Is the VSS Pin (pin 103) connected internally to GND_Shield?

[Stefan Schmidt]

Dear Nordic Support Team,

 In the datasheet of the nrf9160 (nRF9160_PS_v2.2.pdf) the VSS pin appears in the section 6.4.5.1 GPIO Electrical Specification, but I cannot find the information if it is internally connected to GND_Shield.

Best regards

Stefan

[Øyvind]

Hello Stefan, 

No sure what you are referring to. The GND_Shield pins of the nRF9160 are connected to Ground, similarly the VSS is connected to Ground. This is described in chapter 10 Hardware and layout. 

Kind regards,
Øyvind

[Stefan Schmidt]

Hi Øyvind,

thanks for your reply. I know that they have to be connected them to ground, my question is: Are they connected in the nrf9160?

I tried to read VCC_GPIO on a custom board and was wondering why the readings are so far away from the 1.778 V I was measuring with an external Multimeter. The readings are outside the 4% DC error, and always lower than 1.779 V. I increased the aquisition time to 40 µs, but this did not solve the issue.

When the modem was active the VCC_GPIO read with the ADC was even lower, so I thought that ground bouncing might be the issue and that it would be better to have the VSS pin routed to the star ground point separately from the GND_Shield pins (which I suspected to be the ground pins for the modem).

But now I have set up this sample zephyr/samples/drivers/adc on a nRF9160-dk and this is what I read for the 1.8 V VCC_GPIO:

- adc@e000, channel 1: 7621 = 1674 mV

I use this devicetree setup for this channel:

    channel@1 {
        reg = <1>;
        zephyr,gain = "ADC_GAIN_1_6";
        zephyr,reference = "ADC_REF_INTERNAL";
        zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
        zephyr,input-positive = <NRF_SAADC_VDD>;
        zephyr,resolution = <14>;
        zephyr,oversampling = <8>;
    };

Do you have an idea why the reading is so wrong?

[Stefan Schmidt]

7433.adc.zip

Hi Øyvind,

 please find attached the zipped adc folder from zephyr/samples/drivers. I am using nRF Connect SDK v2.6.1, and simply zipped the adc folder after deleting the build directory.

I have build it with 'west build -b nrf9160dk_nrf9160_ns', in the prj.conf you can see that I am using Segger RTT for logging.

[Øyvind]

Hello, and thanks for providing your project. I see the same result and inquired internally. This does look like a known issue, which there is currently no solution to. 

Kind regards,
Øyvind

[Stefan Schmidt]

Hi, thank you for your reply. Please let me know when you get feedback.

Is this issue somehow related to the fact that a common ground plane is used? Instead of a ground plane for the modem supply and a separate one for the VDD_GPIO?

Best regards

Stefan

[Stefan Schmidt]

Ping. Not sure, if my editing triggered an update on your side.

[Øyvind]

Thanks, I see the update. Have forwarded to our R&D team.