How can I read out the actual value of the power supply, using the low power comparator?

[Giannis Anastasopoulos]

Dear all,

I need to read out the voltage of the battery that supplies my nrf52. From what I have read I see that there are 2 options: LPCOMP and ADC.

Due to restrictions that I have because of my hardware, I cannot add any exteranl components in order to add the voltage divider. So I need to use the LPCOMP.

I have checked the SDK example, but I don't see how to actually read out the value. Searching around has only revealed to me there is a way to trigger an event when the supply voltage falls under a certain value. But what do I need to do in order to measure the value of the supply voltage?

[Øyvind]

Hello,

I have checked the SDK example, but I don't see how to actually read out the value

What example did you look at? The Proximity Application (examples\ble_peripheral\ble_app_proximity) measures the battery level every 2 minutes using ADC. If there is a change in the battery level, the application sends the current level as a notification.

/**@brief Function for handling the ADC interrupt.
 *
 * @details  This function will fetch the conversion result from the ADC, convert the value into
 *           percentage and send it to peer.
 */
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;
        uint16_t          batt_lvl_in_milli_volts;
        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, 1);
        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);
        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);
        }
    }
}

Hope this helps.

Kind regards,
Øyvind

[awneil]

Giannis Anastasopoulos said:

I don't see how to actually read out the value

The "LPCOMP" is a comparator - so it does not give you a "read out of the voltage"

A comparator just tells you that the input is above or below the reference.

Giannis Anastasopoulos said:

I cannot add any exteranl components in order to add the voltage divider.

If your battery voltage exceeds the allowable input pin voltage, then you will still need a divider - whether you use the comparator or the ADC

[Giannis Anastasopoulos]

So, from what I understand, I can simply read the voltage directly from the pin, without any voltage divider, as long as my supplied voltage is less than 3.3V, correct?

[awneil]

Check the Product Specification for the actual limit (which is also related to VDD) - as long as you don't exceed that, you're OK.

You may also need to adjust the ADC gain so as not to exceed its internal input range