nRF91 power saving modes

[Holger Staiger]

A question about the power saving modes PSM and eDRX. A device is mostly in deep sleep and wakes up cyclically for mobile communication.

It is better:

Completely disconnect the nRF91 from the power supply and thus dial into the mobile network every time you communicate

or to supply the nRF91 with voltage during the deep sleep phases and to use the power saving modes

[Stian Røed Hafskjold]

It's hard to give a definite answer to your question. The system off current of the entire chip is 1.4 µA, and the PSM current is 4 µA. That means that you can save maximum 2.6 µA in average, if you consider the best case parameters (which would basically mean not sending any data at all). As soon as you start waking up to send data, the overhead of starting up the modem and scanning for the network will quickly make the system OFF option a worse alternative.

We have some numbers suggesting that the wakeup interval where you get similar current consumption between the two options is about 1 day. In PSM mode you will use about 6.4 µA in average, and in system off mode you will get 6.12 µA in average. But again this depends greatly on the network parameters, reception etc. For instance, how long does it take to connect, what is the TX power needed, which DRX parameters are supported by the network, what is the active time after the connection, etc.

So unless you are uploading data very infrequent, maybe once a week, I think the safest is to go for PSM mode. You can also consider a more dynamic approach based on network parameters.

You should also take into consideration which PSM interval the network is supporting. If you design an application with data upload once every day and the network only supports 3 hour PSM interval, then maybe you should have the possibility to turn off the modem.

...

After writing the above I see now that you wrote "completely disconnect the nRF91 from the power supply". I guess the answer will be the same except that you can also save the 1.4 µA system OFF current. But you will of course not be able to use the application CPU on the nRF91 and would need a separate MCU (In case you didn't know, the nRF91 has a separate cortex M33 application CPU which is not used by the modem stack).

[Holger Staiger]

So in the end it is only a question of energy consumption which is better: using energy saving modes or switching off completely.

Are there no disadvantages worldwide if you always completely switch off the chip after a transmission.

[Stian Røed Hafskjold]

It would take a little longer to send the data, since it needs to search for the network and connect. But other than that, and current consumption, I don't think there are any disadvantages. You connect to the network and transfer whatever data you need, and turn off. In PSM mode the UE is also unreachable by the network, as if it was turned off.