linux-kernel - Re: [PATCH v4 1/6] dt-bindings: adc: ad7173: add support for ad411x

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Date: Fri, 7 Jun 2024 10:29:24 +0300
From: "Ceclan, Dumitru" <mitrutzceclan@...il.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Dumitru Ceclan via B4 Relay
 <devnull+dumitru.ceclan.analog.com@...nel.org>, dumitru.ceclan@...log.com,
 Lars-Peter Clausen <lars@...afoo.de>,
 Michael Hennerich <Michael.Hennerich@...log.com>,
 Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>, David Lechner <dlechner@...libre.com>,
 linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/6] dt-bindings: adc: ad7173: add support for ad411x

On 06/06/2024 22:58, Jonathan Cameron wrote:
> On Wed, 5 Jun 2024 09:54:31 +0300
> "Ceclan, Dumitru" <mitrutzceclan@...il.com> wrote:
> 
>> On 03/06/2024 23:00, Jonathan Cameron wrote:
>>> On Mon, 3 Jun 2024 12:46:10 +0300
>>> "Ceclan, Dumitru" <mitrutzceclan@...il.com> wrote:
>>>   
>>>> On 01/06/2024 21:35, Jonathan Cameron wrote:  
>>>>> On Fri, 31 May 2024 22:42:27 +0300
>>>>> Dumitru Ceclan via B4 Relay <devnull+dumitru.ceclan.analog.com@...nel.org> wrote:
>>>>>     
>>>>>> From: Dumitru Ceclan <dumitru.ceclan@...log.com>  
>>
>> ...
>>
>>>>>> +          Supported only by AD7172-2, AD7172-4, AD7175-2, AD7175-8, AD7177-2:
>>>>>> +            19: ((AVDD1 − AVSS)/5)+
>>>>>> +            20: ((AVDD1 − AVSS)/5)−    
>>>>>
>>>>> That's what it says on the datasheet (so fine to copy that here) but I'm curious, what does
>>>>> that mean in practice?  How can we have negative and postive signals of the difference
>>>>> between two power supply voltages where I'm fairly sure AVDD1 always greater than AVSS.
>>>>>    
>>>>
>>>> I have not tested that as I do not have a model that supports this wired up.
>>>> If I had to guess they are the same signal but one should be connected to the
>>>> positive input, one to the negative input...but I could be wrong.  
>>>
>>> If they are, then as far as I we are concerned is this one channel with two
>>> representations depending on whether it is 1st or 2nd in the list?
>>> Can we use one number and hide that detail in the driver?
>>>
>>> Seems odd though if that is the case.
>>>
>>> I guess if we find out later this is the case we can tighten the binding to
>>> enforce the right one instead of squashing them to one value, but that
>>> is a bit ugly.  Any chance of digging out the info?  If not we can go ahead
>>> but ideally answering things like this make a our life easier in the long run.
>>>
>>> Jonathan
>>>   
>>
>> "(Avdd1/Avss)/5+ as positive input and (Avdd/Avss)/5- as negative
>>   this is used for monitoring power supplies, the inputs must be selected in pair"
>> Perhaps it's an internal voltage divider...? I dunno
>>
>> So it seems like this cannot be used as a common mode voltage input.
>> I'll restrict the driver to only allow these inputs paired together
>> and rename the define for these selections.
> Most mysterious :)  I'd be interested to know what value it reads
> back if you ever get the part.
>

My best guess now is that the reason for /5 is so that you can measure
the AVDD AVSS difference using the internal 2.5V reference.
So for AVDD 5V, AVSS 0V using the internal ref it would read 1V

I'll let you know if I test this
 
> Ah well, great to have gotten that extra detail even if it leaves
> more questions!
> 
> Jonathan
>