Simplest way to read an ADC input

[leron]

Dear All, I would like a simple way to read the voltage on one ADC pin without using any "adc_event_handler"-like solution. I tried

reading=nrf_adc_convert_single(NRF_ADC_CONFIG_INPUT_2);

but without success.

Thanks

Francesco

[awneil]

The function does not check if the ADC is initialized and powered

See: developer.nordicsemi.com/.../a00754.html

So are you sure that you have correctly initialized and powered the ADC?

[leron]

Let me explain better. This is my main. It is basically the adc example where I tried not to use the interrupts or the handler to have a reading.

#include "nrf.h"
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include "nrf_drv_adc.h"
#include "nordic_common.h"
#include "boards.h"
#include "nrf_log.h"
#include "app_error.h"
#include "nrf_delay.h"
#include "app_util_platform.h"

#define ADC_BUFFER_SIZE 1 /**< Size of buffer for ADC samples. */

static nrf_adc_value_t adc_buffer[ADC_BUFFER_SIZE]; /* ADC buffer. */ 
static nrf_drv_adc_channel_t m_channel_config = NRF_DRV_ADC_DEFAULT_CHANNEL(NRF_ADC_CONFIG_INPUT_2); /* Channel instance. Default configuration used. */

static void adc_event_handler(nrf_drv_adc_evt_t const * p_event) 
{ 
// if (p_event->type == NRF_DRV_ADC_EVT_DONE) 
// { // uint32_t i; 
// for (i = 0; i < p_event->data.done.size; i++) 
// { 
// NRF_LOG_PRINTF("Current sample value: %d\r\n", p_event->data.done.p_buffer[i]); // 
// } 
// } 
}

static void adc_config(void) 
{ 
ret_code_t ret_code; 
nrf_drv_adc_config_t config = NRF_DRV_ADC_DEFAULT_CONFIG;
ret_code = nrf_drv_adc_init(&config, adc_event_handler);
    APP_ERROR_CHECK(ret_code);
nrf_drv_adc_channel_enable(&m_channel_config);
}

int main(void)
{ 
nrf_adc_value_t myvalue; 
LEDS_CONFIGURE(BSP_LED_0_MASK); 
LEDS_OFF(BSP_LED_0_MASK);
adc_config();
UNUSED_RETURN_VALUE(NRF_LOG_INIT());

NRF_LOG_PRINTF("ADC example\r\n");

while(1)
{
APP_ERROR_CHECK(nrf_drv_adc_buffer_convert(adc_buffer,ADC_BUFFER_SIZE));
nrf_drv_adc_sample();
nrf_delay_ms(100);
LEDS_INVERT(BSP_LED_0_MASK);\
myvalue=nrf_adc_convert_single(NRF_ADC_CONFIG_INPUT_2);
NRF_LOG_PRINTF("Currently my value: %d\r\n",myvalue);
}

}

I would like to know more details about:

1. "APP_ERROR_CHECK(nrf_drv_adc_buffer_convert(adc_buffer,ADC_BUFFER_SIZE));": without this it does not work

2: if I understand properly first a measurement is triggered with nrf_drv_adc_sample() while nrf_adc_convert_single makes accessible the first of the measurements. Am I correct?

Thanks

[Sigurd]

First of all, event-driven ADC is recommended, so that the CPU can be turned off during inactive periods. But if you don’t want that, here is a very simple way to read the ADC without using any events, using the nrf_drv_adc_sample_convert function:

#include "nrf.h"
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include "nrf_drv_adc.h"
#include "nordic_common.h"
#include "boards.h"
#include "nrf_log.h"
#include "app_error.h"
#include "nrf_delay.h"
#include "app_util_platform.h"


static nrf_adc_value_t       adc_value; /**< ADC buffer. */
static nrf_drv_adc_channel_t m_channel_config = NRF_DRV_ADC_DEFAULT_CHANNEL(NRF_ADC_CONFIG_INPUT_2); 

/**
 * @brief Function for main application entry.
 */
int main(void)
{
    UNUSED_RETURN_VALUE(NRF_LOG_INIT());
    NRF_LOG_PRINTF("ADC example\r\n");

    while (true)
    {
        nrf_drv_adc_sample_convert(&m_channel_config,&adc_value);
        NRF_LOG_PRINTF("ADC buffer: %d\r\n",adc_value);
        nrf_delay_ms(100);
    }
}
/** @} */

[leron]

Thanks Sigurd, how long does it take to have a measurement ready? Can I shorten the nrf_delay_ms() to 10ms (of course after removing the UART)?

[Sigurd]

How long time before the measurement is ready depend on what bit mode you use on the ADC. The default configuration uses 10-bit, and takes 68 μs to sample. If you use 8-bit mode it takes 20 μs (50K samples per second). The nrf_delay_ms() is just there to not overflow the uart.