Bootloaders
The bootloader is a firmware image that is responsible for booting the application. The bootloader chooses an application firmware image to boot and verifies it to ensure validity. The image validation can be used to ensure chain of trust.
Using a bootloader is closely related to device firmware update (DFU).
There are two types of bootloaders used by the nRF Connect SDK:
Immutable first-stage bootloaders that cannot be upgraded through DFU and run after each reset.
Upgradable second-stage bootloaders that can perform DFU to both themselves and the application, and are booted by the first-stage bootloader.
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:
Bootloader |
Can be first-stage |
Can be second-stage |
Key type support |
Public key revocation |
SMP updates by the application |
Downgrade protection |
Versioning |
Update methods (supported by nRF Connect SDK) |
|---|---|---|---|---|---|---|---|---|
Yes |
No |
No |
Yes |
Dual-slot direct-xip |
||||
Yes |
Yes (only with NSIB as first-stage) |
No |
Yes |
Yes |
Image swap - single primary Dual-slot direct-xip |
See the following user guides to learn more: