From: Liam Beguin <lvb@xxxxxxxxxx> An ADC is often used to measure other quantities indirectly. This binding describe one case, the measurement of a temperature through the voltage across an RTD resistor such as a PT1000. Signed-off-by: Liam Beguin <lvb@xxxxxxxxxx> Reviewed-by: Rob Herring <robh@xxxxxxxxxx> --- .../iio/afe/temperature-sense-rtd.yaml | 101 ++++++++++++++++++ 1 file changed, 101 insertions(+) create mode 100644 Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml diff --git a/Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml b/Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml new file mode 100644 index 000000000000..336ce96371db --- /dev/null +++ b/Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml @@ -0,0 +1,101 @@ +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause) +%YAML 1.2 +--- +$id: http://devicetree.org/schemas/iio/afe/temperature-sense-rtd.yaml# +$schema: http://devicetree.org/meta-schemas/core.yaml# + +title: Temperature Sense RTD + +maintainers: + - Liam Beguin <liambeguin@xxxxxxxxx> + +description: | + RTDs (Resistance Temperature Detectors) are a kind of temperature sensors + used to get a linear voltage to temperature reading within a give range + (usually 0 to 100 degrees Celsius). + + When an io-channel measures the output voltage across an RTD such as a + PT1000, the interesting measurement is almost always the corresponding + temperature, not the voltage output. This binding describes such a circuit. + + The general transfer function here is (using SI units) + + V = R(T) * iexc + R(T) = r0 * (1 + alpha * T) + T = 1 / (alpha * r0 * iexc) * (V - r0 * iexc) + + The following circuit matches what's in the examples section. + + 5V0 + ----- + | + +---+----+ + | R 5k | + +---+----+ + | + V 1mA + | + +---- Vout + | + +---+----+ + | PT1000 | + +---+----+ + | + ----- + GND + +properties: + compatible: + const: temperature-sense-rtd + + io-channels: + maxItems: 1 + description: | + Channel node of a voltage io-channel. + + '#io-channel-cells': + const: 0 + + excitation-current-microamp: + description: The current fed through the RTD sensor. + + alpha-ppm-per-celsius: + description: | + alpha can also be expressed in micro-ohms per ohm Celsius. It's a linear + approximation of the resistance versus temperature relationship + between 0 and 100 degrees Celsius. + + alpha = (R_100 - R_0) / (100 * R_0) + + Where, R_100 is the resistance of the sensor at 100 degrees Celsius, and + R_0 (or r-naught-ohms) is the resistance of the sensor at 0 degrees + Celsius. + + Pure platinum has an alpha of 3925. Industry standards such as IEC60751 + and ASTM E-1137 specify an alpha of 3850. + + r-naught-ohms: + description: | + Resistance of the sensor at 0 degrees Celsius. + Common values are 100 for PT100, 500 for PT500, and 1000 for PT1000 + +additionalProperties: false +required: + - compatible + - io-channels + - excitation-current-microamp + - alpha-ppm-per-celsius + - r-naught-ohms + +examples: + - | + pt1000_1: temperature-sensor0 { + compatible = "temperature-sense-rtd"; + #io-channel-cells = <0>; + io-channels = <&temp_adc1 0>; + + excitation-current-microamp = <1000>; /* i = U/R = 5 / 5000 */ + alpha-ppm-per-celsius = <3908>; + r-naught-ohms = <1000>; + }; +... -- 2.32.0.452.g940fe202adcb
