SAADC read on VDD giving a dip in value every 1 to 1.5 hours for approx 2 mins

[Ameer Usman]

So I am using nRF52811 and I have noticed the battery voltage going at a constant rate and suddenly for a time of 2 mins the Voltage value (ADC) dips and it comes back to normal after approx 2 mins (sometimes very exact).

What could be the potential cause of it?

The code for the event is:

void saadc_event_handler(nrf_drv_saadc_evt_t const * p_event)
{
    if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
    {
        nrf_saadc_value_t adc_result;
        
        uint8_t           percentage_batt_lvl;
        uint32_t          err_code;

        adc_result = p_event->data.done.p_buffer[0];

        err_code = nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer, SAADC_SAMPLES_IN_BUFFER);
        APP_ERROR_CHECK(err_code);

        batt_lvl_in_milli_volts = ADC_RESULT_IN_MILLI_VOLTS(adc_result);  // + DIODE_FWD_VOLT_DROP_MILLIVOLTS;
//        percentage_batt_lvl = battery_level_in_percent(batt_lvl_in_milli_volts);

//        err_code = ble_bas_battery_level_update(&m_bas, percentage_batt_lvl, BLE_CONN_HANDLE_ALL);

        NRF_LOG_INFO("-------Battery Voltage in mV: %d----------", batt_lvl_in_milli_volts);

        if ((err_code != NRF_SUCCESS) &&
            (err_code != NRF_ERROR_INVALID_STATE) &&
            (err_code != NRF_ERROR_RESOURCES) &&
            (err_code != NRF_ERROR_BUSY) &&
            (err_code != BLE_ERROR_GATTS_SYS_ATTR_MISSING)
           )
        {
            APP_ERROR_HANDLER(err_code);
        }
    }
}

And initialization is:

static void adc_configure(void)
{
    ret_code_t err_code = nrf_drv_saadc_init(NULL, saadc_event_handler);
    APP_ERROR_CHECK(err_code);

    nrf_saadc_channel_config_t config = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_VDD);
//    nrf_saadc_channel_config_t config = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0);
    
    err_code = nrf_drv_saadc_channel_init(0, &config);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(&adc_buf[0], SAADC_SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(&adc_buf[1], SAADC_SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);
}

Let me know if more information is needed.

[Otso Jousimaa]

Have you checked if there is loading on battery before sampling? For example the radio can cause significant voltage droop on coin cells, there's some oscilloscope captures at blog.ruuvi.com/ruuvi-firmware-part-13-battery-measurement-synchronized-to-radio-a0a4dba1e002

[Ameer Usman]

So so I am sampling on radio activity enable:

rc = radio_notification_init(3, NRF_RADIO_NOTIFICATION_TYPE_INT_ON_ACTIVE , NRF_RADIO_NOTIFICATION_DISTANCE_800US); //Changed from Active to Inactive to combat Voltage drop on radio activity
APP_ERROR_CHECK(rc);

Would performing the sampling on radio activity disable fix the issue:

rc = radio_notification_init(3, NRF_RADIO_NOTIFICATION_TYPE_INT_ON_INACTIVE , NRF_RADIO_NOTIFICATION_DISTANCE_800US); //Changed from Active to Inactive to combat Voltage drop on radio activity
APP_ERROR_CHECK(rc);

[Otso Jousimaa]

Measuring on radio activity disable is how we do it at Ruuvi. Below is the screen capture of sampling (GPIO, blue) and battery voltage (VDD, red). I can't say for sure if it will fix your issue, but it has been very reliable for us on CR2477 batteries.

[Ameer Usman]

Great, Thank you :)