How to measure current with an external power supply on nRF52840?

[SimonJ]

Hi,

I have a nRF52840 DK that I want to measure sleep current on, but I find it a bit confusing with all the switches etc. After reading through the documentation I have the following setup:

SW10: ON
SW6: nRF ONLY
SW9: VDD
P21: Power analyzer

But what about SW8? And should a usb cable be connected to J2 to prevent leakage current from nRF to the debugger? Is it possible to use the LEDs in this configuration?

Thank you!

[Stian Røed Hafskjold]

Yes, SW6 open the switches between the chip and the debugger, LEDs and UART interface.

If SW10 in ON, the chip is powered from P21 -> VSRC_NRF -> SW9 (VDD pos.) -> VDD_nRF/VDD_HV

If SW10 in OFF, the chip is powered from VDD -> VSRC_NRF -> SW9 (VDD pos.) -> VDD_nRF/VDD_HV.

[SimonJ]

Thank you for verifying.

Now I have a test setup based on the ble_pwr_profiling example in the SDK. I have modified the code to go directly into connectable advertisement mode and have set the interval to 100 ms.

Between the advertisements I get two short current pulses at approximately 2.7 mA. What is that?

Between the current pulses the average current is 4.6 uA, I would expect it to be less? How can I get the datasheet number of 2.35 uA? Or is it possible to go lower when running the BLE stack?

[Stian Røed Hafskjold]

The pulses you see between the advertising events are probably the ULP regulator operating in refresh mode. https://devzone.nordicsemi.com/f/nordic-q-a/38101/the-chip-s-current-consumption-is-not-expected/147109#147109

You need to add LF clock and RTC current to the idle current when the softdevice is enabled. You can also try to turn off the RAM blocks to save idle current.

[SimonJ]

Thank you.

When I measure current, should I include the refresh mode pulses or are the datasheet numbers without them?

Will the BLE stack function properly if I disable RAM blocks? I want to measure a realistic scenario where I do advertisements every 100 ms and 1 s, like a beacon. 

Should I use the DCDC or LDO to achieve lowest power?

What current consumption can I expect?

[Stian Røed Hafskjold]

The refresh spikes are included in the datasheet numbers and are part of the idle current.

The BLE stack will function properly if you disable some (not all) RAM blocks. You can read more about the softdevice RAM requirement here, and how to adjust it. The "RAM size" variable must be adjusted to the size of the physical RAM (minus the size required by the softdevice, i.e. what is left over for the application).

DCDC gives you the lowest power consumption.

Please check out the Online Power Profiler to get an estimate on the power consumption while using the softdevice.