Redbear BLE nano / CR2450 / Battery level

Question

Hello, 
 I'm powering a Readbear BLE nano using a CR2450 coin cell battery. The firmware application is written using mbed. How can I measure the battery level of the CR2450 over time using the Nordic mbed API and libraries? 
 Thanks,
Chris

Ole Bauck · Accepted Answer

It looks from here and here that the analogIn API can only be configured to use the ADC input pins and reference VDD 1/3 prescaler. I would recommend to make your own init and read function with VBG reference and VDD/3 input:

void adc_init()
{
    NRF_ADC->ENABLE = ADC_ENABLE_ENABLE_Enabled;
    NRF_ADC->CONFIG = (ADC_CONFIG_RES_10bit << ADC_CONFIG_RES_Pos) |
                      (ADC_CONFIG_INPSEL_SupplyOneThirdPrescaling << ADC_CONFIG_INPSEL_Pos) |
                      (ADC_CONFIG_REFSEL_VBG << ADC_CONFIG_REFSEL_Pos) |
                      (ADC_CONFIG_PSEL_Disabled << ADC_CONFIG_PSEL_Pos) |
                      (ADC_CONFIG_EXTREFSEL_None << ADC_CONFIG_EXTREFSEL_Pos);
}

uint16_t adc_read()
{
    NRF_ADC->EVENTS_END  = 0;
    NRF_ADC->TASKS_START = 1;

    while (!NRF_ADC->EVENTS_END) {
    }

    return (uint16_t)NRF_ADC->RESULT; // 10 bit
}

Correct me if some of this is wrong as I did not test it.