default adc config with comments

&tscadc {
    status = "okay";

    adc {
        // Configure one or more (up to 8) steps for the adc to execute:


        // For each step, the channel to sample.
        //  range: 0 .. 7
        ti,adc-channels = <0 1 2 3 4 5 6 7>;
        //
        // BeagleBone Black (and most other variants):
        //  ch 0    P9.39
        //  ch 1    P9.40
        //  ch 2    P9.37
        //  ch 3    P9.38
        //  ch 4    P9.33
        //  ch 5    P9.36
        //  ch 6    P9.35
        //  ch 7    measures 0.5 * VDD_3V3B with 2.4 kΩ source impedance
        //
        // PocketBeagle:
        //  ch 0    P1.19
        //  ch 1    P1.21
        //  ch 2    P1.23
        //  ch 3    P1.25
        //  ch 4    P1.27
        //  ch 5    P2.35 via 10k/10k voltage divider
        //  ch 6    P1.02 via 10k/10k voltage divider
        //  ch 7    P2.36 via pmic mux
        //
        // The divider used on PocketBeagle channels 5 and 6 makes the effective voltage V_eff and
        // source impedance Z_eff seen by the adc on these channels depend on the voltage V_src and
        // impedance Z_src of the source connected to the corresponding pin as follows:
        //
        //  V_eff = V_src / (2 + Z_src / (10 kΩ))
        //  Z_eff = 5 kΩ * (1 + Z_src / (Z_src + 20 kΩ))
        //      ≈ 5 kΩ + Z_src / 4    for small values of Z_src (up to 2 kΩ or so)


        // For each step, number of adc clock cycles to wait between setting up muxes and sampling.
        //  range: 0 .. 262143
        //  optional, default is 152 (XXX but why?!)
        ti,chan-step-opendelay = <152 152 152 152 152 152 152 152>;
        //`
        // XXX is there any purpose to set this nonzero other than to fine-tune the sample rate?


        // For each step, how many times it should sample to average.
        //  range: 1 .. 16, must be power of two (i.e. 1, 2, 4, 8, or 16)
        //  optional, default is 16
        ti,chan-step-avg = <16 16 16 16 16 16 16 16>;
        //
        // If you're using periodic sampling (using the iio block device rather than sysfs) then
        // you should consider setting this to 1 and if desired reduce the samplerate in userspace
        // instead since averaging isn't a particularly good low-pass filter.
        //
        // If you're using sysfs to occasionally read a value, then the default value of 16 will
        // still get you the most accurate readings.


        // For each step, number of adc clock cycles to sample minus two.
        //  range: 0 .. 255   (resulting in sampling time of 2 .. 257 cycles)
        //  optional, default is 0
        ti,chan-step-sampledelay = <0 0 0 0 0 0 0 0>;
        //
        // If this is set too low, accuracy will deteriorate when the thing you're measuring has a
        // high source impedance.  The maximum source impedance recommended (by erratum 1.0.32) is:
        //  (2 + sampledelay) * 2.873 kΩ - 0.2 kΩ
        // which means that the default should be fine for source impedance up to 5.5 kΩ.
        //
        // (This seems to ensure the sampling time is at least 21 times the RC constant, based on
        // the 5.5 pF nominal capacitance specified in the datasheet.)


        // After sampling, conversion time is 13 adc clock cycles.
        //
        // The adc clock frequency is 3 MHz, therefore the total time per step in microseconds is:
        //  ( opendelay + avg * ( 2 + sampledelay + 13 ) ) / 3
        //
        // If all steps use the same timings then the sample rate will be:
        //  3 MHz / ( opendelay + avg * ( 2 + sampledelay + 13 ) ) / number_of_steps
        //
        // The highest samplerate obtainable (avg=1, opendelay=0, sampledelay=0) is therefore:
        //  200 kHz / number_of_steps
        //  = 25 kHz    when using all 8 steps.
        //
        // Using avg=16 reduces that to:
        //  12.5 kHz / number_of_steps
        //  = 1.5625 kHz    when using all 8 steps.
        //
        // Using the default values (avg=16, opendelay=152, sampledelay=0) reduces that to:
        //  7.653 kHz / number_of_steps
        //  = 0.9566 kHz    when using all 8 steps.
    };
};