115 lines
3.6 KiB
YAML
115 lines
3.6 KiB
YAML
# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
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%YAML 1.2
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---
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$id: http://devicetree.org/schemas/iio/afe/temperature-transducer.yaml#
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$schema: http://devicetree.org/meta-schemas/core.yaml#
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title: Temperature Transducer
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maintainers:
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- Liam Beguin <liambeguin@gmail.com>
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description: |
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A temperature transducer is a device that converts a thermal quantity
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into any other physical quantity. This binding applies to temperature to
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voltage (like the LTC2997), and temperature to current (like the AD590)
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linear transducers.
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In both cases these are assumed to be connected to a voltage ADC.
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When an io-channel measures the output voltage of a temperature analog front
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end such as a temperature transducer, the interesting measurement is almost
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always the corresponding temperature, not the voltage output. This binding
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describes such a circuit.
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The general transfer function here is (using SI units)
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V(T) = Rsense * Isense(T)
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T = (Isense(T) / alpha) + offset
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T = 1 / (Rsense * alpha) * (V + offset * Rsense * alpha)
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When using a temperature to voltage transducer, Rsense is set to 1.
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The following circuits show a temperature to current and a temperature to
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voltage transducer that can be used with this binding.
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VCC
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-----
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+---+---+
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| AD590 | VCC
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+---+---+ -----
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| |
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V proportional to T +----+----+
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| D+ --+ |
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+---- Vout | LTC2997 +--- Vout
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| D- --+ |
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+---+----+ +---------+
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| Rsense | |
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+---+----+ -----
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| GND
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-----
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GND
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properties:
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compatible:
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const: temperature-transducer
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io-channels:
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maxItems: 1
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description: |
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Channel node of a voltage io-channel.
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'#io-channel-cells':
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const: 0
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sense-offset-millicelsius:
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description: |
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Temperature offset.
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This offset is commonly used to convert from Kelvins to degrees Celsius.
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In that case, sense-offset-millicelsius would be set to <(-273150)>.
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default: 0
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sense-resistor-ohms:
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description: |
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The sense resistor.
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By default sense-resistor-ohms cancels out the resistor making the
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circuit behave like a temperature transducer.
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default: 1
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alpha-ppm-per-celsius:
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description: |
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Sometimes referred to as output gain, slope, or temperature coefficient.
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alpha is expressed in parts per million which can be micro-amps per
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degrees Celsius or micro-volts per degrees Celsius. The is the main
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characteristic of a temperature transducer and should be stated in the
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datasheet.
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additionalProperties: false
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required:
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- compatible
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- io-channels
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- alpha-ppm-per-celsius
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examples:
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- |
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ad950: temperature-sensor-0 {
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compatible = "temperature-transducer";
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#io-channel-cells = <0>;
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io-channels = <&temp_adc 3>;
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sense-offset-millicelsius = <(-273150)>; /* Kelvin to degrees Celsius */
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sense-resistor-ohms = <8060>;
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alpha-ppm-per-celsius = <1>; /* 1 uA/K */
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};
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- |
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znq_tmp: temperature-sensor-1 {
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compatible = "temperature-transducer";
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#io-channel-cells = <0>;
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io-channels = <&temp_adc 2>;
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sense-offset-millicelsius = <(-273150)>; /* Kelvin to degrees Celsius */
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alpha-ppm-per-celsius = <4000>; /* 4 mV/K */
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};
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...
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