PSM floor current is 60uA not 4uA

[Frikkie Badenhorst]

Hi there, 

We are running a power consumption test using PSM mode, and we are however not measuring the 4uA floor current as expected.

We are requesting a active time of 6 minutes, and TAU time of 30minutes. We are also using eDRX.

However after the active time has passed, the device does not go into a low power state. There also appears to be a lot of noise:

I can confirm that the network does assign us the correct timers, as we get the following message in:

+CEREG: 1,"0001","01A2D002",7,,,"00100110","11100000"

Kindly advise what can be the issue here?

[Achim Kraus]

I guess, if you check GSMA Energy Efficiency for Mobile IoT , page 11, figure 1, you will recognize the difference between "active time" (T3324, or ) and "Inactivity timer". Others use the terms "PSM Activity Timer" and "RRC Activity Timer". So you selected the phase when switching from RRC Connected to Idle, but sleeping is the reached after Idle (in you case 6 minutes). Just wait and measure later.

[Frikkie Badenhorst]

Hi Achim,

I did not select the phase where it goes from RRC connect to Idle. I selected the phase during which idle time (as you can see in the figure, 6 minutes of multiple eDRX cycles occured) goes to sleeping. The "inactivity timer" is present in the very first spike, I am just zoomed out a lot.

I have waited more than the 30minutes periodic TAU, which comes in successfully after 30minutes as expected. So I guess there is indeed an issue with the PSM.

The current in the sleeping state sits around 60uA.

[Frikkie Badenhorst]

I can confirm that it is not the SIM card. Voltage over the sim card is 1.8V until the device enters PSM sleep mode, where it drops to 0V. There is also negligible current flowing throught the sim in this state.

[Achim Kraus]

> where it drops to 0V. 

Great, so only 1 and 2 left.

If these two are verified, then maybe

4: using eDRX and PSM together doesn't work as expected.

[Stian Røed Hafskjold]

Achim Kraus said:

4: using eDRX and PSM together doesn't work as expected.

I have not seen any issues with the modem itself, related to current consumption in PSM. eDRX (RRC idle) is part of the PSM request. It will stay in eDRX during the "active time" before entering PSM and is a very common use case. The problem is most likely in the application.

I tried the NCS 1.9.0 version of the application, and I had to specifically change uart2 status to "disabled" in order to get the current down. This was the only change necessary. Unfortunately I was not able to get the NCS 1.8.0 version to run, it's just crashing during boot, and I will have to investigate why this happens. Anyways, 1.8.0 is quite old now so it would be great if you could try out a newer version of NCS.

By the way, I see that you are using the PPK. A new PPK firmware update was released today that was targeting issues with showing the idle current correctly when the current is drawn in bursts, which is often the case with buck/boost converters implementing burst mode. The reported idle current was higher than the actual current consumption. This is now fixed. Although I understand that this is probably not the case here since the hex-file I created earlier did show the correct current consumption. Just wanted to mention it. The PPK firmware will be updated automatically when the PPK software is updated through nRF connect for desktop.

[Achim Kraus]

> By the way, I see that you are using the PPK. A new PPK firmware update was released today

Great. I will do that tomorrow.

I don't use eDRX and I'm also not aware of any issues in PSM without eDRX. Therefore I mentioned "4" just in the case, that 1 and 2 are also not responsible for the larger current. We will see, if verifying 1 and 2 already answers the topic.

What may speedup this topic would be a prebuild "serial LTE modem". I guess using that with the AT/modem settings applied by Frikkie should give some more answers.

[dejans]

Hi,

Frikkie Badenhorst let us know the results of your testing.

Best regards,
Dejan

[Frikkie Badenhorst]

Hi all,

Apologies for the delay in response - we had other priorities to attend to first. However, looking forward to get back onto this and solve it as we think PSM might be a feature that we can use. (We are still considering PSM vs just turning the modem of completely).

Anyways, we have also manually disabled the uarts and will start testing with it, hopefully it solves the issue. 

Will keep you updated.

[dejans]

Hi,

Thank you for the update.

Best regards,
Dejan

[Frikkie Badenhorst]

Hi Dejan,

Good news, as Stian Røed Hafskjold advised, it was indeed the uart2 that had to be manually disabled. We also disable serial logging, and we are able to measure a PSM current of 4uA.

We do however have 3 more questions regarding the power profile. Please see power profile and description below:

Section 1: Attach procedure including RRC inactivity timer

Section 2: EDRX paging time window

Section 3: eDRX interval

Section 4: AT#XSLEEP=2 command sent to modem to turn of uart1

Section 5: PSM sleep mode entered with floor current of 4 uA

The floor current (as specified by the long line in red) during the entire RRC Idle state (where eDRX is active) sits around 40ua.

1) Is the floor current of 40uA in the RRC Idle state expected, or can that be optimized as well with some project config changes? I think the 40uA makes sense as I think SIM is still active in this state?

2) There seems to be a random spike (section highlighted by the power profiler app) every 30 seconds between eDRX cycles. Do you have any idea what this might be? It is important for us to understand the power profile perfectly.

3) How can an external MCU know when the modem goes into and exits from the PSM mode? Is there for example an unsolicited response code which we can enable?

Thanks,

Frikkie

[dejans]

Hi Frikkie,

Frikkie Badenhorst said:

1) Is the floor current of 40uA in the RRC Idle state expected, or can that be optimized as well with some project config changes? I think the 40uA makes sense as I think SIM is still active in this state?

In PSM, it should be possible to achieve floor current of around 4us. This is expected for SIM card which is shut down. However, if SIM card is not switched off due to being used frequently, power consumption may rise. Reference value in the Online Power Profiler for SIM stop clock current is around 30 us, which is relatively close to what you see. Some SIM cards are required to go to sleeping state before being completely shut down to prevent flash wear-out. Newer cards for IoT applications have this sleeping time set to around 1 minute or less, while other can have this interval at around 10 minutes.

Frikkie Badenhorst said:

2) There seems to be a random spike (section highlighted by the power profiler app) every 30 seconds between eDRX cycles. Do you have any idea what this might be? It is important for us to understand the power profile perfectly.

I would need to get back to you when I get more information related to this question.

Frikkie Badenhorst said:

3) How can an external MCU know when the modem goes into and exits from the PSM mode? Is there for example an unsolicited response code which we can enable?

There is modem sleep notification and periodic TAU notification. You can read more about modem sleep notifications in the documentation.

Best regards,
Dejan

[dejans]

Hi Frikkie,

The spike that can be seen is switching noise of the PPK when it goes from one measuring region to another. This is expected behavior.

Best regards,
Dejan