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

Mon Jul 21 02:09:06 PDT 2025

> -----Original Message-----
> From: Peng Fan (OSS) <peng.fan at oss.nxp.com>
> Sent: 2025年7月21日 17:39
> To: Nuno S? <noname.nuno at gmail.com>
> Cc: Primoz Fiser <primoz.fiser at norik.com>; Jonathan Cameron
> <jic23 at kernel.org>; David Lechner <dlechner at baylibre.com>; Bough Chen
> <haibo.chen at nxp.com>; Nuno Sa <nuno.sa at analog.com>; Andy Shevchenko
> <andy at kernel.org>; Rob Herring <robh at kernel.org>; Krzysztof Kozlowski
> <krzk+dt at kernel.org>; Conor Dooley <conor+dt at kernel.org>; Shawn Guo
> <shawnguo at kernel.org>; Sascha Hauer <s.hauer at pengutronix.de>;
> Pengutronix Kernel Team <kernel at pengutronix.de>; Fabio Estevam
> <festevam at gmail.com>; linux-iio at vger.kernel.org; imx at lists.linux.dev;
> devicetree at vger.kernel.org; linux-arm-kernel at lists.infradead.org;
> linux-kernel at vger.kernel.org; upstream at lists.phytec.de;
> andrej.picej at norik.com
> Subject: Re: [PATCH 1/2] dt-bindings: iio: adc: imx93: Add calibration properties
> 
> On Mon, Jul 14, 2025 at 05:11:31PM +0100, Nuno S? 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 at baylibre.com> wrote:
> >> >
> >> > > On 7/10/25 2:39 AM, Primoz Fiser wrote:
> >> > > > From: Andrej Picej <andrej.picej at norik.com>
> >> > > >
> >> > > > Document i.MX93 ADC calibration properties and how to set them.
> >> > > >
> >> > > > Signed-off-by: Andrej Picej <andrej.picej at norik.com>
> >> > > > Signed-off-by: Primoz Fiser <primoz.fiser at 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.y
> >> > > > +++ aml
> >> > > > @@ -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.
> 
> Just my understanding, it is hard to use one fixed settings to fit all kinds of
> conditions.
> 
>                  Noise induced from PCB tracks  Electro- magnetic noise
> 		     |                           |
> 		     V                           V
>  ---------
> |SOC(ADC)|   <---------------------------------<- (~) external Signal
>  ---------
>                  ^                 ^
>                  |                 |
>              I/O coupled noise    Internal noise
> 
> 
> So OEM A's board may needs different settings compared with OEM B's board.

The noise on Vref did impact the calibration, we did get report from customer, and IC guys suggested to do like the following patch, what's your comments?

https://patchwork.kernel.org/project/linux-iio/patch/20250423-adcpatch-v1-1-b0e84c27ae98@nxp.com/

Regards
Haibo Chen 
> 
> Regards,
> Peng
> 
> >
> >- Nuno S??
> >> On the other hand, public TRM doesn't give much more information and
> >> >