142 lines
4.7 KiB
YAML
142 lines
4.7 KiB
YAML
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# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
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---
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$id: http://devicetree.org/schemas/hwmon/ntc-thermistor.yaml#
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$schema: http://devicetree.org/meta-schemas/core.yaml#
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title: NTC thermistor temperature sensors
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maintainers:
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- Linus Walleij <linus.walleij@linaro.org>
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description: |
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Thermistors with negative temperature coefficient (NTC) are resistors that
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vary in resistance in an often non-linear way in relation to temperature.
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The negative temperature coefficient means that the resistance decreases
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as the temperature rises. Since the relationship between resistance and
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temperature is non-linear, software drivers most often need to use a look
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up table and interpolation to get from resistance to temperature.
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When used in practice, a thermistor is often connected between ground, a
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pull-up resistor or/and a pull-down resistor and a fixed voltage like this:
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+ e.g. 5V = pull-up voltage (puv)
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+-+
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| | Pull-up resistor
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| | (puo)
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+-+
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|-------------------------o
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+-+ | ^
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| |/ |
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| / |
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|/| Thermistor | Measured voltage (mv)
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/ | | "connected ground"
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/| | |
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+-+ |
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|-------------------------o
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+-+ ^
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| | |
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| | Pull-down resistor | Measured voltage (mv)
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| | (pdo) | "connected positive"
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+-+ |
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| v
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+ GND GND
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The arrangements of where we measure the voltage over the thermistor are
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called "connected ground" and "connected positive" and shall be understood as
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the cases when either pull-up or pull-down resistance is zero.
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If the pull-up resistance is 0 one end of the thermistor is connected to the
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positive voltage and we get the thermistor on top of a pull-down resistor
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and we take the measure between the thermistor and the pull-down resistor.
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Conversely if the pull-down resistance is zero, one end of the thermistor is
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connected to ground and we get the thermistor under the pull-up resistor
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and we take the measure between the pull-up resistor and the thermistor.
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We can use both pull-up and pull-down resistors at the same time, and then
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the figure illustrates where the voltage will be measured for the "connected
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ground" and "connected positive" cases.
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properties:
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$nodename:
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pattern: "^thermistor(.*)?$"
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compatible:
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oneOf:
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- const: epcos,b57330v2103
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- const: epcos,b57891s0103
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- const: murata,ncp15wb473
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- const: murata,ncp18wb473
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- const: murata,ncp21wb473
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- const: murata,ncp03wb473
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- const: murata,ncp15wl333
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- const: murata,ncp03wf104
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- const: murata,ncp15xh103
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- const: samsung,1404-001221
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# Deprecated "ntp," compatible strings
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- const: ntc,ncp15wb473
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deprecated: true
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- const: ntc,ncp18wb473
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deprecated: true
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- const: ntc,ncp21wb473
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deprecated: true
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- const: ntc,ncp03wb473
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deprecated: true
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- const: ntc,ncp15wl333
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deprecated: true
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"#thermal-sensor-cells":
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description: Thermal sensor cells if used for thermal sensoring.
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const: 0
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pullup-uv:
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$ref: /schemas/types.yaml#/definitions/uint32
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description: Pull-up voltage in micro volts. Must always be specified.
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pullup-ohm:
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$ref: /schemas/types.yaml#/definitions/uint32
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description: Pull-up resistance in ohms. Must always be specified, even
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if zero.
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pulldown-ohm:
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$ref: /schemas/types.yaml#/definitions/uint32
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description: Pull-down resistance in ohms. Must always be specified, even
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if zero.
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connected-positive:
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$ref: /schemas/types.yaml#/definitions/flag
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description: Indicates how the thermistor is connected in series with
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a pull-up and/or a pull-down resistor. See the description above for
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an illustration. If this flag is NOT specified, the thermistor is assumed
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to be connected-ground, which usually means a pull-down resistance of
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zero but complex arrangements are possible.
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# See /schemas/iio/adc/adc.yaml
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io-channels:
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maxItems: 1
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description: IIO ADC channel to read the voltage over the resistor. Must
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always be specified.
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required:
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- compatible
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- pullup-uv
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- pullup-ohm
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- pulldown-ohm
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- io-channels
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additionalProperties: false
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examples:
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- |
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thermistor {
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compatible = "murata,ncp18wb473";
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io-channels = <&gpadc 0x06>;
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pullup-uv = <1800000>;
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pullup-ohm = <220000>;
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pulldown-ohm = <0>;
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#thermal-sensor-cells = <0>;
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};
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