Re: [PATCH v3 09/10] dt-bindings: iio: afe: add bindings for temperature-sense-rtd

[Liam Beguin]

On Sun Jul 4, 2021 at 1:02 PM EDT, Jonathan Cameron wrote:
> On Wed, 30 Jun 2021 21:00:33 -0400
> Liam Beguin <liambeguin@xxxxxxxxx> wrote:
>
> > From: Liam Beguin <lvb@xxxxxxxxxx>
> > 
> > An ADC is often used to measure other quantities indirectly. This
> > binding describe one case, the measurement of a temperature through the
> > voltage across an RTD resistor such as a PT1000.
> > 
> > Signed-off-by: Liam Beguin <lvb@xxxxxxxxxx>
> > ---
> >  .../iio/afe/temperature-sense-rtd.yaml        | 101 ++++++++++++++++++
> >  MAINTAINERS                                   |   7 ++
> >  2 files changed, 108 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml
> > 
> > diff --git a/Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml b/Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml
> > new file mode 100644
> > index 000000000000..e23e74e63ec5
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml
> > @@ -0,0 +1,101 @@
> > +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/iio/afe/temperature-sense-rtd.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: Temperature Sense RTD
> > +
> > +maintainers:
> > +  - Liam Beguin <lvb@xxxxxxxxxx>
> > +
> > +description: |
> > +  RTDs (Resistance Temperature Detectors) are a kind of temperature sensors
> > +  used to get a linear voltage to temperature reading within a give range
> > +  (usually 0 to 100 degrees Celsius).
> > +
> > +  When an io-channel measures the output voltage across an RTD such as a
> > +  PT1000, the interesting measurement is almost always the corresponding
> > +  temperature, not the voltage output. This binding describes such a circuit.
> > +
> > +  The general transfer function here is (using SI units)
> > +
> > +    V = R(T) * iexc
> > +    R(T) = r0 * (1 + alpha * T)
> > +    T = 1 / (alpha * r0 * iexc) * (V - r0 * iexc)
> > +
> > +  The following circuit matches what's in the examples section.
> > +
> > +           5V0
> > +          -----
> > +            |
> > +        +---+----+
> > +        |  R 5k  |
> > +        +---+----+
> > +            |
> > +            V 1mA
> > +            |
> > +            +---- Vout
> > +            |
> > +        +---+----+
> > +        | PT1000 |
> > +        +---+----+
> > +            |
> > +          -----
> > +           GND
> > +
> > +properties:
> > +  compatible:
> > +    const: temperature-sense-rtd
> > +
> > +  io-channels:
> > +    maxItems: 1
> > +    description: |
> > +      Channel node of a voltage io-channel.
> > +
> > +  '#io-channel-cells':
> > +    const: 1
>
> Only 1 channel, so this should be 0. For consumers of this driver
> they only need to identify the device, not the device + channel.
>
> https://github.com/devicetree-org/dt-schema/blob/master/schemas/iio/iio.yaml
>

Thanks for pointing that out. will fix.

> > +
> > +  excitation-current-microamp:
> > +    description: The current fed through the RTD sensor.
> > +
> > +  alpha-ppm-per-celsius:
> > +    description: |
> > +      alpha can also be expressed in micro-ohms per ohm Celsius. It's a linear
> > +      approximation of the resistance versus temperature relationship
> > +      between 0 and 100 degrees Celsius.
> > +
> > +      alpha = (R_100 - R_0) / (100 * R_0)
> > +
> > +      Where, R_100 is the resistance of the sensor at 100 degrees Celsius, and
> > +      R_0 (or r-naught-ohms) is the resistance of the sensor at 0 degrees
> > +      Celsius.
> > +
> > +      Pure platinum has an alpha of 3925. Industry standards such as IEC60751
> > +      and ASTM E-1137 specify an alpha of 3850.
> > +
> > +  r-naught-ohms:
> > +    description: |
> > +      Resistance of the sensor at 0 degrees Celsius.
> > +      Common values are 100 for PT100, 500 for PT500, and 1000 for PT1000
> > +
> > +additionalProperties: false
> > +required:
> > +  - compatible
> > +  - io-channels
> > +  - excitation-current-microamp
> > +  - alpha-ppm-per-celsius
> > +  - r-naught-ohms
> > +
> > +examples:
> > +  - |
> > +    pt1000_1: temperature-sensor0 {
> > +        compatible = "temperature-sense-rtd";
> > +        #io-channel-cells = <1>;
> > +        io-channels = <&temp_adc1 0>;
> > +
> > +        excitation-current-microamp = <1000>; /* i = U/R = 5 / 5000 */
> > +        alpha-ppm-per-celsius = <3908>;
> > +        r-naught-ohms = <1000>;
> > +    };
> > +...
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index 9bf553e53f0f..ed80e671a16a 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -8897,6 +8897,13 @@ F:	Documentation/devicetree/bindings/iio/afe/current-sense-shunt.yaml
> >  F:	Documentation/devicetree/bindings/iio/afe/voltage-divider.yaml
> >  F:	drivers/iio/afe/iio-rescale.c
> >  
> > +IIO UNIT CONVERTER (TEMPERATURE)
> > +M:	Liam Beguin <liambeguin@xxxxxxxxx>
> > +R:	Peter Rosin <peda@xxxxxxxxxx>
> > +L:	linux-iio@xxxxxxxxxxxxxxx
> > +S:	Maintained
> > +F:	Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml
>
> I'm not sure we'd normally bother with a MAINTAINERS entry when it is
> just the binding doc
> (as rest is in the driver). The binding doc itself has it's own local
> maintainers entry
> which is the more useful one.
>

You're right, it makes sense to drop the changes to MAINTAINERS as the
information is already contained it the bindings.
I added this after briefly talking about it with Peter.

Thanks again for reviewing these changes,
Liam

> > +
> >  IKANOS/ADI EAGLE ADSL USB DRIVER
> >  M:	Matthieu Castet <castet.matthieu@xxxxxxx>
> >  M:	Stanislaw Gruszka <stf_xl@xxxxx>