Sim card issues (CEREG 90), possibly due to freezing weather

[Sebastiaan Merckx]

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

[Håkon Alseth]

Hi,

 

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.

CEREG: 90 points to communication problems with the SIM itself.

This sounds like a mechanical issue with the SIM card holder itself. Have you tried replacing the SIM holder to see if this has an effect on the stability over temperature?

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).

The supply voltage for the SIM goes off after a given time-period after enabling eDRX or PSM, depending on what your SIM supports (if it supports power down).

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?

I do not think that this is a root-cause of the problems you're seeing.

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"?

If you use PSM/eDRX actively in your application, the SIM communication will only occur on-demand, meaning that the SIM will effectively be reset for each time you communicate with it.

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?

Did both devices have the exact same problem, ie. that CEREG: 90 was logged as the failure reason?

since it's battery based and includes supercapacitors so it's not that easy.

Can you share information related to the battery type that you're using, and the supercaps on your board?

 

Kind regards,

Håkon

[Sebastiaan Merckx]

Hi Håkon,

I'll let our HW engineer answer on the questions about battery type and supercaps, but I already can tell you that the powering circuit is not that easy. Main battery is https://www.rrc-ps.com/power-management/batterieladegeraete/produkt/RRC2130, so dual-cell Li-ion, 4170 mAh. There's a backup battery (single-cell, 1800 mAh) but the backup battery did not have to take over during most of the 1-week outage.

Both devices had exactly the same problem (CEREG:90), the second one was even fixed by just opening and closing the sim socket (not explicitly taking out the card) followed by a reset.

Given your information and Torje's shared experience above, a mechanical issue is indeed more likely (possibly caused indirectly by the cold?)