temperature-transducer.yaml

  1# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
  2%YAML 1.2
  3---
  4$id: http://devicetree.org/schemas/iio/afe/temperature-transducer.yaml#
  5$schema: http://devicetree.org/meta-schemas/core.yaml#
  6
  7title: Temperature Transducer
  8
  9maintainers:
 10  - Liam Beguin <liambeguin@gmail.com>
 11
 12description: |
 13  A temperature transducer is a device that converts a thermal quantity
 14  into any other physical quantity. This binding applies to temperature to
 15  voltage (like the LTC2997), and temperature to current (like the AD590)
 16  linear transducers.
 17  In both cases these are assumed to be connected to a voltage ADC.
 18
 19  When an io-channel measures the output voltage of a temperature analog front
 20  end such as a temperature transducer, the interesting measurement is almost
 21  always the corresponding temperature, not the voltage output. This binding
 22  describes such a circuit.
 23
 24  The general transfer function here is (using SI units)
 25    V(T) = Rsense * Isense(T)
 26    T = (Isense(T) / alpha) + offset
 27    T = 1 / (Rsense * alpha) * (V + offset * Rsense * alpha)
 28
 29  When using a temperature to voltage transducer, Rsense is set to 1.
 30
 31  The following circuits show a temperature to current and a temperature to
 32  voltage transducer that can be used with this binding.
 33
 34           VCC
 35          -----
 36            |
 37        +---+---+
 38        | AD590 |                               VCC
 39        +---+---+                              -----
 40            |                                    |
 41            V proportional to T             +----+----+
 42            |                          D+ --+         |
 43            +---- Vout                      | LTC2997 +--- Vout
 44            |                          D- --+         |
 45        +---+----+                          +---------+
 46        | Rsense |                               |
 47        +---+----+                             -----
 48            |                                   GND
 49          -----
 50           GND
 51
 52properties:
 53  compatible:
 54    const: temperature-transducer
 55
 56  io-channels:
 57    maxItems: 1
 58    description: |
 59      Channel node of a voltage io-channel.
 60
 61  '#io-channel-cells':
 62    const: 0
 63
 64  sense-offset-millicelsius:
 65    description: |
 66      Temperature offset.
 67      This offset is commonly used to convert from Kelvins to degrees Celsius.
 68      In that case, sense-offset-millicelsius would be set to <(-273150)>.
 69    default: 0
 70
 71  sense-resistor-ohms:
 72    description: |
 73      The sense resistor.
 74      By default sense-resistor-ohms cancels out the resistor making the
 75      circuit behave like a temperature transducer.
 76    default: 1
 77
 78  alpha-ppm-per-celsius:
 79    description: |
 80      Sometimes referred to as output gain, slope, or temperature coefficient.
 81
 82      alpha is expressed in parts per million which can be micro-amps per
 83      degrees Celsius or micro-volts per degrees Celsius. The is the main
 84      characteristic of a temperature transducer and should be stated in the
 85      datasheet.
 86
 87additionalProperties: false
 88
 89required:
 90  - compatible
 91  - io-channels
 92  - alpha-ppm-per-celsius
 93
 94examples:
 95  - |
 96    ad950: temperature-sensor-0 {
 97        compatible = "temperature-transducer";
 98        #io-channel-cells = <0>;
 99        io-channels = <&temp_adc 3>;
100
101        sense-offset-millicelsius = <(-273150)>; /* Kelvin to degrees Celsius */
102        sense-resistor-ohms = <8060>;
103        alpha-ppm-per-celsius = <1>; /* 1 uA/K */
104    };
105  - |
106    znq_tmp: temperature-sensor-1 {
107        compatible = "temperature-transducer";
108        #io-channel-cells = <0>;
109        io-channels = <&temp_adc 2>;
110
111        sense-offset-millicelsius = <(-273150)>; /* Kelvin to degrees Celsius */
112        alpha-ppm-per-celsius = <4000>; /* 4 mV/K */
113    };
114...