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
6title: NTC thermistor temperature sensors
7
8maintainers:
9  - Naveen Krishna Chatradhi <ch.naveen@samsung.com>
10  - Linus Walleij <linus.walleij@linaro.org>
11
12description: |
13  Thermistors with negative temperature coefficient (NTC) are resistors that
14  vary in resistance in an often non-linear way in relation to temperature.
15  The negative temperature coefficient means that the resistance decreases
16  as the temperature rises. Since the relationship between resistance and
17  temperature is non-linear, software drivers most often need to use a look
18  up table and interpolation to get from resistance to temperature.
19
20  When used in practice, a thermistor is often connected between ground, a
21  pull-up resistor or/and a pull-down resistor and a fixed voltage like this:
22
23      + e.g. 5V = pull-up voltage (puv)
24      |
25     +-+
26     | |
27     | | Pull-up resistor
28     | | (puo)
29     +-+
30      |-------------------------o
31     +-+ |                      ^
32     | |/                       |
33     | /                        |
34     |/| Thermistor             | Measured voltage (mv)
35     / |                        | "connected ground"
36    /| |                        |
37     +-+                        |
38      |-------------------------o
39     +-+                        ^
40     | |                        |
41     | | Pull-down resistor     | Measured voltage (mv)
42     | | (pdo)                  | "connected positive"
43     +-+                        |
44      |                         |
45      |                         v
46      + GND                     GND
47
48  The arrangements of where we measure the voltage over the thermistor are
49  called "connected ground" and "connected positive" and shall be understood as
50  the cases when either pull-up or pull-down resistance is zero.
51
52  If the pull-up resistance is 0 one end of the thermistor is connected to the
53  positive voltage and we get the thermistor on top of a pull-down resistor
54  and we take the measure between the thermistor and the pull-down resistor.
55
56  Conversely if the pull-down resistance is zero, one end of the thermistor is
57  connected to ground and we get the thermistor under the pull-up resistor
58  and we take the measure between the pull-up resistor and the thermistor.
59
60  We can use both pull-up and pull-down resistors at the same time, and then
61  the figure illustrates where the voltage will be measured for the "connected
62  ground" and "connected positive" cases.
63
64properties:
65  $nodename:
66    pattern: "^thermistor(.*)?$"
67
68  compatible:
69    oneOf:
70      - const: epcos,b57330v2103
71      - const: epcos,b57891s0103
72      - const: murata,ncp15wb473
73      - const: murata,ncp18wb473
74      - const: murata,ncp21wb473
75      - const: murata,ncp03wb473
76      - const: murata,ncp15wl333
77      - const: murata,ncp03wf104
78      - const: murata,ncp15xh103
79      - const: samsung,1404-001221
80      # Deprecated "ntp," compatible strings
81      - const: ntc,ncp15wb473
82        deprecated: true
83      - const: ntc,ncp18wb473
84        deprecated: true
85      - const: ntc,ncp21wb473
86        deprecated: true
87      - const: ntc,ncp03wb473
88        deprecated: true
89      - const: ntc,ncp15wl333
90        deprecated: true
91
92  "#thermal-sensor-cells":
93    description: Thermal sensor cells if used for thermal sensoring.
94    const: 0
95
96  pullup-uv:
97    $ref: /schemas/types.yaml#/definitions/uint32
98    description: Pull-up voltage in micro volts. Must always be specified.
99
100  pullup-ohm:
101    $ref: /schemas/types.yaml#/definitions/uint32
102    description: Pull-up resistance in ohms. Must always be specified, even
103      if zero.
104
105  pulldown-ohm:
106    $ref: /schemas/types.yaml#/definitions/uint32
107    description: Pull-down resistance in ohms. Must always be specified, even
108      if zero.
109
110  connected-positive:
111    $ref: /schemas/types.yaml#/definitions/flag
112    description: Indicates how the thermistor is connected in series with
113      a pull-up and/or a pull-down resistor. See the description above for
114      an illustration. If this flag is NOT specified, the thermistor is assumed
115      to be connected-ground, which usually means a pull-down resistance of
116      zero but complex arrangements are possible.
117
118  # See /schemas/iio/adc/adc.yaml
119  io-channels:
120    maxItems: 1
121    description: IIO ADC channel to read the voltage over the resistor. Must
122      always be specified.
123
124required:
125  - compatible
126  - pullup-uv
127  - pullup-ohm
128  - pulldown-ohm
129  - io-channels
130
131additionalProperties: false
132
133examples:
134  - |
135    thermistor0 {
136      compatible = "murata,ncp18wb473";
137      io-channels = <&gpadc 0x06>;
138      pullup-uv = <1800000>;
139      pullup-ohm = <220000>;
140      pulldown-ohm = <0>;
141      #thermal-sensor-cells = <0>;
142    };
143