nRF9151: disconnect from network and go to low power state

[klaasg]

We are developing a very low power product. In some cases like not getting registered, PSM not being available, ... we want to drop the connection and go to a sleep state with a few uA energy consumption.

The problems we are facing:

• We are using AT+CFUN=0. The problem is that it can take very long. Does the nRF91 have a timeout to this?
  Because at some point, we want to just give up deregistering nicely and shut down the modem to save on energy used.
• Currently we're doing this  "giving up on CFUN=0" after two full minutes. We then call nrf_modem_shutdown(). It appears that this call can also take more than a few seconds. Our devices reboot because that function took too long to return (watchdog).

So our questions are:

• How long can a AT+CFUN=0 at maximum take?
• Say we want to give up deregistering because AT+CFUN=0 is taking too long, can we do that and what is the best way?
• Say we don't want to use the modem for quite some time. Does it make sense to shut down the modem, or is CFUN=0 enough to be at the lowest possible power consumption?

Thank you.

[dejans]

Hi,

Can you provide some details about your use case and your application?

Which network does your device connect to?

How long is your device expected to be in minimum functionality state (AT+CFUN=0)?
How and when do you put your device to fully functional mode?

How long do you expect modem not to be used when you mentioned "Say we don't want to use the modem for quite some time"?

EDIT:
Do you connect to NB-IoT network? 

It is recommended that you use AT+CFUN=4 instead of AT+CFUN=0 due to NVM wear.

Best regards,
Dejan

[klaasg]

Thank you for answering.

Our devices are sometimes stationary and sometimes traveling. They send data over UDP/TCP to a server a few times every day.

• If the device is in an area without network, we want to consume as few power as possible. So if registering fails, we go to sleep for 24 hours and try again after. We want our whole device to consume ~20uA.
• We start a network attach with AT+CFUN=1 and then listen to the notifications to get information. In the end we either get into PSM or we don't, then we do AT+CFUN=0. Not getting onto PSM can happen because of a bad network or because a device was moving (fast).
• Sometimes, we see very long network de-registering times (after +CFUN=0), which can be energy consuming because the modem is active. We would like to prevent this in the best way and quit more harshly.
• We have currently seen this on Telus Mobility Canada and Orange Belgium. We know that PSM currently is not working on Telus Mobility, so that is why the device is doing +CFUN=0 there.

> Do you connect to NB-IoT network?

We are using both NB-IoT and LTE-M. We might see these long de-registering times more on LTE-M, but I have no proof of that.

> It is recommended that you use AT+CFUN=4 instead of AT+CFUN=0 due to NVM wear.

Will CFUN=4 also give us energy consumption of a few micro-amps? We thought that NVM wear would be OK because we have the holdoff of 24 hours if things go wrong.

Thank you!