nRF9160 Modem lock up workaround

[DamoL]

Hi, 

I've noticed an issue with the modem locking up on the nRF9160DK. I'm not sure what causes it, but it always gets err 116 - ETIMEDOUT when initialising the modem and trying to connect to a network. 

The easy fix at the moment is to unplug the DK and plug it back in. If I do, the modem connects within seconds. However, my fear is when this is in our product, it will get into this lock up state and not be able to get out of it. 

My code is loosely based on the Simple MQTT sample. When the modem_configure() function fails it has a retry delay, but without power cycling the board, it will never connect. I tried adding in lte_lc_power_off() and lte_lc_power_on() with the delay inbetween, but this didn't help. 

do {
	err = modem_configure();
	if (err) {
		printk("Retrying in %d seconds\n", CONFIG_LTE_CONNECT_RETRY_DELAY_S);
		lte_lc_power_off();
		k_sleep(K_SECONDS(CONFIG_LTE_CONNECT_RETRY_DELAY_S));
		lte_lc_power_on();
	}
} while (err);

I definitely need a soft way reset the modem when it gets in that state, and I imagine someone has also come across this issue. As for why it gets into it, I am not sure yet, but will try and understand why. 

Thanks, 

Damien

[DamoL]

Just been doing some more digging in the code. 

 I noticed when its trying to connect to a network I can actually query "AT+CEREG?" and recieve information that it actually is connected to the network. The problem is its not always getting a CEREG notification. It fails at the point it tries to take the link semaphore (line 661 of lte_lc.c). I wonder is some other thread is blocking the notification coming through somehow, or how I can get round this. 

I guess one option is to manually check the CEREG status is modem_configure() fails. If so, I assume I would also have to give the link semaphore in case that is used elsewhere. 

[Øyvind]

DamoL said:

And I have attached the bin file from the modem trace. If you could let me know what the trace says that would be helpful.

Looking through the modem trace it looks like your device is receiving 

Cause #17 – Network failure
This EMM cause is sent to the UE if the MME cannot service an UE generated request because of PLMN
failures.

What SIM are you using? Can you please run the AT Client and issue the following commands:

AT%XSYSTEMMODE=0,1,0,2
AT+CFUN=1
AT+CFUN?

What happens if you run standard Asset Tracker v2 on your board? Do you see the same issue?

Can you leave the modem trace running for longer?