NRF9160 Modem and Brownouts

[Thomas]

What level of risk is involved with brownouts for the modem? We have seen some modems in the past which have bricked themselves when a brownout has occurred when performing periodic flash maintenance. Is there any chance of something similar happening on the nrf9160? How robust is the modem to the frequent brownout resets that would be seen when batteries are on the way out?

Kind regards,

Thomas

[Heidi]

Hi! I apologize for the delayed response to your inquiry. 

The modem is quite reliable when it comes to power being cut during file system operations. 

File system sync is done occasionally to all files. Although the sync does not always erase the in-use half of file system, it is recommended that we are conservative and the sync is done at most three times a day (this ensures the 10 000 erase cycle limit is not reached within 10 years).

I have never encountered the modem being bricked. I have forwarded this to the developers though, and I'll update you when I hear back.

Best regards,

Heidi

[Thomas]

Hi Heidi, sorry one more question. When you say it is recommended the sync is done at most 3 times a day, are you saying the modem handles that, or we as users need to ensure it does not do that? If so, how? We don't have to limit the amount of times we power down the modem with AT+CFUN=0 do we?

[Heidi]

I'm not sure. 

I know there is some limit to running AT+CFUN=0, which is why we recommend customers use AT+CFUN=4 if they're going to be powering down very often, but I don't know if this limitation is due to the file-sync or something else.

But as for the 3 times a day maximum, a believe the modem takes care of that. I have asked a developer to double-check so I'll let you know when I get a reply. 

[Thomas]

Thanks for the information Heidi

[Heidi]

Okay, so the modem has internal timers for file system sync which is not controllable by the application.

However, in addition to this, AT+CFUN=0 performs a file system sync every time the application requests it. So yes, you should limit the amount of times you power down the modem with AT+CFUN=0.

You should use AT+CFUN=4 instead if it's merely for powering down, and not for saving configurations to flash. For that, AT+CFUN=0 has to be used.

[Thomas]

Okay great, thank you for clarifying this Heidi

[Heidi]

Hi, I got a response from the developers regarding how the modem handles brownouts. See the response below:

 

The modem internally monitors the battery supply level at the start of RX/TX operations and when it detects the voltage dropping below the fixed 3000 mV level, the modem will disconnect from the network, enter deep sleep offline mode, and will notify the application domain of this event.

The modem will not remove itself out of flight mode until the application re-activates the modem (AT+CFUN=1), and at that time the modem will again resume monitoring the battery supply level.

 EDIT 10th Nov: There is additional support through the AT command %XPOFWARN. When it has been activated, the modem will also check for battery low-level warnings at all times, except when the modem is completely off. If such a warning is triggered, the modem will again disconnect from the network, enter deep sleep offline mode, and notify the application domain of this event.