Current draw jumps ~187 uA after 10 min for no apparent reason.

[jthmp]

We are using the legacy 17.1 nrf sdk with the nrf52840 MCU in tickless idle mode and notice that after about 10.5 minutes the current draw jumps from ~90uA to ~277 uA.

There is no obvious change in our firmware behavior and no unusual activity when the jump happens (that we can see so far, but still looking).

The system is just in idle mode the whole time.

In other versions of our firmware the time can be anywhere from 25.5 minutes to 35 minutes depending on the firmware version. But it's always a consistent time (ex: always 35 min).

We suspect it's a gpio being turned on (like a gpio input with a pullup) but so far we haven't found the culprit. 

We are going to apply errata 246 fix:
https://docs.nordicsemi.com/bundle/errata_nRF52840_Rev3/page/ERR/nRF52840/Rev3/latest/anomaly_840_246.html

but this should only save 40 uA.

What would cause a jump like this?

[hmolesworth]

One possibility is one or more slow or floating inputs start to cause feedthrough leakage which can be of the order of the current observed. To avoid this often the best course is to drive all unused io pins as output low (this has no significant effect on power consumption). Worth checking other pins (such as analogue inputs) for levels which may lie on the threshold around VDD/2 when the SAADC is disabled. I have seen this effect hold for periods of time when the pin voltage rises slowly to the tiny window which causes feedthrough from internal VDD to internal GND which loads slightly the battery voltage and the pin voltage then stops rising and stays in the problem window.

Description of feedthrough mechanism scba004e.pdf

Maybe post a pdf of the schematic for review

[jthmp]

Thank you for your reply hmolesworth,

We will try out your suggestion and get back to you with a schematic snapshot if necessary.

[jthmp]

So, it looks like there are no unused gpio. 

Here are things we have tried:

- reset all gpio to input / no pull on startup before configuring for actual use (didn't fix)

- printed out any gpio config / states changes in a thread to see if the current draw corresponds to a gpio change (no changed detected)

Do you have any suggestions for detecting what peripheral states might have changed that might be effecting this current draw jump?

[hmolesworth]

Probing the io pins with a 'scope would be worth doing, though if there is no 'scope available simply touching each io pin connection with a sharp sewing needle held in a bare hand would indicate any pin voltage issues by a corresponding change in observed current. Your comment about unexpected pullup or pulldown or an input held in an unexpected state with pullup/down enabled would give somewhere close to the observed jump assuming 3V and 13k internal resistance.

If not already doing so, adding timer overflow interrupt processing might catch an unexpected timer issue. If not already doing so, add an io pin toggle on sleep/wake to verify the mark:space ratio remains exactly as expected. Any io pin can be hijacked for that purpose provide it doesn't have a load - pushbutton input is ideal if no external pullup/down.

What is the exact value VDD before and after the jump; is there a measurable difference? Are there any potential phantom power sources attached to io pins?

A schematic is needed to provide more thoughts, either post here or email direct if the circuit is connfidential.

[Edvin]

Hello,

jthmp said:

reset all gpio to input / no pull on startup before configuring for actual use (didn't fix)

You shouldn't need this. By default, all pins are disconnected inputs, which is the state that draws the least amount of current. 

I would try to remove one peripheral at the time (never enable it in the application) until the behavior goes away. That should help pinpointing where the issue is.

Could it be other devices on your PCB that is drawing the current?

Best regards,

Edvin