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