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 # 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 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: 0 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 = <0>; io-channels = <&temp_adc1 0>; excitation-current-microamp = <1000>; /* i = U/R = 5 / 5000 */ alpha-ppm-per-celsius = <3908>; r-naught-ohms = <1000>; }; ...