SAADC Configuration

[Rohit Rajapure]

Hi All,

I am developing one project and there we are using ADC (i.e., SAADC in nRF). I have designed the hardware where I am using Capacitor ( C14, 2.2µF) which is connected to Analog pin and my requirement is I have to read the voltage level.

Here I am attaching details and image,

The voltage level of the input capacitor C14, 2.2µF / 400V to the Step-Down controller must be monitored. A voltage divider is used across the capacitor from R2, 3M3, and R3, 24k. The tapped voltage is measured at pin port P0.04 / AIN2 of the nRF52840. The expected voltage at the input of the controller is between 2.35V and 0V. The voltage at the capacitor must not fall below the value of 100V.

This is my requirement can anyone help how to configure SAADC with respect to my requirement.

Thanks in advance

Rohit R

[Rohit Rajapure]

Hi Karl,

Yes, I am able to read all resolution values.

8bit resolution is sufficient for me so I am going ahead with an 8bit resolution.

Thank you so much for your constant help.

Thanks and Regards

Rohit R

[Karl Ylvisaker]

Hello Rohit,

Rohit Rajapure said:

Yes, I am able to read all resolution values.

Great, I am glad to hear that! 

Rohit Rajapure said:

8bit resolution is sufficient for me so I am going ahead with an 8bit resolution.

All right, that is great! Please keep in mind that you might loose some LSB to noise in your measurements.
So, if you need 8 NOEB, then I would suggest that you use 10 bit resolution.

Rohit Rajapure said:

Thank you so much for your constant help.

It is no problem at all, I am happy to help!

Please do not hesitate to open a new ticket if you should encounter any issues or questions in the future.

Good luck with your development!

Best regards,
Karl

[Rohit Rajapure]

Hi Karl,

I need one help,

I am using saadc_sampling_trigger(); trigger the ADC samples. But I am getting 1 sample.

I have tried by changing the buffer size than my code not working.

What I need is, I want to read multiple samples when I trigger ADC?

Can you please help how to do that? below is my code let me know what I did wrong.

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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 1

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[1][SAMPLES_IN_BUFFER];
static nrf_ppi_channel_t     m_ppi_channel;
static uint32_t              m_adc_evt_counter;

bool boolEventHandled = true;

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;
        //ui16CurrentADCValue = p_event->data.done.p_buffer[0]; 
         
        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",p_event->data.done.p_buffer[i]);
        }
        m_adc_evt_counter++;

        boolEventHandled = true;
    }

}


void saadc_init(void)
{
        ret_code_t err_code;
    nrf_drv_saadc_config_t saadc_config;
    saadc_config.resolution = NRF_SAADC_RESOLUTION_8BIT;   //8 bit resolution
    saadc_config.interrupt_priority = APP_IRQ_PRIORITY_LOW;
    
    err_code = nrf_drv_saadc_init(&saadc_config, saadc_callback);
    APP_ERROR_CHECK(err_code);
    
   //singale shot and channel config 
   nrf_saadc_channel_config_t channel_config;
   channel_config.reference = NRF_SAADC_REFERENCE_INTERNAL;
   channel_config.gain = NRF_SAADC_GAIN1_4;
   channel_config.acq_time = NRF_SAADC_ACQTIME_10US;
   channel_config.mode = NRF_SAADC_MODE_SINGLE_ENDED;
   channel_config.pin_n = NRF_SAADC_INPUT_DISABLED; 
   channel_config.pin_p = NRF_SAADC_INPUT_AIN2;
   channel_config.resistor_n = NRF_SAADC_RESISTOR_DISABLED;
   channel_config.resistor_p = NRF_SAADC_RESISTOR_DISABLED;

    err_code = nrf_drv_saadc_channel_init(2, &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);
   
}

void saadc_sampling_trigger(void)
{
    ret_code_t err_code;
    err_code = nrf_drv_saadc_sample(); // Check error
    APP_ERROR_CHECK(err_code);
    boolEventHandled = false;
}

/**
 * @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();

    saadc_init();

    NRF_LOG_INFO("SAADC HAL simple example started.");


    while (1)
    {   
        if(boolEventHandled)
        {
          saadc_sampling_trigger();
        }
        nrf_delay_ms(1000);
        NRF_LOG_FLUSH();
    }
}


/** @} */

Thanks and Regards

Rohit R

[Karl Ylvisaker]

Hello Rohit R,

Rohit Rajapure said:

I am using saadc_sampling_trigger(); trigger the ADC samples. But I am getting 1 sample.

I have tried by changing the buffer size than my code not working.

The nrfx_saadc_sample() function triggers the START task or SAMPLE task register, depending on weather the driver is configured in low power mode or not. If you want to use a timer to trigger ADC sampling yo could trigger the task directly via PPI instead. This way you get precise timing, as it happens in HW and won't be delayed by a potential high priority interrupt.
If I have misunderstood you, and you would just like to get a single sample by calling a single function, you should instead use the nrfx_saadc_sample_convert function for this.

Rohit Rajapure said:

What I need is, I want to read multiple samples when I trigger ADC?

Have you seen the SAADC example from the SDK? It demonstrates how to set up the SAADC's sampling to trigger by a timer through PPI.
What are your requirements for the SAADC - do you intend to perform a sampling ( or multiple samples ? ) each second, have I understood this correct?
If you are doing a periodic sampling, I would highly recommend setting it to trigger by a timer through PPI, as demonstrated in the SDK example.

On a general note, I would also recommend that you do some power management in your main while loop, so as to reduce power consumption.
In most examples - including the SAADC peripheral example - you can see this implemented as the idle_state_handler function.
Currently, it seems that you are using a 1000 ms delay in the main loop to time your measurements - this is not recommended, as the delay NOP CPU instructions are wasted, and the delay function will not give you an exact delay, if you intend to add BLE communication to your project later on. 

Best regards,
Karl

[Rohit Rajapure]

Hi Karl,

Thanks for the feedback,

Karl Ylvisaker said:

What are your requirements for the SAADC - do you intend to perform a sampling ( or multiple samples ? ) each second, have I understood this correct?

- yes, I want multiple sample in around 6msec.

- I do not want to use timer or PPI channels, I need just a normal operation of SAADC operation.

- I had gone through the SDK example so with reference to that example I have modified my program without using timer or PPI.

- My requirement for SAADC previously was read single sample so I have configured and in attached code I am able to get that. But in my final project, requirement case is increased, means in 1 case I need single sample and in 2 case I need multiple samples (as in my first statement). So I want keeping same configuration how I can achieve my both requirement (case 1 and case 2).

Thanks and Regards

Rohit R