nrf9160 modem : Could not get response prefix, error: -22

[Varouzan]

Hello,

Developing on Zephyr 1.2 and modem fw 1.2.

Developing a client LTE application. If I run the below sequence of commands periodically every 2 min:

lte_lc_offline();
lte_lc_init();
lte_lc_connect();

 eventually lte_lc_connect(); returns the errors below.

 Could not get response prefix, error: -22
 Could not get network registration status

After this the modem seems to be stuck in a bad state , as I can't get it to reconnect to the network running the above sequence.

Is there a good way to recover that modem, and what does that error mean excactly?

Thank you.

[Hakon]

Hello,

 

If I run the below sequence of commands periodically every 2 min:

 How many times do you need to run this before the modem becomes unresponsive? The modem is supposed to lock if you try to reboot it too many times in rapid succession.

[Varouzan]

Around 5 times.

Is there a way to unlock it?

[Hakon]

If I run the below sequence of commands periodically every 2 min

What do you do in between the intervals? Do you sleep or busy wait, or do you use timer interrupts?

[Varouzan]

We have an lte client thread that wakes up based on a timer interrupt. The thread is suspended when not active.

Our system also has a separate thread fro GPS.

Is there a "better" way to reconnect to the network? That doesn't cause this problem?

Or a way to unlock the modem?

[Hakon]

I will need to reproduce this. This is most likely a bug and not intended behavior.

[Hakon]

I ran your code from the system work queue and the modem does not lock. Can you send me your project so I can test it?

[Varouzan]

Hello Hakon,

Sorry for the late reply. That would be hard for me to do.

However I was able to reproduce the bug pretty consistently by doing the following, so maybe you will be able to do so too.

lte_lc_init();
lte_lc_connect();

client_fd = socket(AF_INET, SOCK_STREAM, 0);
bind(lte_client_fd, (struct sockaddr *)&local_addr, sizeof(local_addr));
fcntl(lte_client_fd, F_GETFL);
connect(lte_client_fd, (struct sockaddr *)&socket_info,  sizeof(struct sockaddr_in));

lte_lc_offline();

Doing this on repeat causes the error described on the 8th time.

Connecting and disconnecting without creating a socket does not cause this problem for me, as it probably didn't for you. It only appears I a socket is created and connect() is called.So assume creating a socket effects this somehow?

A have also tried closing the socket, both before and after going into offline mode, but the end result was the same and the error remained.

Does this offer any more insight?

Thank you.

[Hakon]

Ok, I will try to reproduce this as well.

[Hakon]

I tested it and creating a socket doesn't seem to make a difference. Can you try creating a modem trace of the issue by following these instructions?

[Varouzan]

So I ported part of my project to run on the nordic dev kit since I was using a custom PCB and I captured the modem trace. You can get the file from the link below.

https://github.com/varouzan/nordic_debug_trace_files/blob/main/trace-2021-02-18T21-09-29.452Z.bin

Running the code on the dev kit I was not able to see the "Could not get response prefix, error: -22" error message, however the modem eventually stops connecting to the LTE network. This happened after around 15 minutes of connecting to LTE, creating a socket, and disconnecting.

[Hakon]

Thank you, I will submit the trace for analysis.

[Hakon]

Developing on Zephyr 1.2 and modem fw 1.2.

 What is the exact modem firmware version? 1.2.0?