temperature-sense-rtd.yaml

  1# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
  2%YAML 1.2
  3---
  4$id: http://devicetree.org/schemas/iio/afe/temperature-sense-rtd.yaml#
  5$schema: http://devicetree.org/meta-schemas/core.yaml#
  6
  7title: Temperature Sense RTD
  8
  9maintainers:
 10  - Liam Beguin <liambeguin@gmail.com>
 11
 12description: |
 13  RTDs (Resistance Temperature Detectors) are a kind of temperature sensors
 14  used to get a linear voltage to temperature reading within a give range
 15  (usually 0 to 100 degrees Celsius).
 16
 17  When an io-channel measures the output voltage across an RTD such as a
 18  PT1000, the interesting measurement is almost always the corresponding
 19  temperature, not the voltage output. This binding describes such a circuit.
 20
 21  The general transfer function here is (using SI units)
 22
 23    V = R(T) * iexc
 24    R(T) = r0 * (1 + alpha * T)
 25    T = 1 / (alpha * r0 * iexc) * (V - r0 * iexc)
 26
 27  The following circuit matches what's in the examples section.
 28
 29           5V0
 30          -----
 31            |
 32        +---+----+
 33        |  R 5k  |
 34        +---+----+
 35            |
 36            V 1mA
 37            |
 38            +---- Vout
 39            |
 40        +---+----+
 41        | PT1000 |
 42        +---+----+
 43            |
 44          -----
 45           GND
 46
 47properties:
 48  compatible:
 49    const: temperature-sense-rtd
 50
 51  io-channels:
 52    maxItems: 1
 53    description: |
 54      Channel node of a voltage io-channel.
 55
 56  '#io-channel-cells':
 57    const: 0
 58
 59  excitation-current-microamp:
 60    description: The current fed through the RTD sensor.
 61
 62  alpha-ppm-per-celsius:
 63    description: |
 64      alpha can also be expressed in micro-ohms per ohm Celsius. It's a linear
 65      approximation of the resistance versus temperature relationship
 66      between 0 and 100 degrees Celsius.
 67
 68      alpha = (R_100 - R_0) / (100 * R_0)
 69
 70      Where, R_100 is the resistance of the sensor at 100 degrees Celsius, and
 71      R_0 (or r-naught-ohms) is the resistance of the sensor at 0 degrees
 72      Celsius.
 73
 74      Pure platinum has an alpha of 3925. Industry standards such as IEC60751
 75      and ASTM E-1137 specify an alpha of 3850.
 76
 77  r-naught-ohms:
 78    description: |
 79      Resistance of the sensor at 0 degrees Celsius.
 80      Common values are 100 for PT100, 500 for PT500, and 1000 for PT1000
 81
 82additionalProperties: false
 83required:
 84  - compatible
 85  - io-channels
 86  - excitation-current-microamp
 87  - alpha-ppm-per-celsius
 88  - r-naught-ohms
 89
 90examples:
 91  - |
 92    pt1000_1: temperature-sensor0 {
 93        compatible = "temperature-sense-rtd";
 94        #io-channel-cells = <0>;
 95        io-channels = <&temp_adc1 0>;
 96
 97        excitation-current-microamp = <1000>; /* i = U/R = 5 / 5000 */
 98        alpha-ppm-per-celsius = <3908>;
 99        r-naught-ohms = <1000>;
100    };
101...