MCUBoot for On-Chip OAD (Stack Library)

This section describes the behavior of the MCUBoot for on-chip OAD where the combined image approach is used. This approach requires two full application and stack library pairs on the target device. In an on-chip OAD system, there are only two image types that can be executed safely:

  • Persistent application (version 1.0.0): Permanently resident application that implements OAD profile

  • User application (version higher than): OAD upgradeable application that implements OAD reset service and user functionality

For more information about OAD image header, refer to the MCUBoot OAD Image Header section of this chapter. The roles and responsibilities of each application in an on-chip system is defined in the OAD Application section.

In summary, the user application are responsible for updating the image version number flash field in the OAD image header, and resetting the device when appropriate; while the persistent application will be resposible of storing the new image.

Based on the image version number field in the image header, the secure bootload will decide which image is most fit to run. The following will describe the TI’s software solution using MCUBoot process .

@startuml participant oaddist as "OAD Distributor" participant pers as "Persistent Application" participant user as "User Application" participant mcubl as "MCUBoot Secure Booloader" user --> oaddist : Advertising oaddist --> user : Connection Indication oaddist <--> user : Service Discovery rnote over oaddist Discover Reset Service and Characteristics end note oaddist --> user : Request OAD transfer\ntrigger reset user -> user : Change the image version number\nto 0 in flash user -> user : Reset user -> mcubl : After reset, OAD target goes\ninto MCUBoot Secure Bootloader mcubl -> mcubl : Detect the version number in\nPersistent Application is\nhigher than User Application mcubl -> pers : Boot into Persistent Application pers --> oaddist : Advertising oaddist --> pers : Connection Indication oaddist <--> pers : Service Discovery rnote over oaddist Discover OAD Service and Characteristics end note oaddist --> pers : Get OAD block size pers --> oaddist : OAD block size rsp oaddist --> pers : Send Image Identity pers --> oaddist : Image Identity Success rnote over pers The Persistent Application does not check the image identity as this will is checked by the MCUBoot Secure Bootloader end note group Start Transfering New Image oaddist --> pers : OAD start pers --> oaddist : Notify on OAD_IMG_BLOCK \ncharacteristic ... ... Repeat until finishing transfering new image ... ... end oaddist --> pers : OAD Enable pers -> pers : Reset pers -> mcubl : After reset, OAD target goes\ninto MCUBoot Secure Bootloader mcubl -> mcubl : Detect the version number in\nUser Application is\nhigher than Persistent Application rnote over mcubl MCUBoot Secure Bootloader validates the new image end note mcubl -> user : Jump to new User Application image @enduml

Figure 218. MCUBoot OAD sequence diagram.

In order to determine which image is best to run, secure bootloader takes the following measures:

  1. At startup, the secure bootloader check identify image header on Primary slot - check magic number(ih_magic) 0x96F3B83D.

  2. Then bootloader further check identify image header on Secondary slot - check magic number(ih_magic) 0x96F3B83D.

  3. Compare versions (ih_ver) and select Primary if Primary version >= Secondary version

  4. Secure bootloader will now go to certain flash addresses based on which MCUBoot design you choose (Direct XIP or Overwrite only, please see MCUBoot section for more information) in search for a valid Image Identification value. This is done in the following way:

    • Read the version number in the image header, then check for its validity (integrity check, signature verification etc.)

    • If the image is invalid MCUboot erases its memory slot and starts to validate the other image. After a successful validation of the selected image the bootloader chain-loads it.

Note

BLE5-Stack currently only supports Direct XIP method.