-xip

See the following user guides to learn more:

Subpages:

• Secure bootloader chain
  • Chain of trust
  • Architecture
• Adding a bootloader chain
  • Adding an immutable bootloader
  • Adding an upgradable bootloader
• Testing the bootloader chain
• Using external flash memory partitions
• Customizing the bootloader
  • Using custom project configurations
  • Assigning Kconfig fragments
• Firmware updates
  • Signature keys
  • Configuring image versions