RE: SAADC samples the same as ground value for LM35 sensor.

haakonsh — Tue, 28 May 2019 08:54:20 GMT

Then you've most likely blown the ESD diode on that pin since you've applied a voltage higher than VDD + 0.3V.

I suggest you try another analog input, this time you need to make sure that you're applying a voltage that's within the specification of the device.

RE: SAADC samples the same as ground value for LM35 sensor.

TrebleStick — Mon, 27 May 2019 14:45:30 GMT

It's 3V

RE: SAADC samples the same as ground value for LM35 sensor.

haakonsh — Mon, 27 May 2019 14:42:49 GMT

What voltage is VDD at?

You've probably exceeded the maximum rated voltage for the gpio, and consequently blown an ESD fuse.
See I/O pin voltage in Absolute maximum ratings.

RE: SAADC samples the same as ground value for LM35 sensor.

TrebleStick — Sun, 26 May 2019 12:13:16 GMT

Measuring a proper bench voltage source varying it from 0-5V produced some strange results. Regardless of the configuration of the SAADC channel, reading only changed when input voltage exceeded 3.65V. This either maxed out the reading or put it within a range (depending on the gain and reference voltage) that is usable to detect voltage change from 3.7V+. Any ideas why this might be the case? It sounds like a hardware issue to me.

I am using the development board so power is supplied by USB and taking the 0-5V rails to power the temperature sensor, it's a very simple sensor and the measured signal from that on both volt meter and oscilloscope comes out as expected.

RE: SAADC samples the same as ground value for LM35 sensor.

haakonsh — Thu, 23 May 2019 13:15:19 GMT

I suggest you use the SAADC to measure a proper voltage source first. This will tell you if there's noise in the SAADC or if it's in the temperature sensor's signal.

You should also check your power supply and make sure that you adhere to the reference design of the nRF52 and the temperature sensor's.

RE: SAADC samples the same as ground value for LM35 sensor.

TrebleStick — Wed, 22 May 2019 19:57:36 GMT

Hi, thank you for the suggestions:

1. I added the offset calibration to start up, the values coming through average at about 80 for the first reading and then return to the usual values around 1900. For calibrating every second degree Celsius change in temperature: Firstly if the ADC can't detect the temperature change because of the noise how would you be able to trigger such an event? Secondly would this mean uninitialising and reinitialising the ADC? Running calibration will not work unless the ADC is idle

2. This was already implemented before, forgot to mention.

3. I tried 16x oversampling onwards along side a resolution of 14-bits (as is suggested in the documentation) and there was no difference in results apart from the expected slow down as oversampling increased.

Thanks in advance :)

*EDIT* - code followed by debug prints below

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 *    Nordic Semiconductor ASA integrated circuit.
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/** @file
 * @defgroup nrf_adc_example main.c
 * @{
 * @ingroup nrf_adc_example
 * @brief ADC Example Application main file.
 *
 * This file contains the source code for a sample application using ADC.
 *
 * @image html example_board_setup_a.jpg "Use board setup A for this example."
 */

#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include "nrf.h"
#include "nrf_drv_saadc.h"
#include "nrf_drv_ppi.h"
#include "nrf_drv_timer.h"
#include "boards.h"
#include "app_error.h"
#include "nrf_delay.h"
#include "app_util_platform.h"
#include "nrf_pwr_mgmt.h"

#include "nrf_log.h"
#include "nrf_log_ctrl.h"
#include "nrf_log_default_backends.h"

#define SAMPLES_IN_BUFFER 5
volatile uint8_t state = 1;

static const nrf_drv_timer_t m_timer = NRF_DRV_TIMER_INSTANCE(0);
static nrf_saadc_value_t     m_buffer_pool[2][SAMPLES_IN_BUFFER];
static nrf_ppi_channel_t     m_ppi_channel;
static uint32_t              m_adc_evt_counter;


void timer_handler(nrf_timer_event_t event_type, void * p_context)
{

}


void saadc_sampling_event_init(void)
{
    ret_code_t err_code;

    err_code = nrf_drv_ppi_init();
    APP_ERROR_CHECK(err_code);

    nrf_drv_timer_config_t timer_cfg = NRF_DRV_TIMER_DEFAULT_CONFIG;
    timer_cfg.bit_width = NRF_TIMER_BIT_WIDTH_32;
    err_code = nrf_drv_timer_init(&m_timer, &timer_cfg, timer_handler);
    APP_ERROR_CHECK(err_code);

    /* setup m_timer for compare event every 400ms */
    uint32_t ticks = nrf_drv_timer_ms_to_ticks(&m_timer, 400);
    nrf_drv_timer_extended_compare(&m_timer,
                                   NRF_TIMER_CC_CHANNEL0,
                                   ticks,
                                   NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK,
                                   false);
    nrf_drv_timer_enable(&m_timer);

    uint32_t timer_compare_event_addr = nrf_drv_timer_compare_event_address_get(&m_timer,
                                                                                NRF_TIMER_CC_CHANNEL0);
    uint32_t saadc_sample_task_addr   = nrf_drv_saadc_sample_task_get();

    /* setup ppi channel so that timer compare event is triggering sample task in SAADC */
    err_code = nrf_drv_ppi_channel_alloc(&m_ppi_channel);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_ppi_channel_assign(m_ppi_channel,
                                          timer_compare_event_addr,
                                          saadc_sample_task_addr);
    APP_ERROR_CHECK(err_code);
}


void saadc_sampling_event_enable(void)
{
    ret_code_t err_code = nrf_drv_ppi_channel_enable(m_ppi_channel);

    APP_ERROR_CHECK(err_code);
}


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);

        int i;
        uint32_t data_average = 0;
        NRF_LOG_INFO("ADC event number: %d", (int)m_adc_evt_counter);
        SEGGER_RTT_printf(0, "ADC event: %d -> ", (int)m_adc_evt_counter); //swap for debug print

        for (i = 0; i < SAMPLES_IN_BUFFER; i++)
        {
            NRF_LOG_INFO("%d", p_event->data.done.p_buffer[i]);
            SEGGER_RTT_printf(0,"%d:", p_event->data.done.p_buffer[i]); //swap for debug print
            data_average += p_event->data.done.p_buffer[i];
        }
        //Produce average of the buffered data
        data_average = data_average/i+1;
        SEGGER_RTT_printf(0, " AVERAGE : %u\n", data_average);
        
        m_adc_evt_counter++;
        
    }
}


void saadc_init(void)
{
    ret_code_t err_code;
    nrf_saadc_channel_config_t channel_config =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0);

    err_code = nrf_drv_saadc_init(NULL, saadc_callback);
    APP_ERROR_CHECK(err_code);

    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);

}


/**
 * @brief Function for main application entry.
 */
int main(void)
{
    uint32_t err_code = NRF_LOG_INIT(NULL);
    APP_ERROR_CHECK(err_code);

    NRF_LOG_DEFAULT_BACKENDS_INIT();

    ret_code_t ret_code = nrf_pwr_mgmt_init();
    APP_ERROR_CHECK(ret_code);

    //calibrate on start up
    err_code = nrfx_saadc_calibrate_offset();
    SEGGER_RTT_printf(0, "Calibration Error Code %x\n", err_code);
    APP_ERROR_CHECK(err_code);

    saadc_init();
    saadc_sampling_event_init();
    saadc_sampling_event_enable();
    NRF_LOG_INFO("SAADC HAL simple example started.");
    
    while (1)
    {
        nrf_pwr_mgmt_run();
        NRF_LOG_FLUSH();
    }
}


/** @} */







//----------debug print out from above main----------------//
Calibration Error Code 0
ADC event: 0 -> -69:319:130:33:-8: AVERAGE : 82
ADC event: 1 -> 16:1849:1847:1845:1848: AVERAGE : 1482
ADC event: 2 -> 1836:1858:1851:1852:1858: AVERAGE : 1852
ADC event: 3 -> 1852:1842:1843:1840:1841: AVERAGE : 1844
ADC event: 4 -> 1853:1849:1852:1841:1843: AVERAGE : 1848
ADC event: 5 -> 1846:1852:1837:1845:1845: AVERAGE : 1846
ADC event: 6 -> 1838:1845:1842:1847:1845: AVERAGE : 1844
ADC event: 7 -> 1853:1839:1836:1848:1850: AVERAGE : 1846
ADC event: 8 -> 1846:1838:1844:1852:1857: AVERAGE : 1848
ADC event: 9 -> 1856:1850:1859:1855:1854: AVERAGE : 1855
ADC event: 10 -> 1867:1860:1858:1854:1857: AVERAGE : 1860
ADC event: 11 -> 1849:1858:1861:1845:1842: AVERAGE : 1852

RE: SAADC samples the same as ground value for LM35 sensor.

haakonsh — Wed, 22 May 2019 07:43:00 GMT

You should do three things:

Run offset calibration on start-up, and every second degree Celsius change in temperature.
Ensure that you've configured the SAADC to sample in the correct operating range of your signal. I suggest you try a gain of 1, and use the internal 0.6V reference.
Enable oversampling of your measurement, at least 10X.