Modem 30 minute lockup

[Burt]

This is driving me totally nuts. There are inconsistent pieces of information in documents and code. I am getting a modem reset loop problem. The documentation says the timer does not run when the modem has no power. That tells me I have to leave power on in order to get out of this ridiculous mess that cuts me off from any and all LTE-M and NB-IoT networks. The message from some asset tracker code tells me I can get out by doing a power reset, but that does not work.

How in blazes do I get out of this fricking mess????? This is at least one day down the drain.

How about 5 minutes rather than 30 minutes. How about something that simply restores the modem?

Did I put a bug into the code? I can't imagine how, although I do put in printfs.

Thanks.

Burt S.

[Achim Kraus]

Sometimes long network searches may be caused by multi-imsi-SIM-cards with auto-switching. You may check that by reading the IMSI (AT+CIMI) frequently, e.g. every minute. If it changes, the modem usually restarts the network search. It's only one cause, but sometimes a nasty one. Even if it doesn't apply in your case, it's good to know it.

[Burt]

Thanks, Achim, yes, this is good to know. If you have examples of multi-imsi-SIM-cards with auto-switching, please let me know that. We use a Twilio Super SIM which is multi-imsi; I guess it is auto-switching, but I don't know when I should expect the imsi to auto-switch. We also use Soracom SIMs; I don't know whether or not those are multi-imsi.

[Achim Kraus]

The easiest and fastest way is just to check the IMSI frequently in the app. You may also ask your SIM card provider, but that may take more time and is sometimes not that easy.

[Achim Kraus]

Burt said:

If you have examples of multi-imsi-SIM-cards with auto-switching, please let me know that.

I've tested a couple of them.

Some switch after 2 minutes, which may cause larger issues, especially, if both NB-IoT and LTE-M should be considered in the network search. E.g. in a region with LTE-M only a modem with an config "NB-IoT preferred" and 2 minutes network search timeout will hardly work. To escape that you need to detect the imsi switch (by reading the IMSI and compare it with the last one). If it happens, it may be a good idea to swap the preference (here from NB-IoT to LTE-M) after 6-10 minutes network search without success.

Some others switch after 10 minutes. Here you need some patience. E.g. if there are 3 imsi on the SIM, it may take 20 minutes to have the right one active.

Some of the SIM card providers offer also SIMs, where it's possible for the App to select one of the available imsis. That helps a lot, if you know, which one is for your region and how to switch it.