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

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

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?

On the other hand, public TRM doesn't give much more information and
input from NXP would be highly desired.

@Haibo Chen your thoughts?

BR,
Primoz

> 
>>
>>
> 

-- 
Primoz Fiser
phone: +386-41-390-545
email: primoz.fiser@norik.com
--
Norik systems d.o.o.
Your embedded software partner
Slovenia, EU
phone: +386-41-540-545
email: info@norik.com