Rak3172 at 450µA in deep sleep!?

LPWAN Modules / EVBs
1

Hi,
I can’t figure out why my rak3172 uses 450µA in deep sleep while it should end up below 10µA.

I use 1 voltage divider (1M, 1.5M) for battery voltage detection; this should consume around 1µA.

I use 1 SHT40 sensor whose GND-pin gets set to HIGH after measuring, so that it shuts off. Anyway it should only consume a few µA even if left powered.

I use a timer as per the low power example and the following lines to sleep (like 5min) between sending packets with new sensor values:

api.system.lpm.set(1);
api.system.lpmlvl.set(2);
api.system.sleep.all();

in “loop”:

api.system.scheduler.task.destroy();

That’s all. The lpmlvl.set(2) command and setting the ADC-reading pin for the voltage divider to INPUT_PULLDOWN instead of INPUT brought it down from 630µA to 450µA, I’m glad I found both these hints.

What else can I do? I use a custom PCB with the rak3172 module and the above mentioned peripherals, nothing else connected.

2

It is honestly the first time I hear that someone sets GND to VDD level to “shut-off” a sensor.

I don’t know the SHT40 in detail, but usually Sensirion sensors have an option to shut them down by command over I2C to minimize power consumption.

If that is not enough, then VDD of the sensor could be cut off with a load switch.

Other than that, I have no idea where the 450uA consumption in your system would come from.

3

Thanks @beegee

I did some measurements, and setting the sensor’s V_in to GND would have drawn more power, so instead I set the GND pin to VDD. The sensor sees 3,xV on both ends that way. On the SHT40 it doesn’t make much sense current-wise, but other sensors for which the PCB was universally made would consume more.

Anyway, eliminating this function in code and letting run the SHT40 in standby, with no heater configured etc., the overall current doesn’t change. The sensor would draw 1-2µA only anyway, so it can’t be the issue I’m facing.

I had made a design with a load switch, as per a design recommendation by TI including a ultra low power timer (forgot the name), but in total this draws more current than just using the 3172 in deep sleep with a timer.

Could there be any part of the 3172 which isn’t sleeping properly?

I tried setting all unused pins to LOW and also to set the ADC to off per command, even disable Serial, but nothing worked. (I guess they all sleep in deep sleep anyway, but just to be sure…)

4

IMHO, you should not set any VDD or GND pin to its opposite value.
I think by doing what you do, you might open some paths were electricity flows where it shouldn’t.

If you need to shut it down and the implemented sleep function of the sensor is not sufficient, cut its VDD supply with a load switch.

Did you measure your consumption if you keep the sensor powered up, just set it into sleep mode or stopping it from doing measurements.

5

Yes, same current. I figured the sensor goes to sleep by itself anyway. But even if actively running, it wouldn’t draw that much current.

6

So I rebuilt the sensor node - new PCB, new parts, all newly soldered.

Same code, just uploaded. Now the sleep current is 5.94µA.

I’m keeping the SHT40 active for now. Perhaps my disabling via GND to HIGH damaged something?

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