SAADC sample value range on nRF52 dev kit

[Tim]

Hello ...

I'm porting our product from RF Digital's Simblee to Laird's BL651 (nRF52810). A phototransistor measures light, is connected to an analog pin, and is sampled occasionally. The circuit also has a comparator, and GPIOTE and PPI are used to detect LoToHi transition and call an interrupt routine.

I'm having trouble understanding how to configure SAADC to obtain appropriate analog readings of the phototransistor value. Simblee used the Arduino IDE and hid much of the complexities of analog-to-digital conversion. The only code necessary was:

pinMode(photoTransistorAnalogPin, INPUT);
analogValue = analogRead(photoTransistorAnalogPin);

The range of analog values we'd get on our Simblee boards was roughly 25-952. When I hook up our phototransistor circuit to the nRF52 dev kit and use SAADC to sample the analog value, the range I get (same light input range) is roughly 178-753. Here's the code (SAADC is also used to sample battery level):

#define SAMPLES_IN_SAADC_BUFFER 2
static nrf_saadc_value_t adc_buf[SAMPLES_IN_SAADC_BUFFER];

void saadc_event_handler(nrfx_saadc_evt_t const * p_event)
{
    if (p_event->type == NRFX_SAADC_EVT_DONE)
    {
        uint32_t          err_code;
        err_code = nrfx_saadc_buffer_convert(p_event->data.done.p_buffer, SAMPLES_IN_SAADC_BUFFER);
        APP_ERROR_CHECK(err_code);

        battery_level = p_event->data.done.p_buffer[0];
        analog_level = p_event->data.done.p_buffer[1];

	    .
	    .
	    .

    }
}

static void saadc_configure(void)
{
    nrfx_saadc_config_t saadc_config = NRFX_SAADC_DEFAULT_CONFIG;
    ret_code_t err_code = nrfx_saadc_init(&saadc_config, saadc_event_handler);
    APP_ERROR_CHECK(err_code);


    nrf_saadc_channel_config_t channel0_config = NRFX_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_VDD);
    nrf_saadc_channel_config_t channel1_config = NRFX_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN1);

    err_code = nrfx_saadc_channel_init(0, &channel0_config);
    APP_ERROR_CHECK(err_code);
    err_code = nrfx_saadc_channel_init(1, &channel1_config);
    APP_ERROR_CHECK(err_code);

    err_code = nrfx_saadc_buffer_convert(adc_buf, SAMPLES_IN_SAADC_BUFFER);
    APP_ERROR_CHECK(err_code);
}

Later when user requests battery level or analog value:

ret_code_t err_code;
err_code = nrfx_saadc_sample();
APP_ERROR_CHECK(err_code);

Even with the phototransistor completely removed from the circuit, I still get an analog value of 164.

I'm new to SAADC. Appreciate any guidance.

Thanks,

Tim

[Jared]

Hi,

The value that you get is dependent on the specific ADC that you're using. You can therefore expect to get a different value when you use the nRF. However, you should have a value that varies accordingly to what is connected to the SAADC input so something is not correct. The code that you posted seems correct, so i'm guessing that you're not using the correct pin. See this case.

regards

Jared 

[Tim]

Thanks Jared. I mistakenly had the analog input configured for internal pull-up resistor. Change to:

nrf_gpio_cfg_input(PHOTOTRANSISTOR_ANALOG_PIN,NRF_GPIO_PIN_NOPULL);

Now I'm getting correct analog values.

Tim