SAADC output not making sense

Question

I have my custom board wired to the nRF52 DK. Using the SAADC example code located in GitHub I am trying to set up SAADC to measure the coin cell battery that will power the board. I am testing it against the power fed in from the DK which is about 2.9 volts. However the output is not what I expect. My code is as follows:

void saadc_init(void)
{
    ret_code_t err_code;
    nrf_drv_saadc_config_t saadc_config;
    nrf_saadc_channel_config_t channel_config;

    //Configure SAADC
    saadc_config.interrupt_priority = APP_IRQ_PRIORITY_LOW;                               //Set SAADC interrupt to low priority.
	
    //Initialize SAADC
    err_code = nrf_drv_saadc_init(&saadc_config, saadc_callback);                         //Initialize the SAADC with configuration and callback function. The application must then implement the saadc_callback function, which will be called when SAADC interrupt is triggered
    APP_ERROR_CHECK(err_code);
		
    //Configure SAADC channel
    channel_config.reference = NRF_SAADC_REFERENCE_INTERNAL;                              //Set internal reference of fixed 0.6 volts
    channel_config.gain = NRF_SAADC_GAIN1_6;                                              //Set input gain to 1/6. The maximum SAADC input voltage is then 0.6V/(1/6)=3.6V. The single ended input range is then 0V-3.6V
    channel_config.mode = NRF_SAADC_MODE_SINGLE_ENDED;                                    //Set SAADC as single ended. This means it will only have the positive pin as input, and the negative pin is shorted to ground (0V) internally.
    channel_config.pin_p = NRF_SAADC_INPUT_VDD;                                          //Select the input pin for the channel. AIN0 pin maps to physical pin P0.02.
    channel_config.pin_n = NRF_SAADC_INPUT_DISABLED;                                      //Since the SAADC is single ended, the negative pin is disabled. The negative pin is shorted to ground internally.
    channel_config.resistor_p = NRF_SAADC_RESISTOR_DISABLED;                              //Disable pullup resistor on the input pin
    channel_config.resistor_n = NRF_SAADC_RESISTOR_DISABLED;                              //Disable pulldown resistor on the input pin

    //Sets saadc channel
    err_code = nrf_drv_saadc_channel_init(0, &channel_config);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[0],SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);
    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[1],SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);
}

I then print it out in the callback just like in the online example

/**@This is SAADC callback for battery reading.
 */
void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
{
    if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
    {
        ret_code_t err_code;				
        err_code = nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer, SAMPLES_IN_BUFFER);
        APP_ERROR_CHECK(err_code);
        printf("ADC event number: %d\r\n",(int)m_adc_evt_counter);
        for (int i = 0; i < SAMPLES_IN_BUFFER; i++)
        {
            printf("%d\r\n", p_event->data.done.p_buffer[i]);
        }
        m_adc_evt_counter++;

        //Uinint saadc
        nrf_drv_saadc_uninit();
        NRF_SAADC->INTENCLR = (SAADC_INTENCLR_END_Clear << SAADC_INTENCLR_END_Pos);
        NVIC_ClearPendingIRQ(SAADC_IRQn);
    }
}

However in the console I am seeing a figure of 200 give or take.

I thought I ought to be seeing a figure of

2.9 volts * (1 / 6 ) / 0.6 * 1024 = 825.

What am I missing?

Stefan Birnir Sverrisson · Accepted Answer

Hi Richie 
 I cant see what you are missing. I take the saadc_low_power example and configure it to sample on NRF_SAADC_INPUT_VDD with 10 bit resolution and I get value around 820. I cant see any other change in your configuration that you are showing. Try to sample the standard example and gradually make changes and then you will realize why you get the strange results.