SAADC does not sample

RE: SAADC does not sample

Turbo J — Mon, 05 Aug 2019 14:09:02 GMT

LOL: This is an example on how to not use a pointer variable.

This is how it is supposed to look:

static volatile int16_t nrf_adc_result;

int16_t readResult(){
        NRF_SAADC->RESULT.PTR = (uint32_t)&nrf_adc_result;

        nrf_saadc_task_trigger(NRF_SAADC_TASK_START);
        while (nrf_saadc_event_check(NRF_SAADC_EVENT_STARTED) == 0);
        nrf_saadc_task_trigger(NRF_SAADC_TASK_SAMPLE);
        while (nrf_saadc_event_check(NRF_SAADC_EVENT_END) == 0);
        nrf_saadc_event_clear(NRF_SAADC_EVENT_END);
        if (nrf_adc_result <0)
        {
            nrf_adc_result = 0;
        }

      return nrf_adc_result;
}

Note that you can eliminate the pointer variable altogether here.

I also notices that I was mistaken above: You need to read at least two books: One for basic C programming, one for programming C on a microcontroller like the ARM Cortex-M variants. The latter tells you about interrupts and the use of volatile among many other details.

RE: SAADC does not sample

eBrock — Mon, 05 Aug 2019 12:59:04 GMT

Your explanation was useful. Your insult was not.

RE: SAADC does not sample

Turbo J — Thu, 01 Aug 2019 20:55:51 GMT

int16_t * adcResult = 0;

That points to flash address zero, which is part of the MBR and countains the lower half of the inital stack pointer.

You should point it to something in RAM like:

static int16_t nrf_adc_result = 0;
int16_t * adcResult = &nrf_adc_result;

Also, get a book about the C programming language and read it. This is a very basic beginner question, and C cannot be learned properly just from forum posts.

RE: SAADC does not sample

eBrock — Thu, 01 Aug 2019 18:32:22 GMT

What exactly looks like a NULL pointer?

RE: SAADC does not sample

Turbo J — Thu, 01 Aug 2019 18:13:02 GMT

That looks a lot like a NULL pointer to me.

Hopefully OP set adcResult pointer to a more sensible RAM location in code not shown here.

RE: SAADC does not sample

eBrock — Thu, 01 Aug 2019 18:03:30 GMT

OK, I have figured this out. My code had several problems. The first problem, which made it very difficult to debug,

was that the SAMPLE task only executes once and then apparently the SAADC stops. So, when I want to sample

again, I have to start the SAADC and then I can start the SAMPLE task. The second problem is that I needed to configure

the SAADC result address as &adcResult. The third problem is that I was assuming the placement of the 8-bit value

and the sign bit. Both were wrong. There is missing information from the nRF52832 data sheet. Even for 8-bit value,

the sign bit is bit 15 of the result. Also, when configured for an 8-bit value, the SAADC result is not little endian.

Here are the changes I made to the code:

NRF_SAADC->RESULT.PTR = (uint32_t)&adcResult;

        nrf_saadc_task_trigger(NRF_SAADC_TASK_START);
        while (nrf_saadc_event_check(NRF_SAADC_EVENT_STARTED) == 0);
        nrf_saadc_task_trigger(NRF_SAADC_TASK_SAMPLE);
        while (nrf_saadc_event_check(NRF_SAADC_EVENT_END) == 0);
        nrf_saadc_event_clear(NRF_SAADC_EVENT_END);
        nrf_adc_result = (uint16_t)adcResult;
        if (nrf_adc_result >> 15 == 1)
        {
            nrf_adc_result = 0;
        }

RE: SAADC does not sample

eBrock — Thu, 01 Aug 2019 13:05:49 GMT

I forgot to include other details...

I'm using SES for IDE and the nRF52 Dev board. I've already tried many variations of pointer declaration just to see if it would effect the result.

I've looked at the SDK example as well as other examples and I think my code should work.

Here is the definition code:

#include <stdbool.h>
#include <stdint.h>
#include <string.h>
#include "sdk_common.h"
#include "nrf52.h"
#include "nrf_delay.h"
#include "nrf_gpio.h"
#include "nrf_saadc.h"

#define LED_1           17                      // I/O for LED 1 = P0.17
#define LED_2           18                      // I/O for LED 2 = P0.18
#define LED_3           19                      // I/O for LED 3 = P0.19
#define LED_4           20                      // I/O for LED 4 = P0.20

static uint16_t nrf_adc_result = 0;
int16_t * adcResult = 0;

RE: SAADC does not sample

Turbo J — Wed, 31 Jul 2019 22:42:03 GMT

Missing defintition of "adcResult", which is likely used wrong here.

Note that there is SAADC example code in the SDK.