NRF9160 DK ADC long sample times

[nordic_matt]

I'm working with the NRF9160 DK and have found the sample times for the ADC to be abnormally long. I am sampling three channels and seeing a total time for the ADC read of 80+us when I'm expecting something on the order of 15us. I have experimented with many different settings and configurations and not found any way to improve it. 

Here is my configuration

#include <hal/nrf_saadc.h>
#define ADC_DEVICE_NAME DT_ADC_0_NAME
#define ADC_RESOLUTION 10
#define ADC_GAIN ADC_GAIN_1_6
#define ADC_REFERENCE ADC_REF_INTERNAL
#define ADC_ACQUISITION_TIME ADC_ACQ_TIME(ADC_ACQ_TIME_MICROSECONDS, 3)
#define ADC_1ST_CHANNEL_ID 0
#define ADC_1ST_CHANNEL_INPUT NRF_SAADC_INPUT_AIN0
#define ADC_2ND_CHANNEL_ID 2
#define ADC_2ND_CHANNEL_INPUT NRF_SAADC_INPUT_AIN2
#define ADC_3RD_CHANNEL_ID 3
#define ADC_3RD_CHANNEL_INPUT NRF_SAADC_INPUT_AIN3

static const struct adc_channel_cfg m_1st_channel_cfg = {
.gain = ADC_GAIN,
.reference = ADC_REFERENCE,
.acquisition_time = NRF_SAADC_ACQTIME_3US,
.channel_id = ADC_1ST_CHANNEL_ID,
#if defined(CONFIG_ADC_CONFIGURABLE_INPUTS)
.input_positive = ADC_1ST_CHANNEL_INPUT,
#endif
};

static const struct adc_channel_cfg m_2nd_channel_cfg = {
.gain = ADC_GAIN,
.reference = ADC_REFERENCE,
.acquisition_time = NRF_SAADC_ACQTIME_3US,
.channel_id = ADC_2ND_CHANNEL_ID,
#if defined(CONFIG_ADC_CONFIGURABLE_INPUTS)
.input_positive = ADC_2ND_CHANNEL_INPUT,
#endif
};

static const struct adc_channel_cfg m_3rd_channel_cfg = {
.gain = ADC_GAIN,
.reference = ADC_REFERENCE,
.acquisition_time = NRF_SAADC_ACQTIME_3US,
.channel_id = ADC_3RD_CHANNEL_ID,
#if defined(CONFIG_ADC_CONFIGURABLE_INPUTS)
.input_positive = ADC_3RD_CHANNEL_INPUT,
#endif
};

Here is the channel set up:

adc_dev = device_get_binding("ADC_0");
if (!adc_dev) {
printk("device_get_binding ADC_0 failed\n");
}
err = adc_channel_setup(adc_dev, &m_1st_channel_cfg);
if (err) {
printk("Error in adc setup: %d\n", err);
}

err = adc_channel_setup(adc_dev, &m_2nd_channel_cfg);
if (err) {
printk("Error in adc setup: %d\n", err);
}

err = adc_channel_setup(adc_dev, &m_3rd_channel_cfg);
if (err) {
printk("Error in adc setup: %d\n", err);
}

And here is where I'm sampling:

static int adc_sample(void)
{
int ret;

const struct adc_sequence_options options = {
.interval_us = 0,
.extra_samplings = 0,

};

const struct adc_sequence sequence = {
.channels = BIT(ADC_1ST_CHANNEL_ID) | BIT(ADC_2ND_CHANNEL_ID) | BIT(ADC_3RD_CHANNEL_ID),
.buffer = m_sample_buffer,
.buffer_size = sizeof(m_sample_buffer),
.resolution = ADC_RESOLUTION,
.oversampling = 0,
.calibrate = false,
.options = &options,
};

if (!adc_dev) {
return -1;
}
gpio_pin_write(gpio_dev, 10, 1);
ret = adc_read(adc_dev, &sequence);
gpio_pin_write(gpio_dev, 10, 0);

/*printk("ADC read err: %d\n", ret);

/* Print the AIN0 values */
/*for (int i = 0; i < BUFFER_SIZE; i++) {
float adc_voltage = 0;
adc_voltage = (float)(((float)m_sample_buffer[i] / 1023.0f) *
3600.0f);
printk("ADC raw value: %d\n", m_sample_buffer[i]);
printf("Measured voltage: %f mV\n", adc_voltage);
}
*/
return ret;
}

[Håkon Alseth]

Hi,

 

I can confirm that there is overhead in the zephyr API that causes the SAADC to run a bit slower. There's semaphores for locking and unlocking in the adc driver (adc_nrfx_saadc.c), which goes to sleep (or yields), and it takes a bit of time to wake up on events and run the code as well. I see approx. 35 us overhead on a single measurement.

20 us acq = 56 us

10 us acq = 46 us

3 us acq = 41 us

 

Note that it also takes some microseconds for the conversion to be available as well.

 

Kind regards,

Håkon

[nordic_matt]

So, the best approach is to avoid using Zephyr for time critical measurements? Direct register addressing?  Do you have some example code you could post?

[Håkon Alseth]

nordic_matt said:

So, the best approach is to avoid using Zephyr for time critical measurements?

Everything has its use-case. If you require a higher sampling rate, I would recommend using a timer + PPI to trigger the SAADC. By including the nrfx_saadc.h (and ppi) into your project, you should be able to tie these together. If you search for "saadc ppi" (or other variations) on this forum, you should find snippets on how to set it up.

 

Kind regards,

Håkon

[nordic_matt]

I tried working with nrfx_saadc.h and including it and had lots of trouble. I'm not sure how to get it to build with SES and for the build to find the file. I get errors like:

1> C:\Nordic\ncs\nrf\samples\nrf9160\adc\build_nrf9160_pca10090ns/../src/main.c:68: undefined reference to `nrfx_saadc_init'

I tried going through the Zephyr menuconfig to get it to find the nrfx_saadc.h file but to no avail. Searching around I found plenty of examples of how to work with the old SDK but since this is 9160, they need modification. I also found this example: 

https://devzone.nordicsemi.com/nordic/cellular-iot-guides/b/getting-started-cellular/posts/nrf-connect-sdk-tutorial

It is using 

#include <hal/nrf_saadc.h>

So, the question is, What is the right way to go about this? I see that nrfx_saadc.h includes hal/nrf_saadc.h. But then I see in the Zephyr structure that it is including adc_nrfx_saadc.h. I'm new to this whole Zephyr structure and want to work with best practices. Could you provide a little guidance?

[Håkon Alseth]

If you keep the project "as-is", meaning that all the zephyr related ADC drivers are enabled, then all the nrfx-saadc components are also enabled (sources + headers). Remove the zephyr adc driver functions in your application, then include nrfx_saadc.h in your main.c file. This should then not give you any undefined references.

If you want to use the event handler, you have to add the interrupt wrapper for the OS, as done here for the PDM:

https://devzone.nordicsemi.com/f/nordic-q-a/51164/pdm-hardware-exception/204916#204916

 

Its not very easy to use nrfx directly, and this is something that I'm reporting internally so that we can improve this in the future.

 

Kind regards,

Håkon

[nordic_matt]

I'm not sure why I'm getting undefined references. Here is my main.c file. Maybe there's something obvious? 

/*
 * Copyright (c) 2018 Nordic Semiconductor ASA
 *
 * SPDX-License-Identifier: LicenseRef-BSD-5-Clause-Nordic
 */

#include <gpio.h>
#include <net/socket.h>
#include <nrf9160.h>
#include <stdio.h>
#include <string.h>
#include <uart.h>
#include <adc.h>
#include <zephyr.h>
#include <nrfx.h>
#include <nrfx_saadc.h>
//#include <hal/nrf_saadc.h>
//#include <nrfx_dppi.h> 


struct device *gpio_dev;

#define SAADC_DEFAULT_CONFIG                                               \
{                                                                               \
    .resolution         = (nrf_saadc_resolution_t)NRFX_SAADC_CONFIG_RESOLUTION, \
    .oversample         = (nrf_saadc_oversample_t)NRFX_SAADC_CONFIG_OVERSAMPLE, \
    .interrupt_priority = NRFX_SAADC_CONFIG_IRQ_PRIORITY,                       \
    .low_power_mode     = NRFX_SAADC_CONFIG_LP_MODE                             \
}

#define SAADC_DEFAULT_CHANNEL_CONFIG_SE(PIN_P) \
{                                                   \
    .resistor_p = NRF_SAADC_RESISTOR_DISABLED,      \
    .resistor_n = NRF_SAADC_RESISTOR_DISABLED,      \
    .gain       = NRF_SAADC_GAIN1_6,                \
    .reference  = NRF_SAADC_REFERENCE_INTERNAL,     \
    .acq_time   = NRF_SAADC_ACQTIME_3US,           \
    .mode       = NRF_SAADC_MODE_SINGLE_ENDED,      \
    .burst      = NRF_SAADC_BURST_DISABLED,         \
    .pin_p      = (nrf_saadc_input_t)(PIN_P),       \
    .pin_n      = NRF_SAADC_INPUT_DISABLED          \
}

nrf_saadc_channel_config_t saadc_channel_config = SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0);
static bool adcCalibrateDone;


#define BUFFER_SIZE 3
static nrf_saadc_value_t m_sample_buffer[BUFFER_SIZE];

static void saadc_callback(nrfx_saadc_evt_t const * p_event){

    if ( p_event->type == NRFX_SAADC_EVT_DONE )
    {
       printk("ADC raw value: %d\n", m_sample_buffer[0]);
    }
    else if( p_event->type == NRFX_SAADC_EVT_CALIBRATEDONE )
    {
      adcCalibrateDone = true;
    }
}

void InitAdcModule(void)
{
    nrfx_err_t err_code;
    nrfx_saadc_config_t drv_saadc_cfg = SAADC_DEFAULT_CONFIG;
		
    err_code = nrfx_saadc_init( &drv_saadc_cfg, saadc_callback );
    if(err_code != NRFX_SUCCESS){
        //Blah error
    }
	
    adcCalibrateDone = false;
    err_code = nrfx_saadc_calibrate_offset( );
    if(err_code != NRFX_SUCCESS){
        //Blah error
    }
	
    if( err_code == NRFX_SUCCESS )
    {
	err_code = 0;
	while( !adcCalibrateDone );  
    }
	
}

//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);
//}

static int adc_sample(void)
{
	int ret;


	/*printk("ADC read err: %d\n", ret);

	/* Print the AIN0 values */
	/*for (int i = 0; i < BUFFER_SIZE; i++) {
		float adc_voltage = 0;
		adc_voltage = (float)(((float)m_sample_buffer[i] / 1023.0f) *
				      3600.0f);
		printk("ADC raw value: %d\n", m_sample_buffer[i]);
		printf("Measured voltage: %f mV\n", adc_voltage);
	}
*/
	return ret;
}


static inline void nrf_delay_us(u32_t microsec)
{
	NRF_TIMER1_NS->TASKS_CLEAR = 1;
	if (microsec < 2 )
		return;
	NRF_TIMER1_NS->CC[0] = (microsec << 4) - 26;
	NRF_TIMER1_NS->PRESCALER = 0;
	NRF_TIMER1_NS->TASKS_START = 1;
	while (NRF_TIMER1_NS->EVENTS_COMPARE[0] == 0)
		;
	NRF_TIMER1_NS->EVENTS_COMPARE[0] = 0;
	NRF_TIMER1_NS->TASKS_STOP = 1;	
}

int main(void)
{
	nrfx_err_t err;
        nrf_saadc_value_t  adc_value;

	printk("nrf91 saadc sampling AIN0 (P0.13)\n");
	printk("Example requires secure_boot to have ");
	printk("SAADC set to non-secure!\n");
	printk("If not; BusFault/UsageFault will be triggered\n");

//        gpio_dev = device_get_binding(DT_GPIO_P0_DEV_NAME);
//
//        if (!gpio_dev) {
//		printk("Cannot bind gpio device");
//		return -ENODEV;
//	}
//
//        err = gpio_pin_configure(gpio_dev, 10, GPIO_DIR_OUT);//Set pin 0 to output


        //Startup the high frequency clock
        NRF_CLOCK_NS->TASKS_HFCLKSTART = 1;
	while (NRF_CLOCK_NS->EVENTS_HFCLKSTARTED == 0);

        //Configure the timer
//        NRF_TIMER1_NS->TASKS_CLEAR = 1;
//	NRF_TIMER1_NS->PRESCALER = 0;//Run at 16MHz

        InitAdcModule();

	while (1) {

		err = nrfx_saadc_sample();

		if (err) {
			printk("Error in adc sampling: %d\n", err);
		}
		k_sleep(500);

	}
}

Maybe it's my prj.conf? I started with the asset tracker application and haven't changed the prj.conf much? Maybe I should just start over?

CONFIG_BSD_LIBRARY=y
CONFIG_GPIO=y
CONFIG_SERIAL=y
CONFIG_STDOUT_CONSOLE=y
CONFIG_UART_INTERRUPT_DRIVEN=y
CONFIG_TEST_RANDOM_GENERATOR=y
CONFIG_NETWORKING=y
CONFIG_NET_BUF_USER_DATA_SIZE=1
CONFIG_NET_SOCKETS_OFFLOAD=y
CONFIG_NET_SOCKETS=y
CONFIG_NET_SOCKETS_POSIX_NAMES=y
CONFIG_NET_RAW_MODE=y
CONFIG_TRUSTED_EXECUTION_NONSECURE=y
CONFIG_LOG=n
CONFIG_LOG_DEFAULT_LEVEL=4
CONFIG_HEAP_MEM_POOL_SIZE=1024
CONFIG_LTE_LINK_CONTROL=n
CONFIG_ADC=y
CONFIG_ADC_0=y
CONFIG_ADC_NRFX_SAADC=y


CONFIG_MAIN_STACK_SIZE=4096