Re: [PATCH 1/2] dt-bindings: iio: adc: imx93: Add calibration properties

[Primoz Fiser <primoz.fiser@norik.com>]

Hi Nuno,

On 14. 07. 25 18:11, Nuno Sá wrote:
> On Mon, 2025-07-14 at 07:56 +0200, Primoz Fiser wrote:
>> Hi all,
>>
>> On 13. 07. 25 17:02, Jonathan Cameron wrote:
>>> On Thu, 10 Jul 2025 10:46:44 -0500
>>> David Lechner <dlechner@baylibre.com> wrote:
>>>
>>>> On 7/10/25 2:39 AM, Primoz Fiser wrote:
>>>>> From: Andrej Picej <andrej.picej@norik.com>
>>>>>
>>>>> Document i.MX93 ADC calibration properties and how to set them.
>>>>>
>>>>> Signed-off-by: Andrej Picej <andrej.picej@norik.com>
>>>>> Signed-off-by: Primoz Fiser <primoz.fiser@norik.com>
>>>>> ---
>>>>>  .../bindings/iio/adc/nxp,imx93-adc.yaml       | 21 +++++++++++++++++++
>>>>>  1 file changed, 21 insertions(+)
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/iio/adc/nxp,imx93-
>>>>> adc.yaml b/Documentation/devicetree/bindings/iio/adc/nxp,imx93-adc.yaml
>>>>> index c2e5ff418920..d1c04cf85fe6 100644
>>>>> --- a/Documentation/devicetree/bindings/iio/adc/nxp,imx93-adc.yaml
>>>>> +++ b/Documentation/devicetree/bindings/iio/adc/nxp,imx93-adc.yaml
>>>>> @@ -52,6 +52,27 @@ properties:
>>>>>    "#io-channel-cells":
>>>>>      const: 1
>>>>>  
>>>>> +  nxp,calib-avg-en:
>>>>> +    default: 1
>>>>> +    description:
>>>>> +      Enable or disable calibration averaging function (AVGEN).
>>>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>>>> +    enum: [ 0, 1 ]
>>>>> +
>>>>> +  nxp,calib-nr-samples:
>>>>> +    default: 512
>>>>> +    description:
>>>>> +      Selects number of samples (NRSMPL) to be used during calibration.
>>>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>>>> +    enum: [ 16, 32, 128, 512 ]
>>>
>>> Allow 1 as a value and drop the enabled above.   Averaging over 1 sample
>>> is same as no averaging and gives simpler binding.
>>>
>>>>> +
>>>>> +  nxp,calib-t-sample:
>>>>> +    default: 22
>>>>> +    description:
>>>>> +      Selects sample time (TSAMP) of calibration conversions in ADC
>>>>> clock cycles
>>>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>>>> +    enum: [ 8, 16, 22, 32 ]
>>>>> +
>>>>>  required:
>>>>>    - compatible
>>>>>    - reg  
>>>>
>>>> This seem like things that should be set at runtime rather than
>>>> in the devicetree. Unless there is some justification on why
>>>> these values depend on how the chip is wired up?
>>
>> It depends how ADC 1.8V Vref is wired up, especially how noisy it is.
>>
>>>
>>> Further to that, I'd like to see some explanation of why we care
>>> to change it at all. Is it ever a bad idea to enable averaging and
>>> pick a large number of samples for calibration?
>>
>> This is a snippet from the i.MX93 TRM, chapter Analog-to-Digital
>> Converter (SAR_ADC) describing calibration steps:
>>
>> 1. Wait for deassertion of functional reset.
>> 2. Configure SAR controller operating clock (MCR[ADCLKSE] = 0).
>> 3. Bring ADC out of Power-down state (MCR[PWDN] = 0).
>> 4. Configure desired calibration settings (default values kept for
>> highest accuracy maximum time).
>> • MCR[TSAMP]: Sample time for calibration conversion
>> • MCR[NRSMPL]: Number of samples in averaging
>> • MCR[AVGEN]: Averaging function enable in calibration
>> 5. Run calibration by writing a one to MCR[CALSTART].
>> 6. Check calibration run status in MSR[CALBUSY]—wait until MSR[CALBUSY]
>> = 0; alternatively, MSR[ADCSTAT] can be
>> used to check status.
>> 7. Check calibration pass/fail status in MSR[CALFAIL] field. If
>> MSR[CALFAIL] = 1 then calibration failed. Detailed status
>> can be checked in CALSTAT.
>>
>>
>> See point 4).
>>
>> Not sure why would there be an option to configure i.MX93 ADC
>> calibration parameters if one use-case (max accuracy max time) to rule
>> them all?
>>
> 
> Sometimes HW guys just want to give you some options. Does not mean we have to
> use them all :).
> 
> I guess what Jonathan is interested in, is to understand in what conditions the
> defaults are no good for the calibration? If we can have a set of values that
> should pretty much always work, no need to further complicate the bindings or
> the driver.

In case you have a noisy Vref you can adjust the parameters to pass the
calibration and have a working ADC.

The trade-off is a less precise ADC but at least a working one.

In ideal case you would have Vref supplied by the dedicated LDO and tons
of decoupling caps, but in real-world you have it connected to a noisy
SMPS and you need to adjust the parameters accordingly.

That's it :)

BR,
Primoz

-- 
Primoz Fiser
phone: +386-41-390-545
email: primoz.fiser@norik.com
--
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