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?