Re: [PATCH 1/6] dt-bindings: adc: ad7173: add support for ad411x

[Ceclan, Dumitru]

On 03/04/2024 18:22, David Lechner wrote:
> On Wed, Apr 3, 2024 at 2:50 AM Ceclan, Dumitru <mitrutzceclan@xxxxxxxxx> wrote:
>> On 02/04/2024 00:16, David Lechner wrote:
>>> On Mon, Apr 1, 2024 at 2:37 PM David Lechner <dlechner@xxxxxxxxxxxx> wrote:
>>>> On Mon, Apr 1, 2024 at 10:10 AM Dumitru Ceclan via B4 Relay
>>>> <devnull+dumitru.ceclan.analog.com@xxxxxxxxxx> wrote:
>>>>> From: Dumitru Ceclan <dumitru.ceclan@xxxxxxxxxx>
>>>>>
>> ...
>>
>>>>>      properties:
>>>>>        reg:
>>>>> +        description:
>>>>> +          Reg values 16-19 are only permitted for ad4111/ad4112 current channels.
>>>>>          minimum: 0
>>>>> -        maximum: 15
>>>>> +        maximum: 19
>>>> This looks wrong. Isn't reg describing the number of logical channels
>>>> (# of channel config registers)?
>>>>
>>>> After reviewing the driver, I see that > 16 is used as a way of
>>>> flagging current inputs, but still seems like the wrong way to do it.
>>>> See suggestion below.
>>>>
>>>>>        diff-channels:
>>>>> +        description:
>>>>> +          For using current channels specify only the positive channel.
>>>>> +            (IIN2+, IIN2−) -> diff-channels = <2 0>
>>>> I find this a bit confusing since 2 is already VIN2 and 0 is already
>>>> VIN0. I think it would make more sense to assign unique channel
>>>> numbers individually to the negative and positive current inputs.
>>>> Also, I think it makes sense to use the same numbers that the
>>>> registers in the datasheet use (8 - 11 for negative and 12 to 15 for
>>>> positive).
>>>>
>>>> So: (IIN2+, IIN2−) -> diff-channels = <13 10>
>>> Thinking about this a bit more...
>>>
>>> Since the current inputs have dedicated pins and aren't mix-and-match
>>> with multiple valid wiring configurations like the voltage inputs, do
>>> we even need to describe them in the devicetree?
>>>
>>> In the driver, the current channels would just be hard-coded like the
>>> temperature channel since there isn't any application-specific
>>> variation.
>>  Sure, but we still need to offer the user a way to configure which
>> current inputs he wants and if they should use bipolar or unipolar coding.
> From the datasheet, it looks like only positive current input is
> allowed so I'm not sure bipolar applies here. But, yes, if there is
> some other variation in wiring or electrical signal that needs to be
> describe here, then it makes sense to allow a channel configuration
> node for it.

AD4111 datasheet pg.29:
When the ADC is configured for bipolar operation, the output
code is offset binary with a negative full-scale voltage resulting
in a code of 000 … 000, a zero differential input voltage resulting in
a code of 100 … 000, and a positive full-scale input voltage
resulting in a code of 111 … 111. The output code for any
analog input voltage can be represented as
Code = 2^(N – 1) × ((V_IN × 0.1/V REF) + 1)
The output code for any input current is represented as
Code = 2^(N − 1) × ((I_IN × 50 Ω/V REF) + 1)

I would say bipolar applies here, not a great idea because of the limitation on
 the negative side (Input Current Range min:−0.5 max:+24 mA) so still, the option
 is available.