nRF91 modem watchdog fault when re-activating LTE

[JordanYates]

I have an application with a shipping mode that when enabled/disabled calls `conn_mgr_all_if_connect/conn_mgr_all_if_disconnect`.
These functions eventually end up in `lte_net_if.c`, running `lte_lc_func_mode_set` with either `LTE_LC_FUNC_MODE_ACTIVATE_LTE` or `LTE_LC_FUNC_MODE_DEACTIVATE_LTE`.

Connecting to the LTE network on boot works fine, as does entering shipping mode.
The modem fault occurs when attempting to re-activate the modem after it has already been active.
If less than a minute has passed since the modem was de-activated, it re-activates fine and the application continues as per normal.
If over a minute has passed, the modem faults more or less immediately with:

[00:01:26.754,638] <inf> app: Device activated, enable LTE
[00:01:26.982,513] <err> nrf_modem: Modem has crashed, reason 0x2, PC: 0x468de

This issue is 100% reproducible (happens every time), always with the same error code and program counter.
The error code corresponds with NRF_MODEM_FAULT_HW_WD_RESET.

Replacing `LTE_LC_FUNC_MODE_ACTIVATE_LTE` and `LTE_LC_FUNC_MODE_DEACTIVATE_LTE` with `LTE_LC_FUNC_MODE_NORMAL` and `LTE_LC_FUNC_MODE_OFFLINE` does not change the behavior of the fault.

nrfxlib version: Tag v3.1.1 (Commit 3b210a24d3bc7ecfc268e0feab6436306b11e7cb)
Modem firmware: mfw_nrf91x1_2.0.4
Modem model: nRF9151-LACA

[Syed Maysum Abbas Zaidi]

Hi,

Thanks for the detailed report. NRF_MODEM_FAULT_HW_WD_RESET (0x2) means the modem firmware has become unresponsive and its internal hardware watchdog has reset the modem. And from your description, this is triggered when LTE is deactivated for a longer period and then re-activated. Can you try fully shutting down and re-initializing the modem in the following way as mentioned in the documentation:

• When entering shipping mode: stop LTE users and call nrf_modem_shutdown()
• When exiting shipping mode: call nrf_modem_init() and then reconnect LTE

If the issue still reproduces after using this sequence, please enable modem traces and share them so it can be investigated further.

Best Regards,
Syed Maysum

[JordanYates]

> Can you try fully shutting down and re-initializing the modem in the following way as mentioned in the documentation:

I unintentionally left out some information in the original post. The activation and de-activation code paths look like this:

		LOG_INF("Device activated, enable LTE");
		conn_mgr_all_if_up(false);
		conn_mgr_all_if_connect(false);
		...
		LOG_INF("Device de-activated, disable LTE");
		conn_mgr_all_if_disconnect(false);
		conn_mgr_all_if_down(false);

The connect/disconnect calls do indeed call the functional modes I mentioned, but the `if_up` and `if_down` calls get routed down to lte_net_if_enable and lte_net_if_disable, which are already fully shutting down and re-initialising the modem.

This was confirmed by adding extra logging inside those functions, but also by the trace module logging (see below).

> If the issue still reproduces after using this sequence, please enable modem traces and share them so it can be investigated further.

I have enabled modem traces (attached below), however I am doubtful the trace will be useful in practice, since the tracing module suspends in response to the modem powering down, and there is no logic to re-enable it once it comes back up.

[00:00:01.783,386] <inf> modem_trace_backend: Modem_trace RTT backend channel 1
[00:00:01.783,416] <inf> nrf_modem_lib_trace: Trace thread ready
[00:00:01.784,942] <inf> nrf_modem_lib_trace: Trace level override: 2
...
[00:00:26.409,027] <inf> app: Device de-activated, disable LTE
[00:00:27.173,004] <inf> epacket_udp: Network disconnected
[00:00:27.189,422] <inf> nrf_modem_lib_trace: Modem was turned off, no more traces
...
[00:02:02.850,982] <inf> app: Device activated, enable LTE
[00:02:03.280,090] <err> nrf_modem: Modem has crashed, reason 0x2, PC: 0x468de
[00:02:03.280,120] <err> modem_monitor: Modem fault, rebooting in 2 seconds...

 1770250638_nrf_modem_trace.zip

After removing the calls to `conn_mgr_all_if_up/down` (and therefore not shutting down the modem entirely), I am able to transition between shipping and active modes without faults. However this doesn't explain the root cause of this issue, which is how a hardware watchdog can expire less than a second after booting from the shutdown state.

[JordanYates]

> We tested the same shutdown, wait and re-initialization sequence in a clean nRF Connect SDK v3.1.0 environment on nRF9161DK. In this setup, the modem reliably re-initializes and reconnects after the wait period, and we do not see an NRF_MODEM_FAULT_HW_WD_RESET (0x2).

Was any attempt made to run the minimal reproducible sample that you asked for and I provided?

> This indicates the watchdog reset is not caused by the modem firmware or the shutdown delay itself, but is likely related to how LTE and the modem are being controlled in the application when using the connection-manager path.

That is not a surprise (in that extra work needs to happen to trigger the fault, e.g. UDP packets).

> As next steps to isolate the issue in your setup, we recommend first ensuring that all components that may access the modem are fully stopped before calling conn_mgr_all_if_down(). In particular, please verify that all LTE sockets are closed, any DNS/REST/HTTP clients are stopped, no AT commands are in progress, and modem tracing (if enabled) is idle.

So the suggestion is to throw out all the integrations with Zephyr APIs that Nordic has written? I have little interest in attempting to re-architecture my networking flow to diagnose a fault in a component I don't control and can't debug just because you haven't run the sample I have already spent time providing.

> We also recommend running the LTE down/up sequence from a dedicated application context (for example an application thread or work item), rather than directly from a network or connection-manager callback.

You can already see this is being done in the provided example.

> to confirm whether the watchdog reset is specific to the connection-manager integration.

This is a watchdog failure inside the modem core. How could it have anything to do with the connection manager? The modem literally faults before `nrf_modem_lib_init` even returns.

[Syed Maysum Abbas Zaidi]

Hi,

Thanks for the detailed feedback. I understand your point, and I agree that reworking application architecture just to debug a modem fault isn’t ideal.

I are currently trying to build and run the exact minimal sample you shared, using your west workspace. While setting this up in a clean environment, I am hitting a TF-M build error on nrf9161dk/nrf9161/ns. I am checking it internally and once we have the sample building, we’ll proceed with reproducing the modem watchdog issue and report back.

Best Regards,
Syed Maysum

[JordanYates]

> I are currently trying to build and run the exact minimal sample you shared, using your west workspace. While setting this up in a clean environment, I am hitting a TF-M build error on nrf9161dk/nrf9161/ns.

Can you share the build error? A clean environment build works fine locally for me.

(.zephyr_venv) jordan@TAURUS:~/code$ mkdir fault_demo
(.zephyr_venv) jordan@TAURUS:~/code$ cd fault_demo/
(.zephyr_venv) jordan@TAURUS:~/code/fault_demo$ west init -m [email protected]:Embeint/infuse-sdk.git
(.zephyr_venv) jordan@TAURUS:~/code/fault_demo$ cd infuse-sdk/
(.zephyr_venv) jordan@TAURUS:~/code/fault_demo/infuse-sdk$ git checkout example/nrf91_modem_fault
(.zephyr_venv) jordan@TAURUS:~/code/fault_demo/infuse-sdk$ west update
(.zephyr_venv) jordan@TAURUS:~/code/fault_demo/infuse-sdk$ west build -b nrf9161dk/nrf9161/ns ./samples/nrf91_modem_fault/
...
[506/506] Linking C executable zephyr/zephyr.elf
Memory region         Used Size  Region Size  %age Used
           FLASH:      193308 B       704 KB     26.81%
             RAM:       79684 B       168 KB     46.32%
     RetainedMem:          0 GB        256 B      0.00%
        IDT_LIST:          0 GB        32 KB      0.00%
Generating files from /home/jordan/code/fault_demo/infuse-sdk/build/zephyr/zephyr.elf for board: nrf9161dk

[Syed Maysum Abbas Zaidi]

Hi,

Thanks for sharing your build steps.

On my side (Windows environment), I set up a clean workspace and followed essentially the same flow as you:

mkdir fault_demo
cd fault_demo
west init -m https://github.com/Embeint/infuse-sdk.git
west update
cd infuse-sdk
git checkout example/nrf91_modem_fault
west update
west build -b nrf9161dk/nrf9161/ns samples/nrf91_modem_fault -p always

However, the build fails during the TF-M stage with:

zephyr/modules/trusted-firmware-m/nordic/include/RTE_Device.h:14:10:
fatal error: zephyr/devicetree.h: No such file or directory

The file does exist in the workspace, so it appears that the TF-M compile step is not picking up Zephyr’s include path on my host. I’m checking this internally to proceed with reproducing the watchdog issue and will get back to you by tomorrow.

Best Regards,
Syed Maysum

[Syed Maysum Abbas Zaidi]

Hi,

I’ve now been able to reproduce the behavior on our side using the provided nrf91_modem_fault example on nrf9161dk.

(For transparency: on Windows we initially hit a TF-M build issue related to symlink handling in the Zephyr workspace. After enabling Windows Developer Mode and ensuring Git symlinks were properly supported, the project built correctly.)

The sequence behaves as you described:

• LTE connects correctly on boot then entering shipping mode works fine
• If LTE is reactivated within ~1 minute, it reconnects successfully
• If LTE is reactivated after ~60+ seconds of deactivation, the modem consistently crashes with: 
  

  [00:01:23.234,283] <err> nrf_modem: Modem has crashed, reason 0x2, PC: 0x467da

The issue is fully reproducible and occurs consistently with the same fault reason and program counter. Since this appears to be modem-side behavior during functional mode reactivation, we are escalating this internally for further investigation.

I will update you as soon as we have feedback from the modem team.

Best Regards,
Syed Maysum

[JordanYates]

Hi Syed,

Thank you for persisting with getting the reproducing sample working and for escalating internally.

The Windows error is interesting, it would be amazing if you could submit a PR upstream to update https://docs.zephyrproject.org/latest/services/tfm/overview.html with the error that can occur on Windows and how to fix it, as I am certain you will not be the only person to run into it.

[Syed Maysum Abbas Zaidi]

Hi,

Thanks for the suggestion. The issue we observed was related to Windows symlink handling. If symlinks are not enabled before cloning the workspace, Git may check them out as regular files, which causes TFM include path issues during build. Enabling Windows Developer Mode and ensuring symlinks are properly created resolved the problem on our side.

We will review this further and consider contributing if needed.

Best Regards,
Syed Maysum

[Syed Maysum Abbas Zaidi]

Hi,

This is to update you that our internal team were able to reproduce the issue and determined that the modem crash is linked to the RTT logging functionality. As the crash occurs when RTT logging is enabled and the modem is restarted after a specific idle period (around 65 seconds). However disabling RTT logging prevents the crash.

Workaround could be to disable RTT logging for testing scenarios that involve modem shutdown and restart. And since RTT is primarily a development tool, we assess that this issue is unlikely to affect your end products.

We are continuing to investigate this issue and will inform as soon as we have any update on it. Thanks

Best Regards,
Syed Maysum

[JordanYates]

Hi Syed, thanks for the update and the root cause analysis. It is good to know that the issue is triggered by RTT, and I agree with your comment that it is unlikely to affect end products.

It would still be good to have resolved, as crashes during development both waste time investigating the issue and hinders testing the application once the issue is known.

[Syed Maysum Abbas Zaidi]

Hi,

Yes, our team are looking into it and we will keep you updated. Thanks

Best Regards,
Syed Maysum