Re: [PATCH v4 10/10] dt-bindings: iio: afe: add bindings for temperature transducers
From: Rob Herring
Date: Mon Jul 12 2021 - 12:12:07 EST
On Tue, Jul 06, 2021 at 12:09:42PM -0400, Liam Beguin 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 a temperature transducer (either voltage or current).
>
> Signed-off-by: Liam Beguin <lvb@xxxxxxxxxx>
> ---
> .../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@xxxxxxxxxx>
> +
> +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
This is a io-channel consumer and producer?
> +
> + 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?
> +
> + 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
>
>