NRF 52840 DK Battery Level Reading

[Radio]

Hi All,

I am trying to implement a battery level reading app using NRF52840 DK. It seems I am receiving a timeout from saadc.

In order to run on the Lithium battery, should we set the switch to VDD or Li-Po?

Thanks!!

[Susheel Nuguru]

Hi Radio,

I am guessing this is about SW9. SW9 is used for selecting the power source.

• Li-Po: The nRF chip is powered by the Li-Po header on VDDH (high voltage regulator, REG0). The VDD domain on the DK has the same voltage as the output of REG0 (using a voltage follower, 1.8V as default)
• USB: Same as above except that the chip is powered by the nRF USB connector.
• VDD: Chip is powered from VDD domain, disabling the high voltage regulator. VDD domain is either USB, external supply or battery holder. When SW10 is ON, the chip is always powered from external supply.

So setting it on VDD should give correct measurements.

[Radio]

Hi Susheel,

Currently I have connected nRF52840 DK to the USB. Li-Po battery also in place. SW9 selected as VDD. SW10 is OFF.

In my code I am using NRF_SAADC_INPUT_VDD config as for my SAADC channel (This time I am now using the SAADC example given by Nordic).

In this scenario, I measure VDD as 3.0 V and is it correct to say that VDD value is from the USB power supply?

If I really want to measure the Li-Po power, should I simply select Li-Po in SW9 and use NRF_SAADC_INPUT_VDD?

Should I disconnect USB cable before doing so?

Thanks!!

[Susheel Nuguru]

Radio.

There is no battery reading circuitry on the DK and if you are connecting the VDD to the saadc then you will just measure the output of REG0 regulator and not the battery level.

For LiPo, you need to make a voltage divider circuitry over VDDH and GND and make this as the source to one GPIO which is used by saadc to sample.

[Radio]

Hi Susheel,

Thanks for the reply. I still have few more things to clarify.

In the nrf52840 product specification document it states:

"Normal voltage mode is entered when the supply voltage is connected to both the VDD and VDDH pins (pin VDD shorted to pin VDDH)."

"High voltage mode is entered when the supply voltage is only connected to the VDDH pin and the VDD pin is not connected to any voltage supply"

1. In our DK, when I select SW9 switch to VDD, does it short VDD and VDDH  together (Normal mode)?

2. And, when I select Li-Po in SW9, it disconnects VDD pin with any power supply, only VDDH is connected (High Mode)?

3. Is there is away I can run the nRF chip only by using the Li-Po battery?

Thanks! 

[Stian Røed Hafskjold]

Hi, Susheel is on vacation so I will answer your questions.

1. Yes
2. Yes, and VDDH is powered through the LiPo header
3. No, because the LiPo header is powering everything else on the DK as well. You will have to power VDDH on the LiPo header and measure the current going through the VDDH current measurement header to isolate the current to the nRF52

[Radio]

Hi Stian,

Thanks for the reply. In the nrF52840 DK, there is a switch to select "nRF Only" | "Default".

Currently set to "Default". May I know:

1. What is the purpose of this switch?

2. If it only powers the nRF chip? If so from where does it get power? 

Thanks!

[Stian Røed Hafskjold]

1. It disconnects the nRF52 GPIOs and SWD interface from the rest of the DK, so you don't get any leakage current when measuring on the nRF chip.
2. The nRF chip gets its power from the USB, LiPo or the external power header. The problem is that these domains power the rest of the chip

You can power the nRF directly from the external power header, if you set SW10 (VEXT->nRF) to ON. The problem is that this only works in normal voltage mode. For high voltage mode there is no way to power only the chip.

[Radio]

Hi Stian,

Thank you very much for the information.