Continuous periodic nRF52840 ADC sampling with Zephyr

[bojan]

Hello, guys.

We are using Zephyr RTOS (NCS v1.4.2) with nRF52840 development kit.

The idea is to have continuous ADC sampling on two ADC channels. So far, I was able to properly initialize the ADC device, and get one single ADC read with the following piece of code:

const struct adc_sequence sequence = {
	.channels = BIT(ADC_1ST_CHANNEL_ID),
	.buffer = m_sample_buffer,
	.buffer_size = sizeof(m_sample_buffer),
	.resolution = ADC_RESOLUTION,
};

ret = adc_read(adc_dev, &sequence);
if (ret) {
    printk("adc_read() failed with code %d\n", ret);
}

There is an issue even with this piece of code because, when I call adc_read(adc_dev, &sequence) function, only the first value of m_sample_buffer is updated, the rest of the m_sample_buffer values remain 0. What I am missing here?

Other than that, is there any way that we configure ADC device to do periodic sampling in the background without our intervention?

Thanks in advance for your time and efforts.

Sincerely,

Bojan.

[Einar Thorsrud]

Hi Bojan,

You cannot use the Zephyr ADC API if you want to trigger sampling regularly without CPU intervention due to the general nature of the Zephyr APIs. In that case you need to use a TIMER to trigger the SAADC via PPI, and for that you need to use the nrfx drivers directly.

I do not have an example which does that at hand, but how to use nrfx drivers in general in Zephyr is demonstrated by the nrfx use example. You can combine that with this unofficial SAADC example, which demonstrates using the latest nrfx API to sample multiple channels regularly based on a timer (it is for the nRF5 SDK, but the nrfx API is the same regardless).

Referring to the example, note that if you do not want to handle the sample set every time you just need to increase the buffer length, as you will get an interrupt ever time the buffer is filled. Also, you may not need to use double buffering as demonstrated, if you have time to process the buffer on time without it.

Br,

Einar

[xproto]

Hello, Its been three years so If I may ask. Is there an updated example for zephyr ADC continuous periodic sampling?

[Einar Thorsrud]

Hi,

There are quite a few examples for using the SAADC nrfx driver directly now, which you can find under modules/hal/nordic/nrfx/samples/src/nrfx_saadc/. advanced_non_blocking_internal_timer is probably most relevant in this case.

[xproto]

Thank you and I'm sorry I was not able to point out that I'm using nrf52832. Which is not supported in the link you have shared.

[Einar Thorsrud]

Hi,

No, that is correct. But even if the samples does not work out of the box on the nRF52 DK, the principles are the same and can e used also on the nRF52832.

[xproto]

Ok good to know Einar since it is indicated not supported and confirmed over testing on the board. I'll try editing this code and get back to you If I still have question (maybe new thread). Thank you again!