Question about the nRF Connect SDK v2.1.0 - Zephyr adc example using nRF52840 (PCA10056)

Question

I am trying to understand the ADC example using PCA10056. In the DT file, there are two different ADCs in the device tree arduino_adc and zephyr,user:

adc: adc@40007000 {
                        compatible = "nordic,nrf-saadc";
                        reg = <0x40007000 0x1000>;
                        interrupts = <7 NRF_DEFAULT_IRQ_PRIORITY>;
                        status = "okay";
                        #io-channel-cells = <1>;
                        #address-cells = <1>;
                        #size-cells = <0>;
                        channel@0 {
                                reg = <0>;
                                zephyr,gain = "ADC_GAIN_1_6";
                                zephyr,reference = "ADC_REF_INTERNAL";
                                zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
                                zephyr,input-positive = <NRF_SAADC_AIN1>;
                                zephyr,resolution = <12>;
                        };

                        channel@1 {
                                reg = <1>;
                                zephyr,gain = "ADC_GAIN_1_6";
                                zephyr,reference = "ADC_REF_INTERNAL";
                                zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
                                zephyr,input-positive = <NRF_SAADC_VDD>;
                                zephyr,resolution = <14>;
                                zephyr,oversampling = <8>;
                        };

                        channel@7 {
                                reg = <7>;
                                zephyr,gain = "ADC_GAIN_1_5";
                                zephyr,reference = "ADC_REF_VDD_1_4";
                                zephyr,vref-mv = <750>;
                                zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
                                zephyr,input-positive = <NRF_SAADC_AIN6>;
                                zephyr,input-negative = <NRF_SAADC_AIN7>;
                                zephyr,resolution = <12>;
                        };

                };
...
arduino_adc: analog-connector {
                compatible = "arduino,uno-adc";
                #io-channel-cells = <1>;
                io-channel-map = <0 &adc 1>,
                                 <1 &adc 2>,
                                 <2 &adc 4>,
                                 <3 &adc 5>,
                                 <4 &adc 6>,
                                 <5 &adc 7>;
        };

zephyr,user {
                io-channels = <&adc 0>, <&adc 1>, <&adc 7>;
        };

I can see the following code that initializes 3 zephyr_user channels

#if !DT_NODE_EXISTS(DT_PATH(zephyr_user)) || \
    !DT_NODE_HAS_PROP(DT_PATH(zephyr_user), io_channels)
#error "No suitable devicetree overlay specified"
#endif

#define DT_SPEC_AND_COMMA(node_id, prop, idx) \
    ADC_DT_SPEC_GET_BY_IDX(node_id, idx),

/* Data of ADC io-channels specified in devicetree. */
static const struct adc_dt_spec adc_channels[] = {
    DT_FOREACH_PROP_ELEM(DT_PATH(zephyr_user), io_channels,
    DT_SPEC_AND_COMMA)
    };

I have the following questions regarding the DT and the initialization code:

1. What is the difference between the arduino_adc vs the zephyr_user ADC?
2. Based on the PCA10056 pin assignment(https://infocenter.nordicsemi.com/index.jsp?topic=%2Fps_nrf52840%2Fpin.html), AIN0 - AIN7 are analog input pins if I would like to use pins other than 0, 1, and 7. How can I initialize the other pins using the device tree?
3. Can anyone helps me understand the following properties in the node adc@40007000?
   zephyr,gain;
   zephyr,reference;
   zephyr,vref-mv;
   zephyr,acquisition-time;
   zephyr,input-positive;
   zephyr,input-negative;
   zephyr,oversampling = <8>; // Is this mean the result is an average of 8 sample values?
   zephyr,resolution;
4. After I checked the pin assignment, it seems there is no analog out. I am wondering why is that or maybe I missed something.

Answer 1

How can I initialize the other pins using the device tree?

Duplicate what you already have for adc_0 or 1 or 7, then make the reg = <1> and the channel@1 lines match the other channel numbers you want to use (valid range 0-7)

I would suggest you get the nrf52840 "product spec" pdf from the Nordic website, and read the chapter on the SAADC peripheral.... then nearly all of the named labels will make sense to you (gain, positive and negative input etc)