Reading Analog Data in Zephyr with a nrf52840

It turned out to be much harder to read the ADC in Zephyr than I expected. As usual the documentation is slim and the syntax was a little strange partly because of the way the nordic nrf52 devices do analog input.

What’s key to know is that there’s one ADC (run as a nrf,saadc — successive approximation ADC) and it has 8 channels, corresponding to the 8 possible analog input pins (AIN0…AIN7). To do an analog read you specify which analog channel to use (out of the 8) and that defines which physical pin is used.

I added the following to prj.conf in the project to enable the adc sections during compiles,

CONFIG_ADC=y
CONFIG_ADC_ASYNC=y

and, i enabled it on the board by adding this to myboard.dtsi:

&adc { /* ADC */
 status = "okay";
};

which enables the nrf52840 peripheral named ‘adc’.

I also added an (optional) alias so that the adc name applies somewhat hardware-independently. This is at the top of myboard.dtsi:

aliases {
   ... (other stuff)
   adcctrl = &adc;
   ...
};

Having the alias lets us use a couple of zephyr macros to get the device name

DT_LABEL(DT_ALIAS(adcctrl))

in the source code.

Source Code

Working C code is below