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Modem Firmware Update (FOTA)

Nordic Semiconductor's cellular SoCs contain two independently-updateable firmware components. To update them with Memfault, we use a separate Memfault Project for the modem firmware:

ComponentUpdated viaMemfault Project
Application firmwareMCUboot swapYour app project
Modem firmwarenRF modem bootloaderA dedicated modem project
Prerequisites

Step 1: Create a Modem Firmware Project​

  1. Log in to app.memfault.com.
  2. Create a new project (e.g. my-product-modem).
  3. Copy the Project Key from Settings → Project Keys. You will need this in your firmware's prj.conf.

Step 2: Update Kconfig Settings​

Add the following to your prj.conf:

prj.conf
# Enable built-in modem FOTA support (Memfault SDK ≥ 1.39.0)
CONFIG_MEMFAULT_FOTA_MODEM_UPDATE=y

# Project key for the dedicated modem Memfault project
CONFIG_MEMFAULT_FOTA_MODEM_PROJECT_KEY="your-modem-project-key-here"

The optional CONFIG_MEMFAULT_FOTA_MODEM_SOFTWARE_TYPE (default "mfw") can be set if you used a different --software-type when uploading.

Step 3: Integrate into Your FOTA Loop​

Choose the integration path that fits your application.

Option A: Automatic (via existing periodic FOTA check)​

For automatic modem FOTA, set CONFIG_MEMFAULT_PERIODIC_FOTA_CHECK=y. The periodic upload handler will call memfault_zephyr_fota_start() and memfault_zephyr_fota_modem_start() on a regular interval, performing app and modem updates when deployed.

Option B: Manual​

Call the FOTA functions directly at a time of your choosing:

// Check for app update first; if none pending, also checks for modem update:
int rv = memfault_zephyr_fota_start();

// Or check for a modem update only:
rv = memfault_zephyr_fota_modem_start();

Step 4: Upload Modem Firmware to Memfault​

Nordic ships modem firmware as delta packages — a binary that upgrades from one specific version to the next. Download the modem firmware package for your target device:

The delta update filename contains the source and target versions, e.g. mfw_nrf91x1_update_from_2.0.3_to_2.0.4.bin will update from mfw_nrf91x1_2.0.3 to mfw_nrf91x1_2.0.4. This file will be uploaded directly to Memfault without modification.

Full modem firmware updates

Nordic also provides full modem firmware packages (~2.1 MB) that can update from any version but require external flash storage on the device. See Nordic's full modem firmware update documentation for additional guidance.

To upload the modem firmware to Memfault, create a new Delta Release in your modem firmware project. Set the From Version and To Version fields to match the source and target versions in the modem firmware filename, and upload the .bin file as the release payload. Match the Hardware Version field to the hw_version you use when uploading device data to Memfault (e.g. nrf9160), and see note below about the Software Type field.

You'll likely need to create a new Hardware Version in the modem project before uploading the release artifact. Navigate to Settings → Hardware Versions:

Creating a new hardware version for the modem project

Preferably, you can upload the release payload using the Memfault CLI, using --delta-from and --delta-to to record the source and target versions and create a delta release:

memfault --org-token YOUR_ORG_TOKEN \
  --org YOUR_ORG --project YOUR_MODEM_PROJECT \
  upload-ota-payload \
  --hardware-version YOUR_HW_VERSION \
  --software-type mfw \
  --delta-from mfw_nrf91x1_2.0.3 \
  --delta-to mfw_nrf91x1_2.0.4 \
  mfw_nrf91x1_update_from_2.0.3_to_2.0.4.bin

This both creates the new Delta Release, and uploads the payload to the Release.

Choose your software_type string once and use it consistently

mfw is the recommended convention and matches the SDK default. The value you upload must match CONFIG_MEMFAULT_FOTA_MODEM_SOFTWARE_TYPE on the device (which defaults to "mfw").

The created release should look like this:

Delta release showing From Version mfw_nrf91x1_2.0.3 to mfw_nrf91x1_2.0.4

Activate the release on the default cohort to begin rolling it out.

Verification/Testing​

You can trigger a manual modem-only FOTA from the shell with the mflt fota_modem command:

uart:~$ mflt fota_modem
[00:01:06.124,755] <inf> mflt: Checking modem FOTA (current version: mfw_nrf91x1_2.0.3)
[00:01:06.524,902] <dbg> mflt: memfault_platform_log: DNS lookup for device-nrf.memfault.com = 44.206.80.55
[00:01:10.440,551] <dbg> mflt: memfault_platform_log: OTA check w/ device info: serial=xxx, hardware=nrf9151dk, software_type=mfw, software_version=mfw_nrf91x1_2.0.3
[00:01:13.656,463] <inf> mflt: FOTA Update Available. Starting Download with URL: https://...
[00:01:13.657,348] <inf> mflt: FOTA In Progress
...
# device will reboot and apply modem firmware update, which takes about 2 minutes to complete

After reboot, confirm the new modem version:

uart:~$ at AT+CGMR
mfw_nrf91x1_2.0.4
OK

If modem firmware is up to date, the logs will indicate it:

uart:~$ mflt fota_modem
[00:07:30.581,787] <inf> mflt: Checking modem FOTA (current version: mfw_nrf91x1_2.0.4)
[00:07:31.053,741] <dbg> mflt: memfault_platform_log: DNS lookup for device-nrf.memfault.com = 54.172.47.91
[00:07:34.266,632] <dbg> mflt: memfault_platform_log: OTA check w/ device info: serial=xxx, hardware=nrf9151dk, software_type=mfw, software_version=mfw_nrf91x1_2.0.4
[00:07:36.568,054] <inf> mflt: Modem firmware is up to date
Modem firmware downgrades

While possible, downgrading modem firmware via Memfault OTA is not recommended. Nordic does not publish reverse-direction delta packages: downgrades are only possible over-the-air via full modem update.

To revert to a previous modem version for testing, download the previous modem firmware package, and flash it appropriately. You can use either the nRF Connect for Desktop app, or nrfutil command line app like so:

# install the "91" nrfutil command if not already installed
❯ nrfutil install 91
# perform the modem downgrade
❯ nrfutil 91 modem-firmware-upgrade --firmware ~/Downloads/mfw_nrf91x1_2.0.3.zip --verify --serial-number 1051243117

Troubleshooting​

SymptomLikely cause
modem_info_string_get returns errorLTE not yet attached when the FOTA check runs
OTA check returns 0 (no update) but release is activatedVerify software_type and software_version reported by the device match what was uploaded; check the Processing Log
fota_download_any returns -ENOTSUPCONFIG_DFU_TARGET_MODEM_DELTA not selected (defaults y on nRF91X; check west build -t menuconfig)
Download starts but device reports wrong version after rebootFor delta updates: the source version in the filename must match the modem firmware currently installed on the device
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