xref: /openbmc/linux/Documentation/devicetree/bindings/hwmon/ntc-thermistor.yaml (revision 9a87ffc99ec8eb8d35eed7c4f816d75f5cc9662e)

# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
---
$id: http://devicetree.org/schemas/hwmon/ntc-thermistor.yaml#
$schema: http://devicetree.org/meta-schemas/core.yaml#

title: NTC thermistor temperature sensors

maintainers:
  - Linus Walleij <linus.walleij@linaro.org>

description: |
  Thermistors with negative temperature coefficient (NTC) are resistors that
  vary in resistance in an often non-linear way in relation to temperature.
  The negative temperature coefficient means that the resistance decreases
  as the temperature rises. Since the relationship between resistance and
  temperature is non-linear, software drivers most often need to use a look
  up table and interpolation to get from resistance to temperature.

  When used in practice, a thermistor is often connected between ground, a
  pull-up resistor or/and a pull-down resistor and a fixed voltage like this:

      + e.g. 5V = pull-up voltage (puv)
      |
     +-+
     | |
     | | Pull-up resistor
     | | (puo)
     +-+
      |-------------------------o
     +-+ |                      ^
     | |/                       |
     | /                        |
     |/| Thermistor             | Measured voltage (mv)
     / |                        | "connected ground"
    /| |                        |
     +-+                        |
      |-------------------------o
     +-+                        ^
     | |                        |
     | | Pull-down resistor     | Measured voltage (mv)
     | | (pdo)                  | "connected positive"
     +-+                        |
      |                         |
      |                         v
      + GND                     GND

  The arrangements of where we measure the voltage over the thermistor are
  called "connected ground" and "connected positive" and shall be understood as
  the cases when either pull-up or pull-down resistance is zero.

  If the pull-up resistance is 0 one end of the thermistor is connected to the
  positive voltage and we get the thermistor on top of a pull-down resistor
  and we take the measure between the thermistor and the pull-down resistor.

  Conversely if the pull-down resistance is zero, one end of the thermistor is
  connected to ground and we get the thermistor under the pull-up resistor
  and we take the measure between the pull-up resistor and the thermistor.

  We can use both pull-up and pull-down resistors at the same time, and then
  the figure illustrates where the voltage will be measured for the "connected
  ground" and "connected positive" cases.

properties:
  $nodename:
    pattern: "^thermistor(.*)?$"

  compatible:
    oneOf:
      - const: epcos,b57330v2103
      - const: epcos,b57891s0103
      - const: murata,ncp15wb473
      - const: murata,ncp18wb473
      - const: murata,ncp21wb473
      - const: murata,ncp03wb473
      - const: murata,ncp15wl333
      - const: murata,ncp03wf104
      - const: murata,ncp15xh103
      - const: samsung,1404-001221
      # Deprecated "ntp," compatible strings
      - const: ntc,ncp15wb473
        deprecated: true
      - const: ntc,ncp18wb473
        deprecated: true
      - const: ntc,ncp21wb473
        deprecated: true
      - const: ntc,ncp03wb473
        deprecated: true
      - const: ntc,ncp15wl333
        deprecated: true

  "#thermal-sensor-cells":
    description: Thermal sensor cells if used for thermal sensoring.
    const: 0

  pullup-uv:
    $ref: /schemas/types.yaml#/definitions/uint32
    description: Pull-up voltage in micro volts. Must always be specified.

  pullup-ohm:
    $ref: /schemas/types.yaml#/definitions/uint32
    description: Pull-up resistance in ohms. Must always be specified, even
      if zero.

  pulldown-ohm:
    $ref: /schemas/types.yaml#/definitions/uint32
    description: Pull-down resistance in ohms. Must always be specified, even
      if zero.

  connected-positive:
    $ref: /schemas/types.yaml#/definitions/flag
    description: Indicates how the thermistor is connected in series with
      a pull-up and/or a pull-down resistor. See the description above for
      an illustration. If this flag is NOT specified, the thermistor is assumed
      to be connected-ground, which usually means a pull-down resistance of
      zero but complex arrangements are possible.

  # See /schemas/iio/adc/adc.yaml
  io-channels:
    maxItems: 1
    description: IIO ADC channel to read the voltage over the resistor. Must
      always be specified.

required:
  - compatible
  - pullup-uv
  - pullup-ohm
  - pulldown-ohm
  - io-channels

additionalProperties: false

examples:
  - |
    thermistor {
      compatible = "murata,ncp18wb473";
      io-channels = <&gpadc 0x06>;
      pullup-uv = <1800000>;
      pullup-ohm = <220000>;
      pulldown-ohm = <0>;
      #thermal-sensor-cells = <0>;
    };