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Firmware update in Mbed OS

Mbed OS integrates Pelion Device Management firmware update services, so if you have a Device Management account, you can send binaries to your remote device. You can update the bootloader independently of the application.

Update support in Mbed OS relies on two features:

Updatable binaries

The active firmware, made up of Mbed OS and an application, can be updated only if the binary installed on the device has:

  1. A bootloader, as reviewed below. The bootloader can load a new version of the firmware. If for some reason, the update fails, the bootloader falls back to the firmware version last known to work. As a security feature, the default behavior of the bootloader is to refuse to roll back to old firmware versions once an update succeeds.
  2. Mbed OS with the Device Management Client (which includes the Update client). The clients allow your device to communicate with the Device Management Update service, receive update manifests and firmware, and verify the binary's validity. They are reviewed in details in our Pelion Device Management documentation.

Note: Because some embedded devices don't require remote update capabilities, Mbed OS does not include the Device Management Client by default. You must explicitly import the client to your application.

  1. Permission to access your Device Management account, as well as keys and certificates to verify the firmware's source and validity.

The Mbed OS bootloader

During system startup, a bootloader is responsible for selecting and forwarding control to the next stage in the boot sequence. The Mbed OS bootloader also validates the next stage's sanity and signature before forwarding control to it; if validation fails, the bootloader can select an alternate version of the next stage. It can also install an alternative upon request (in other words, firmware update).

Background: boot sequences and fault tolerance

A boot sequence can have several stages of bootloaders, leading to an application. The different stages (including the application) may need to evolve over time, to add features or bug-fixes. Upgrades are possible for boot sequences with two or more stages: any active stage can replace the next stage in the sequence; when the system restarts, it executes the updated components. Typically, however, the very first stage isn't replaced; because it takes control on startup, a faulty upgrade of this stage can make recovery impossible. This is known as stage 0 bootloader.

To protect against faults in the newly updated components, we store multiple versions (up to a certain maximum number of versions). Each stag is responsible for detecting faults in the following component and rolling it back if it discovers faults. For example, if stage 3 is unstable, needs additional functionality or is otherwise behaving incorrectly, stage 2 (during the next startup sequence) can roll stage 3 back to an earlier version before handing over control. This results in a boot-sequence tree that is traversed in a depth-first order as the system recovers from successive faults.

Most boot sequences are composed of only three stages:

  1. Boot selector (also known as root bootloader or stage zero bootloader): does not get upgraded.
  2. Bootloader: upgradable, with several versions stored on the device.
  3. Application: upgradable, with several versions stored on the device.

Fault tolerance ultimately rests on the sanity of the first-stage bootloader. This bootloader is usually kept minimal to ensure dependable operation.

The Mbed OS bootloader is a hybrid of the boot selector and a bootloader, but it fulfills the requirements of the boot selector: It is small enough to minimize the chance of bugs, but it is advanced enough to install new images.

Managed and unmanaged bootloader tool integration

The Mbed tools (Mbed CLI, Online Compiler) can manage bootloaders where:

  • The bootloader comes before the application in ROM.
  • The application starts immediately after the bootloader.

If the Mbed tool finds a manageable bootloader, the image build process automatically merges the bootloader image with the application image.

If your bootloader does not meet the two requirements of a manageable bootloader, you will need an unmanaged bootloader. The Mbed tools do not automatically combine this bootloader with the application image; you need to write your own scripts to build your full image.

For more information about managed and unmanaged bootloaders, and how to create bootloaders, see the tutorial.

Porting a bootloader

If you're interested in porting the Update client and bootloader to new hardware, please review the porting section.

Security

Note two things about the Mbed OS bootloader's design:

  1. It does not process encrypted off-chip candidate images.
  2. It does not currently verify signatures of candidate images. To save code size and speed up boot time, we use a unique, per-device Message Authentication Code (MAC) to authenticate the firmware in the bootloader: firmware is distributed with a signature, and the Update client verifies the signature and replaces it with a MAC that the bootloader understands.

This means that the default bootloader does not implement secure boot; for high-security applications, further implementation is required. Please see the full bootloader documentation and the porting section on the Pelion Device Management site.

Managing updates with the bootloader

When a device downloads new firmware, it stores it locally (in the storage area) and reboots; the device bootloader then:

  1. Checks the integrity of the active firmware by calculating the hash of the active image and comparing it to the one in the metadata header (the bootloader metadata header is explained below).
  2. Looks for available firmware on the system. There may be more than one image, depending on image sizes.
  3. Chooses the firmware with the latest timestamp.
  4. Checks the integrity of the image in the storage area by checking its hash against its internal metadata header.
  5. Copies the image in the storage area to the active application region if it is applicable.
  6. Forwards control to the start of the active firmware, which contains the Update client (part of Device Management Client). The Update client runs the Pelion Device Management update process:
    1. Receives a notification from one of its update sources.
    2. Downloads the new manifest and parses it to obtain the firmware URI.
    3. Fetches the firmware from one of its update sources. Either the manifest or a cost-ranking algorithm specifies the update source to use.
    4. Writes the firmware into the storage region on the SD card (or external SPI Flash), as a candidate firmware image.
    5. Reboots, handing control back to the bootloader.

Development tool integration with Pelion Device Management Update

Mbed CLI and the Online Compiler implement the Pelion Device Management Update service by directly using the service APIs and the manifest tool. For a device running an updatable application, using Mbed CLI and the Online Compiler's built-in update features can reduce the steps needed to run an update campaign.

Note: The default workflow of the development tools is intended only for development and testing purposes. It is not secure for production.

Your development tool needs to use your Device Management account API key to call the APIs. Once it has access to the APIs, it can generate a manifest, upload the manifest and binary to the server, and deliver the manifest to a device or group of devices. For more information, see the update API documentation.

Note: The tools currently support the update flow for Device Management Client, not Device Management Client Lite.

Tutorials:

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