Correct to shutdown modem and reconnect

[men]

Hi,

My application only needs to be connected to the network for a short period of time to deliver data to a server and then disconnect again. At first everything works as intended (connect/disconnect), but when it try to reconnect it throws an error at "void bsd_recoverable_error_handler(uint32_t error)" with the message "Err: 3".

When connecting lte_lc_init_and_connect() is used and when disconnection firstly lte_lc_offline() and then lte_lc_power_off() is used. If lte_lc_power_off() is skipped the device is able to reconnect.

How would be the correct way to connect/disconnect/reconnect from the LTE network to save power?

[Håkon Alseth]

Hi,

 

 

When connecting lte_lc_init_and_connect() is used and when disconnection firstly lte_lc_offline() and then lte_lc_power_off() is used. If lte_lc_power_off() is skipped the device is able to reconnect.

The lte_lc_offline() translates to CFUN=4, which is perfectly fine. You just need to enable the modem again afterwards.

lte_lc_power_off() will translate to CFUN=0, which will require a power cycle, as stated in the note here:

https://infocenter.nordicsemi.com/topic/ref_at_commands/REF/at_commands/mob_termination_ctrl_status/cfun.html?cp=2_1_3_0

 

Have you tried to use PSM instead? This can be enabled via function "lte_lc_psm_req(true);". Some networks might refuse the interval, so you need to check if the interval is accepted via AT command "AT+CEREG?", but first you need to enable AT+CEREG=5 to get the "added" timers to be reported back when reading.

 

Kind regards,

Håkon

[men]

No I have not tried PSM. Would it be more power efficient when it only needs to connect once a day?

[Håkon Alseth]

Yes, if you set the timer higher, and use the latest modem fw v0.7.0-29, which shall turn off the SIM completely in PSM.

Its now set to 30 mins by default:

https://github.com/NordicPlayground/fw-nrfconnect-nrf/blob/master/drivers/lte_link_control/Kconfig#L31

The tree upper bits sets the interval, and the lower 5 bits set the timer count.

This is shown in the 3gpp TS 24.008 v15.5.0, chapter 10.5.7.4A, and I also copied this table into a PSM example here:

https://github.com/Rallare/fw-nrfconnect-nrf/blob/nrf9160_samples/samples/nrf9160/udp_with_psm/src/main.c#L97

 

The PSM interval should match your expected "receive" window from the network. If you expect to receive/poll data every 2 hours, the PSM interval should be set to this value.

If you only send data, via UDP (thus, never receive), you can set the PSM interval even higher.

If you want to receive every 1 day (24 hours), you can set it to "00111000" (1 hour * 24 = 24h)

 

When PSM is set, you can send data at any time you'd like, just push data and the modem will do the rest for you.

You will then be active for the default 1 minute after triggering a send (https://github.com/NordicPlayground/fw-nrfconnect-nrf/blob/master/drivers/lte_link_control/Kconfig#L39)

Note that the active time and the PSM interval use two different "GPRS timer", so the bits set for active time will differ from the bits set for PSM interval.

 

Kind regards,

Håkon

[men]

That sound great. I will give it a try.

[men]

Hi Håkon,

When I need to access to GPS then I have to set it into offline mode anyway, and make at full reconnect afterwards.

It seems to work at first, but the second time I get the message "Err: 3" and is stuck in lte_lc_init_and_connect(). Do you know what could cause this?