1 # SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
2 ---
3 $id: http://devicetree.org/schemas/hwmon/ntc-thermistor.yaml#
4 $schema: http://devicetree.org/meta-schemas/core.yaml#
5
6 title: NTC thermistor temperature sensors
7
8 maintainers:
9 - Linus Walleij <linus.walleij@linaro.org>
10
11 description: |
12 Thermistors with negative temperature coefficient (NTC) are resistors that
13 vary in resistance in an often non-linear way in relation to temperature.
14 The negative temperature coefficient means that the resistance decreases
15 as the temperature rises. Since the relationship between resistance and
16 temperature is non-linear, software drivers most often need to use a look
17 up table and interpolation to get from resistance to temperature.
18
19 When used in practice, a thermistor is often connected between ground, a
20 pull-up resistor or/and a pull-down resistor and a fixed voltage like this:
21
22 + e.g. 5V = pull-up voltage (puv)
23 |
24 +-+
25 | |
26 | | Pull-up resistor
27 | | (puo)
28 +-+
29 |-------------------------o
30 +-+ | ^
31 | |/ |
32 | / |
33 |/| Thermistor | Measured voltage (mv)
34 / | | "connected ground"
35 /| | |
36 +-+ |
37 |-------------------------o
38 +-+ ^
39 | | |
40 | | Pull-down resistor | Measured voltage (mv)
41 | | (pdo) | "connected positive"
42 +-+ |
43 | |
44 | v
45 + GND GND
46
47 The arrangements of where we measure the voltage over the thermistor are
48 called "connected ground" and "connected positive" and shall be understood as
49 the cases when either pull-up or pull-down resistance is zero.
50
51 If the pull-up resistance is 0 one end of the thermistor is connected to the
52 positive voltage and we get the thermistor on top of a pull-down resistor
53 and we take the measure between the thermistor and the pull-down resistor.
54
55 Conversely if the pull-down resistance is zero, one end of the thermistor is
56 connected to ground and we get the thermistor under the pull-up resistor
57 and we take the measure between the pull-up resistor and the thermistor.
58
59 We can use both pull-up and pull-down resistors at the same time, and then
60 the figure illustrates where the voltage will be measured for the "connected
61 ground" and "connected positive" cases.
62
63 properties:
64 $nodename:
65 pattern: "^thermistor(.*)?$"
66
67 compatible:
68 oneOf:
69 - const: epcos,b57330v2103
70 - const: epcos,b57891s0103
71 - const: murata,ncp15wb473
72 - const: murata,ncp18wb473
73 - const: murata,ncp21wb473
74 - const: murata,ncp03wb473
75 - const: murata,ncp15wl333
76 - const: murata,ncp03wf104
77 - const: murata,ncp15xh103
78 - const: samsung,1404-001221
79 # Deprecated "ntp," compatible strings
80 - const: ntc,ncp15wb473
81 deprecated: true
82 - const: ntc,ncp18wb473
83 deprecated: true
84 - const: ntc,ncp21wb473
85 deprecated: true
86 - const: ntc,ncp03wb473
87 deprecated: true
88 - const: ntc,ncp15wl333
89 deprecated: true
90
91 "#thermal-sensor-cells":
92 description: Thermal sensor cells if used for thermal sensoring.
93 const: 0
94
95 pullup-uv:
96 $ref: /schemas/types.yaml#/definitions/uint32
97 description: Pull-up voltage in micro volts. Must always be specified.
98
99 pullup-ohm:
100 $ref: /schemas/types.yaml#/definitions/uint32
101 description: Pull-up resistance in ohms. Must always be specified, even
102 if zero.
103
104 pulldown-ohm:
105 $ref: /schemas/types.yaml#/definitions/uint32
106 description: Pull-down resistance in ohms. Must always be specified, even
107 if zero.
108
109 connected-positive:
110 $ref: /schemas/types.yaml#/definitions/flag
111 description: Indicates how the thermistor is connected in series with
112 a pull-up and/or a pull-down resistor. See the description above for
113 an illustration. If this flag is NOT specified, the thermistor is assumed
114 to be connected-ground, which usually means a pull-down resistance of
115 zero but complex arrangements are possible.
116
117 # See /schemas/iio/adc/adc.yaml
118 io-channels:
119 maxItems: 1
120 description: IIO ADC channel to read the voltage over the resistor. Must
121 always be specified.
122
123 required:
124 - compatible
125 - pullup-uv
126 - pullup-ohm
127 - pulldown-ohm
128 - io-channels
129
130 additionalProperties: false
131
132 examples:
133 - |
134 thermistor {
135 compatible = "murata,ncp18wb473";
136 io-channels = <&gpadc 0x06>;
137 pullup-uv = <1800000>;
138 pullup-ohm = <220000>;
139 pulldown-ohm = <0>;
140 #thermal-sensor-cells = <0>;
141 };
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