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torvalds
GitHub Repository: torvalds/linux
 Path: blob/master/Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml
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# 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-sense-rtd.yaml#

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$schema: http://devicetree.org/meta-schemas/core.yaml#

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title: Temperature Sense RTD

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maintainers:

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  - Liam Beguin <liambeguin@gmail.com>

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description: |

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  RTDs (Resistance Temperature Detectors) are a kind of temperature sensors

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  used to get a linear voltage to temperature reading within a give range

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  (usually 0 to 100 degrees Celsius).

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  When an io-channel measures the output voltage across an RTD such as a

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  PT1000, the interesting measurement is almost always the corresponding

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  temperature, not the voltage output. This binding describes such a circuit.

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  The general transfer function here is (using SI units)

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    V = R(T) * iexc

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    R(T) = r0 * (1 + alpha * T)

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    T = 1 / (alpha * r0 * iexc) * (V - r0 * iexc)

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  The following circuit matches what's in the examples section.

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           5V0

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          -----

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            |

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        +---+----+

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        |  R 5k  |

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        +---+----+

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            |

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            V 1mA

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            |

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            +---- Vout

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            |

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        +---+----+

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        | PT1000 |

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        +---+----+

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            |

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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-sense-rtd

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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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  excitation-current-microamp:

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    description: The current fed through the RTD sensor.

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  alpha-ppm-per-celsius:

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    description: |

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      alpha can also be expressed in micro-ohms per ohm Celsius. It's a linear

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      approximation of the resistance versus temperature relationship

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      between 0 and 100 degrees Celsius.

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      alpha = (R_100 - R_0) / (100 * R_0)

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      Where, R_100 is the resistance of the sensor at 100 degrees Celsius, and

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      R_0 (or r-naught-ohms) is the resistance of the sensor at 0 degrees

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      Celsius.

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      Pure platinum has an alpha of 3925. Industry standards such as IEC60751

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      and ASTM E-1137 specify an alpha of 3850.

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  r-naught-ohms:

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    description: |

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      Resistance of the sensor at 0 degrees Celsius.

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      Common values are 100 for PT100, 500 for PT500, and 1000 for PT1000

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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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  - excitation-current-microamp

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  - alpha-ppm-per-celsius

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  - r-naught-ohms

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examples:

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  - |

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    pt1000_1: temperature-sensor0 {

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        compatible = "temperature-sense-rtd";

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        #io-channel-cells = <0>;

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        io-channels = <&temp_adc1 0>;

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        excitation-current-microamp = <1000>; /* i = U/R = 5 / 5000 */

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        alpha-ppm-per-celsius = <3908>;

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        r-naught-ohms = <1000>;

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    };

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...

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