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Message-Id: <CCRQ33B4B96F.3IX5IUAK24F49@shaak>
Date: Tue, 13 Jul 2021 00:20:57 -0400
From: "Liam Beguin" <liambeguin@...il.com>
To: "Rob Herring" <robh@...nel.org>
Cc: <peda@...ntia.se>, <jic23@...nel.org>, <lars@...afoo.de>,
<pmeerw@...erw.net>, <linux-kernel@...r.kernel.org>,
<linux-iio@...r.kernel.org>, <devicetree@...r.kernel.org>
Subject: Re: [PATCH v4 10/10] dt-bindings: iio: afe: add bindings for
temperature transducers
On Mon Jul 12, 2021 at 12:11 PM EDT, Rob Herring wrote:
> On Tue, Jul 06, 2021 at 12:09:42PM -0400, Liam Beguin wrote:
> > From: Liam Beguin <lvb@...hos.com>
> >
> > An ADC is often used to measure other quantities indirectly.
> > This binding describe one case, the measurement of a temperature
> > through a temperature transducer (either voltage or current).
> >
> > Signed-off-by: Liam Beguin <lvb@...hos.com>
> > ---
> > .../iio/afe/temperature-transducer.yaml | 111 ++++++++++++++++++
> > 1 file changed, 111 insertions(+)
> > create mode 100644 Documentation/devicetree/bindings/iio/afe/temperature-transducer.yaml
> >
> > diff --git a/Documentation/devicetree/bindings/iio/afe/temperature-transducer.yaml b/Documentation/devicetree/bindings/iio/afe/temperature-transducer.yaml
> > new file mode 100644
> > index 000000000000..b5a4fbfe75e4
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/iio/afe/temperature-transducer.yaml
> > @@ -0,0 +1,111 @@
> > +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/iio/afe/temperature-transducer.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: Temperature Transducer
> > +
> > +maintainers:
> > + - Liam Beguin <lvb@...hos.com>
> > +
> > +description: |
> > + A temperature transducer is a device that converts a thermal quantity
> > + into any other physical quantity. This binding applies to temperature to
> > + voltage (like the LTC2997), and temperature to current (like the AD590)
> > + linear transducers.
> > + In both cases these are assumed to be connected to a voltage ADC.
> > +
> > + When an io-channel measures the output voltage of a temperature analog front
> > + end such as a temperature transducer, 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(T) = Rsense * Isense(T)
> > + T = (Isense(T) / alpha) + offset
> > + T = 1 / (Rsense * alpha) * (V + offset * Rsense * alpha)
> > +
> > + When using a temperature to voltage transducer, Rsense is set to 1.
> > +
> > + The following circuits show a temperature to current and a temperature to
> > + voltage transducer that can be used with this binding.
> > +
> > + VCC
> > + -----
> > + |
> > + +---+---+
> > + | AD590 | VCC
> > + +---+---+ -----
> > + | |
> > + V proportional to T +----+----+
> > + | D+ --+ |
> > + +---- Vout | LTC2997 +--- Vout
> > + | D- --+ |
> > + +---+----+ +---------+
> > + | Rsense | |
> > + +---+----+ -----
> > + | GND
> > + -----
> > + GND
> > +
> > +properties:
> > + compatible:
> > + const: temperature-transducer
> > +
> > + io-channels:
> > + maxItems: 1
> > + description: |
> > + Channel node of a voltage io-channel.
> > +
> > + '#io-channel-cells':
> > + const: 0
Hi Rob,
>
> This is a io-channel consumer and producer?
>
Yes, this is a consumer and a producer.
It consumes a single ADC channel and can be fed to something like hwmon.
> > +
> > + sense-offset-millicelsius:
> > + description: |
> > + Temperature offset. The default is <0>.
> > + This offset is commonly used to convert from Kelvins to degrees Celsius.
> > + In that case, sense-offset-millicelsius would be set to <(-273150)>.
>
> default: 0
>
> > +
> > + sense-resistor-ohms:
> > + description: |
> > + The sense resistor. Defaults to <1>.
> > + Set sense-resistor-ohms to <1> when using a temperature to voltage
> > + transducer.
>
> default: 1
>
> Though why would we set the value to 1 if the default is 1?
>
I can rephrase this. I meant to say that the default will make this
behave like a temperature to voltage transducer.
Liam
> > +
> > + alpha-ppm-per-celsius:
> > + description: |
> > + Sometimes referred to as output gain, slope, or temperature coefficient.
> > +
> > + alpha is expressed in parts per million which can be micro-amps per
> > + degrees Celsius or micro-volts per degrees Celsius. The is the main
> > + characteristic of a temperature transducer and should be stated in the
> > + datasheet.
> > +
> > +additionalProperties: false
>
> Blank line here.
>
> > +required:
> > + - compatible
> > + - io-channels
> > + - alpha-ppm-per-celsius
> > +
> > +examples:
> > + - |
> > + ad950: temperature-sensor-0 {
> > + compatible = "temperature-transducer";
> > + #io-channel-cells = <0>;
> > + io-channels = <&temp_adc 3>;
> > +
> > + sense-offset-millicelsius = <(-273150)>; /* Kelvin to degrees Celsius */
> > + sense-resistor-ohms = <8060>;
> > + alpha-ppm-per-celsius = <1>; /* 1 uA/K */
> > + };
> > + - |
> > + znq_tmp: temperature-sensor-1 {
> > + compatible = "temperature-transducer";
> > + #io-channel-cells = <0>;
> > + io-channels = <&temp_adc 2>;
> > +
> > + sense-offset-millicelsius = <(-273150)>; /* Kelvin to degrees Celsius */
> > + alpha-ppm-per-celsius = <4000>; /* 4 mV/K */
> > + };
> > +...
> > --
> > 2.30.1.489.g328c10930387
> >
> >
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