nrf9160 CFUN=4 PSM network signalling

[plotkin1996]

Hello,

Does nrf9160 mfw1.3.5 perform RACH and network signalling after receiving a CFUN=4 command while in PSM? If so, is it possible to put it offline without turning on RF circuits first?

What I'm trying to do is to force-disable RF communication when I know it will result in a brownout (i. e. temperature is low and battery internal resistance is increased).

[Simonr]

Hi

I discussed what this would do with a couple of colleagues. First off, it's the network that decides what PSM the device will use exactly, but we would expect there to be some radio activity after AT+CFUN=4, even in PSM. At least, there is activity when PSM isn't used first, but you could confirm this if you have a Power Profiler kit or some other way to measure current on the board to see if there are any "radio spikes" after calling CFUN=4.

Also worth noting is that the modem will stop radio activity automatically if the supply voltage drops below a certain level. Check out the battery related AT commands for details. Either way, a brownout is more likely to happen when the radio is used, and not while the device is in an idle state like PSM.

Best regards,

Simon

[plotkin1996]

The situation is like that:

• Modem is in PSM,
• Assume that a TAU update warning is incoming within some minutes (I haven't tried it with actual TAU because the interval is to high),
• Application takes a temperature measurement and discovers that it's pointless to try the TAU, because any radio activity would brownout us,
• So it sends CFUN=4,
• Modem goes into CFUN=4, but before that I still observe the characteristic PRACH spike on power profiler.

So the question is, it it the CFUN=4 that caused PRACH access?

[Simonr]

Hi again

Yes, that must be CFUN=4 causing the power spike you're seeing. But we don't think you need to do this, as the modem will already turn off if it detects that the voltage drops too low. You should also be able to change the voltage level where the modem shuts down with %XVBATLOWLVL if I recall correctly.

Best regards,

Simon