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
7 title: Temperature Sense RTD
8
9 maintainers:
10 - Liam Beguin <liambeguin@gmail.com>
11
12 description: |
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
47 properties:
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
82 additionalProperties: false
83 required:
84 - compatible
85 - io-channels
86 - excitation-current-microamp
87 - alpha-ppm-per-celsius
88 - r-naught-ohms
89
90 examples:
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 ...
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