Re: [PATCH v2 2/8] dt-bindings: iio: dac: maxim,ds4424: add maxim,rfs-ohms property

Conor Dooley
2026-01-27T19:55:32Z

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On Tue, Jan 27, 2026 at 07:49:20PM +0000, Conor Dooley wrote:
> On Tue, Jan 27, 2026 at 07:09:33AM +0100, Oleksij Rempel wrote:
> > The Maxim DS4422/DS4424 and DS4402/DS4404 current DACs determine their
> > full-scale output current via external resistors (Rfs) connected to the
> > FSx pins. Without knowing these values, the full-scale range of the
> > hardware is undefined.
> >
> > Add the 'maxim,rfs-ohms' property to describe these physical components.
> > This property is required to provide a complete description of the
> > hardware configuration.
> >
> > Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
> > ---
> > changes v2:
> > - make maxim,rfs-ohms a required property as the hardware range is undefined
> > without external resistors.
> > - add allOf constraints to enforce 2 vs 4 items in maxim,rfs-ohms based on
> > compatible string.
> > - drop explicit $ref for maxim,rfs-ohms to fix dt_binding_check warning.
> > - update example in binding to include the new required property.
> > ---
> > .../bindings/iio/dac/maxim,ds4424.yaml | 36 +++++++++++++++++++
> > 1 file changed, 36 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/iio/dac/maxim,ds4424.yaml b/Documentation/devicetree/bindings/iio/dac/maxim,ds4424.yaml
> > index efe63e6cb55d..400afd8771aa 100644
> > --- a/Documentation/devicetree/bindings/iio/dac/maxim,ds4424.yaml
> > +++ b/Documentation/devicetree/bindings/iio/dac/maxim,ds4424.yaml
> > @@ -27,9 +27,44 @@ properties:
> >
> > vcc-supply: true
> >
> > + maxim,rfs-ohms:
> > + description: |
> > + Array of resistance values in Ohms for the external Rfs resistors
> > + connected to the FS pins.
> > + - For DS44x2 (2 channels): 2 values required.
> > + - For DS44x4 (4 channels): 4 values required.
> > + Typical values range from 40000 (40 kOhm) to 160000 (160 kOhm).
>
> Add here
> maxItems: 4
> minItems: 2
>
> Are 40kOhm and 160kOhm actual max/min values, or just a suggested range?

Datasheet for the ds4424 seems to imply they're actual max/min values:
"Input resistors (RFS) must be between the speciifed values to ensure the
device meets its accuracy and linearity specifications."
In that case, consider also adding something like:
items:
maximum: 40000
minimum: 160000

Although, that would complain about the 5100 Ohms you're using. How come
the example lies outside of the "typical" range?

> > +
> > required:
> > - compatible
> > - reg
> > + - maxim,rfs-ohms
> > +
> > +allOf:
> > + - if:
> > + properties:
> > + compatible:
> > + contains:
> > + enum:
> > + - maxim,ds4402
> > + - maxim,ds4422
> > + then:
> > + properties:
> > + maxim,rfs-ohms:
>
> > + minItems: 2
>
> Remove this...
>
> > + maxItems: 2
> > + - if:
> > + properties:
> > + compatible:
> > + contains:
> > + enum:
> > + - maxim,ds4404
> > + - maxim,ds4424
> > + then:
> > + properties:
> > + maxim,rfs-ohms:
> > + minItems: 4
>
>
> > + maxItems: 4
>
> ...and this. The outer-most constraints should be in the definition and
> the if/else bit just adjusts whichever bounds it needs to.
>
> Cheers,
> Conor.
>
> pw-bot: changes-requested
>
> >
> > additionalProperties: false
> >
> > @@ -43,6 +78,7 @@ examples:
> > compatible = "maxim,ds4424";
> > reg = <0x10>; /* When A0, A1 pins are ground */
> > vcc-supply = <&vcc_3v3>;
> > + maxim,rfs-ohms = <5100>, <5100>, <5100>, <5100>;
> > };
> > };
> > ...
> > --
> > 2.47.3
> >

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