Do I need to define an Analog input as such in NRF52?

[ndarkness]

Hello,

I was wondering whether or not the standard analog pins on the nrf52832 have to be defined as analog inputs with the gpio methods.

I ask this because I am trying to sample some of those pins (NRF_SAADC_INPUT_AIN4,P0.31 on pca10040 DK for instance) with the SAADC and I don't get the value that I would expect.

My init for the SAADC looks like this

#define SAADC_BUF_SIZE                 (2)                                 
#define SAADC_CHANNELS                 (4)
#define SAADC_INITIAL_CHANNEL          (4) 
nrf_saadc_value_t            bufferSAADC[SAADC_CHANNELS];

//Temperature sensor 4
temperatureReadings[0].adc_pin_no = NRF_SAADC_INPUT_AIN4; // SAADC channel 4

//Temperature sensor 3
temperatureReadings[1].adc_pin_no = NRF_SAADC_INPUT_AIN5; // SAADC channel 5

//Temperature sensor 2
temperatureReadings[2].adc_pin_no = NRF_SAADC_INPUT_AIN6; // SAADC channel 6

//Temperature sensor 1
temperatureReadings[3].adc_pin_no = NRF_SAADC_INPUT_AIN7; // SAADC channel 7

uint32_t saadc_start(temp_meas_param_t tempSensors[])
{
    uint32_t err_code;


    nrf_drv_saadc_config_t saadc_config = NRF_DRV_SAADC_DEFAULT_CONFIG;

    err_code = nrf_drv_saadc_init(&saadc_config, saadc_handler_interrupt);
    APP_ERROR_CHECK(err_code);

    // we calibrate the SAADC before configuring it
    err_code = saadc_calibrate(saadc_config);
    APP_ERROR_CHECK(err_code);

      
    for(uint8_t temperaruteChannel=SAADC_INITIAL_CHANNEL; temperaruteChannel< SAADC_INITIAL_CHANNEL + SAADC_CHANNELS; temperaruteChannel++)
    {
      nrf_saadc_channel_config_t channel_config = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(tempSensors[temperaruteChannel].adc_pin_no);

      channel_config.burst    = NRF_SAADC_BURST_ENABLED;
      channel_config.gain     = SAADC_GAIN1_6;
//      channel_config.acq_time = SAADC_CH_CONFIG_TACQ_10us;

      err_code = nrf_drv_saadc_channel_init(temperaruteChannel, &channel_config);
      APP_ERROR_CHECK(err_code);    
    }

    err_code = nrf_drv_saadc_buffer_convert((nrf_saadc_value_t*)bufferSAADC, SAADC_BUF_SIZE);
    APP_ERROR_CHECK(err_code);

    saadc_is_initialized = true;

    return M_TEMP_STATUS_CODE_SUCCESS;
}

I am using as input for the analog device one of the VDD pins and on of the GND pins of the pca10040DK.

Am I setting something wrong on for the SAADC?

thanks in advance

[Matt]

I think the analog input pins are "fixed" and cannot be remapped to other pins; need Nordic experts to confirm on this one.

[Jørgen Holmefjord]

Hi,

You do not need to configure the pins as analog other than selecting the correct pin in for the SAADC pin configuration registers. The analog input will be configured automatically in the GPIO when it is used for analog peripheral, as described in the GPIO documentation.

Please check the pin assignment documentation of the nRF52832. AIN4 is not located at P0.31, it is located on P0.28. Could this cause the issue you are seeing? If not, please describe the values you are seeing, and what you expect.

Best regards,

Jørgen

[ndarkness]

AIN4 is not located at P0.31, it is located on P0.28. Could this cause the issue you are seeing?I think that was the cause of the issue, I was looking into the analog pins of the nrf52832 DK 10040 labelled as Ax, and there A4,which I asssumed to be the AIN4, is mapped to P0.30. Thanks

[Jørgen Holmefjord]

The Ax/Dx marking of the PCA10040 correlates to the Arduino form-factor pin naming.