SAADC sampling rate

[Aftab]

Hello,

I'm using following code to read ADC values. The minimum sampling time I'm able to read the ADC reliably is 60ms. I want to lower it to say 5ms or 10ms.

Is there any thing I can change in the code to make this happen.

I can lower the sampling time in the nrf_drv_timer_ms_to_ticks(&m_timer, SAMPLING_TIME); function, but then, the ADC values are not reliable. Thanks.

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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 2
volatile uint8_t state = 1;
int16_t adc_buffer[2];
bool adc_conversion_complete=false;

static const nrf_drv_timer_t m_timer = NRF_DRV_TIMER_INSTANCE(1);
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, 250);  //original value
    uint32_t ticks = nrf_drv_timer_ms_to_ticks(&m_timer, 100);
    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_sampling_event_disable(void)
{
    ret_code_t err_code = nrf_drv_ppi_channel_disable(m_ppi_channel);

    APP_ERROR_CHECK(err_code);
}

void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
{
    //nrf_gpio_pin_toggle(12);

    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;
        //NRF_LOG_INFO("ADC event number: %d", (int)m_adc_evt_counter);

        
        for (i = 0; i < SAMPLES_IN_BUFFER; i++)
        {
            //NRF_LOG_INFO("%d: %d", i, p_event->data.done.p_buffer[i]);
            adc_buffer[i] = p_event->data.done.p_buffer[i];
        }
        m_adc_evt_counter++;

        // Indicate ADC conversion complete for continuity scan
        adc_conversion_complete = true;
    }
}


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);
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN2);
    nrf_saadc_channel_config_t channel_config_1 =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN3);

    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_channel_init(1, &channel_config_1);
    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);

}

uint16_t saadc_voltage_get(int ch_num){
    return adc_buffer[ch_num];  // Adjust this divider to get correct ADC value
}

// Where continuity is disabled
uint16_t saadc_battery_volts(){
    if (adc_conversion_complete){
        adc_conversion_complete = false;
        return ((saadc_voltage_get(0)*4)/17 + 1);
    }
    
}

[ovrebekk]

Hi 

It sounds a bit odd that you would be unable to use a faster sampling interval than 60ms. The ADC can sample much faster than this. 

Most likely the issue is related to the interrupt processing, which happens as soon as the ADC buffers fill up, but 5-10ms is not very fast for interrupts either. 

Do you have a lot of other activities in the application that could be delaying the processing of the interrupts?

One thing you can do to reduce the amounf of SAADC interrupts is to increase the size of the ADC buffer, set by the SAMPLES_IN_BUFFER define, so that the SAADC module will do more samples before the buffers fill up and you need to process them. 

Best regards
Torbjørn