nRF5 with input-multiplexer on ADC pin, optimization required

Question

Hey, I am trying to sample from 10 analog sensors and stream the results via bluetooth. Since the nRF5 is on a board that only lets me access 5 Pins, I am using a 16:1 multiplexer connected to a single ADC pin and 4 digital-out pins to control the multiplexer (channel selection).

I changed the Blinky Example to set the digital-out pins and I added code from the ADC Example to read ADC values and send them via UART.

It works so far, but I am not sure if I am doing it correctly since there are so many different ways to make timing, adc,... work. Here is my code:

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

// GENERATED CODE (Digital-out values for MUX channel selection)
const uint8_t PINS[] = {5,4,3,13};
const uint8_t NUM_PINS = 4;
const uint8_t NUM_CHANNELS = 10;

const uint32_t SET_MAP[] = {
0x0,
0x20,
0x10,
0x30,
0x8,
0x28,
0x18,
0x38,
0x2000,
0x2020
};

const uint32_t CLR_MAP[] = {
0x2038,
0x2018,
0x2028,
0x2008,
0x2030,
0x2010,
0x2020,
0x2000,
0x38,
0x18
};
// END GENERATED CODE


// ADC Setup:
// Note: ADC_BUFFER_SIZE has to be equal to NUM_CHANNELS!
#define ADC_BUFFER_SIZE 10                                /**< 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("ADC: %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)
{
	// GPIO:
	// Configure pins as outputs.
	for (int i = 0; i < NUM_PINS; i++)
	{
		nrf_gpio_cfg_output(PINS[i]);
	}

	// ADC:
	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 (true)
    {
		APP_ERROR_CHECK(nrf_drv_adc_buffer_convert(adc_buffer,ADC_BUFFER_SIZE));
				
        for (int i = 0; i < NUM_CHANNELS; i++)
		{
			// Set pins for mux channel selection:
			nrf_gpio_pins_clear(CLR_MAP[i]);
			nrf_gpio_pins_set(SET_MAP[i]);
			// sample from adc;
			nrf_drv_adc_sample();
			
            __SEV();
            __WFE();
            __WFE();

            nrf_delay_ms(10);
            LEDS_INVERT(BSP_LED_0_MASK);
		}
		nrf_delay_ms(1000);
    }
}

It doesnt have to be perfect, but I would like to keep power consumption low and I need stable sampling at around 100hz (10 channels every 10ms). Also I will need to replace the UART with a Bluetooth stream and I need to switch from a nRF51 DK to a nRF52832 (which is a bit different about ADC/SAADC but I dont think its a huge problem).

Am I using the SEV(),WFE(),WFE() correctly?
Do I need a delay between the sampling of the 10 channels (in the code currently 10ms)
Will this approach still work when I add Bluetooth or are there going to be problems with Sleeping/Timing etc.?

Best regards Johannes

Jørgen Holmefjord · Accepted Answer

Hi, 
 
 Yes, this is how the examples in the SDK enters sleep mode. 
 A call to nrf_drv_adc_sample() will assert if the ADC is busy. You should rather check if the ADC is available using nrf_drv_adc_is_busy() before starting sampling. 
 I would not recommend using busy-wait in the main loop for ADC sampling. You should setup a timer (use RTC / App timer for low power), and trigger the ADC sampling in the callback handler. As you mention, the ADC peripheral on the nRF52832 is different from nRF51, and it use a different driver ( nrf_drv_saadc ). You can find a couple of examples using the SAADC on our GitHub . It should not be a problem to get the sample rate you desire along BLE communication. 
 
 Best regards, 
 Jørgen