SAADC VDDHDIV5 input on nRF52840 rev. 1

[Norbert]

Hello,

I got two PCA10056 1.0.0 DKs equipped with an nRF52840 rev. 1 (code AC0). Both do not measure the VCCH voltage correctly (displayed voltage ~2.4V instead of 3.0V). The setup is based on the peripheral->saadc example with the only modification being:

//      NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0);
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_VDDHDIV5);
        channel_config.gain = NRF_SAADC_GAIN1_2;

I chose to increase the input voltage swing on the ADC by configuring the gain stage from 1/6 to 1/2. This should normally give better results.

The displayed result is around 410: 410/1023 * 0.6V * 5 * 2 = 2.4V. I got the same result with the gain stage set to 1/6. The result is consistent across the two boards. Using NRF_SAADC_INPUT_VDD with gain 1/6 yields the correct result. A custom board with a rev. 2 chip works correctly with VDDHDIV5.

Can you confirm this behaviour? I did not see any erratum against explaining this. I can only guess that this behaviour (if confirmed) is a side-effect of erratum 197.

Thanks,

Norbert

[pmochocki]

Karl Ylvisaker said:

Yes, from what I can tell VDDH is not really in use when the nRF is in Normal voltage mode, so I do not think it is a valid source to measure the nRF's supply voltage when not in high voltage mode.

This is very useful and has helped me a lot, as I was wondering why I get strange results when using VDDHDIV5 in Normal voltage mode. To bad that this information is not in the datasheet. I understand that VDDH is not really in use when the nRF is in Normal voltage mode, but VDDH is connected to VDD so users might expect this to work.   

[Karl Ylvisaker]

Hello,

I am glad to hear that you found the answers here useful! :) 

It is indeed not explicitly mentioned in the datasheet - I will write a note about this to our techwriters and see if it is something they can amend if there is a future release of the Product Specification planned.

Thank you for the feedback, we appreciate it! :) 

Best regards,
Karl