Hello,
We have a set of ~10 units in the field (outside) in Belgium, which communicate via NB-IoT, and do an MQTT sync procedure every 4 hours.
During the freezing period of around December 15th, 2 units went offline for about a week (the temperature was around -5 degrees). The network operator (Orange) did not see anything specific occurring. The units have reset themselves periodically (part of the firmware). They "disappeared" not on the same day.
1 of the 2 units resumed some communication on december 19th, but not with the correct intervals (so it definitely still had some trouble). I went on site, and noticed LTE connect errors pointing to "+CEREG: 90" which means issues with the UICC (SIM card). Several manual resets (triggered via the reset line, not by unpowering the system) did not solve the issue. Eventually, I took out the sim card from the slot for a second, and put it back in and it all started working fine.
The second unit never managed to communicate autonomously, but after an on-site intervention, the sim card slot was also opened and re-closed, and the issue was fixed after a reset.
My first assumption is that the cold might have caused some issues: our PCB is in a casing but not yet fully coated / protected. However, it's very strange that the systems couldn't fully recover. So my doubts:
- We never triggered a full power-cycle of the system, since it's battery based and includes supercapacitors so it's not that easy. Could this have made a difference?
- Is it possible that the SIM cards or another part of the electronics get stuck after e.g. a load of invalid commands/responses, and that a power cycle is required to fix the issue? (I have to verify if the 1.8V effectively goes down on a reset).
- I'm using Modem FW 1.3.0 and SDK 1.7.1, which are certainly not the latest ones. Would it make sense to upgrade these?
- Is it possible to reset the sim card via a firmware command? Or would that be done intrinsically on e.g. lte_lc_offline and "AT+CFUN=0"?
- Since it's clearly not easy to reproduce this issue, have there been any similar issues or can you think of a way to reproduce the issue?
Again, the cold temperatures might be related to the issue, since these units have been running for about 6 months without this specific issue, so it would be quite a coincidence that 2 units have the same issue in a time window of only 1 or 2 days, and for a duration of at least 1 week.
Best regards,
Sebastiaan
