nRF9160 modem stuck in searching

[miked531]

The modem on the nrf9160DK (modem fw v1.1.0) appears to get stuck in searching mode. It doesn't happen all the time, and I was able to create it with the following AT commands:

(I want to see registration status changes)
AT+CEREG=2
AT%XSYSTEMMODE=0,1,0,0
AT+CFUN=1
(after some time (minutes?) reports AT+CEREG: 2, ...)
(then I see no more +CEREG status reports and give up)
AT+CFUN=4
(wait a few seconds)
AT+CFUN? reports +CFUN: 4, so it seems to have taken the command
AT+CEREG? reports +CEREG: 0,2,"XXXX","XXXXXXXX",9

This all works just fine with %XSYSTEMMODE=1,0,0,0. After the AT+CFUN=4 completes it will report +CEREG: 0,0,...

I kind of expect the connect to work with either mode, as I have good reception from both AT&T and Verizon, but I'm more concerned that the modem appears to get in an *almost* unrecoverable state. Changing systemmode to 1,0,0,0 followed by AT+CFUN=1 returns to normal behavior.

If it helps in troubleshooting, the AT+CFUN=4 takes ~3 seconds in mode 1,0,0,0 and less than 1 sec in mode 0,1,0,0

This link describes a similar issue: https://devzone.nordicsemi.com/f/nordic-q-a/55154/lte-network-detected-but-never-connected

[miked531]

I will get a modem trace and see what I can find about the SIM, but it may be a few days until I can get that done. Another point to consider is that the same behavior is observed with the iBasis SIM that comes with the DK.

I didn't realize turning on the modem also turned on the SIM, but that makes sense.

Mike

[Martin Lesund]

Hi Mike, 
It would make sense that the iBasis SIM card would have the same behavior if the DT network has not allowed roaming SIMs onto their NB-IoT network.
So I recommend that you get in contact with DT and get an update on which type of SIM card that would allow you on their NB-IoT network.

[miked531]

I am using the DT and iBasis SIMs on an AT&T network (and occaisionally Verizon, depending where I am [again, same beharior]), which we hare already established does not support NB-IoT. Does that change your thinking at all?

[Martin Lesund]

Please contact DT on how to get connected to their NB-IoT network, and if you have the correct DT SIM for that.
When using DT SIM on LTE-M network, do you then roam on the verizon/AT&T network?

[miked531]

Martin,

I feel like we are going down the wrong path by contacting the carrier. Going back to my original issue, I see that AT+CFUN=4 does not seem to completely turn off the modem as expected. AT+CEREG continues to report status 2 (searching for network) after +CFUN=4, and does not change. Additionally, the modem behaves differently after a failed attempt to connect to NB-IoT, where future Cat-M1 connects take 80 seconds vice 5 seconds if I didn't attempt NB-IoT.

I hope to get the modem trace for you by this weekend.