linux-kernel - Re: [PATCH v5 6/6] iio: adc: ad4851: add ad485x driver

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Message-ID: <146a24a7-d7a1-4969-98c0-f621a1709dd7@baylibre.com>
Date: Thu, 7 Nov 2024 10:47:52 -0600
From: David Lechner <dlechner@...libre.com>
To: "Miclaus, Antoniu" <Antoniu.Miclaus@...log.com>,
 "jic23@...nel.org" <jic23@...nel.org>,
 "conor+dt@...nel.org" <conor+dt@...nel.org>,
 "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
 "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>
Subject: Re: [PATCH v5 6/6] iio: adc: ad4851: add ad485x driver

On 11/7/24 10:13 AM, David Lechner wrote:
> On 11/7/24 4:51 AM, Miclaus, Antoniu wrote:


>>> I'm pretty sure that calibscale and calibbias also need to take into
>>> account if resolution boost is enabled or not.
>>
>> Can you please detail a bit on this topic? I am not sure what I should do.
>>
> 
> We haven't implemented oversampling yet in ad4695 yet, so I don't know
> exactly what we need to do either. ;-)
> 
> But this is how I would test it to see if it is working correctly or
> not. We will need to test this with a 20-bit chip since that is the
> only one that will change the _scale attribute when oversampling is
> enabled.
> 
> First, with oversampling disabled (_oversampling_ratio = 1), generate
> a constant voltage of 1V for the input. Read the _raw attribute. Let's
> call this value raw0. Read the _scale attribute, call it scale0 and
> the _offset attribute, call it offset0.
> 
> Then we should have (raw0 + offset0) * scale0 = 1000 mV (+/- some
> noise).
> 
> Then change the offset calibrate to 100 mV. To do this, we reverse
> the calculation 100 mV / scale0 = calibbias (raw units). Write the
> raw value to the _calibbias attribute. Then read the _raw
> attribute again, call it raw0_with_calibbias.
> 
> This time, we should have (raw0_with_calibbias + offset0) * scale0
> = 1100 mV (+/- some noise).
> 
> Then set _calibbias back to 0 and repeat the above by setting the
> _calibscale attribute to 0.90909 (this is 1 / 1.1, which should

Now that I have written this, this has me second-guessing if I
implemented calibscale correctly on ad4695. It would seem more
logical that if we have an actual input voltage of 1 V and a
calibscale of 1.1, then the resulting processed value we read
should be 1100 mV.

Jonathan, can you set me straight? The sysfs ABI docs aren't
clear on this point.

> add 10% to the measured raw value). Read, the _raw attribute again,
> call it raw0_with_caliscale.
> 
> This time, we should have (raw0_with_caliscale + offset0) * scale0
> = 1100 mV (+/- some noise).
> 
> Set _calibscale back to 0. Then set _oversampling_ratio to 2. Read
> _scale and _offset again, call these scale1 and offset1.
> 
> Then repeat the steps above using scale1 and offset1 in the
> calculations. The raw values will be different but the resulting
> processed values (mV) should all be the same if the attributes
> are implemented correctly.
>